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The RISC-V Instruction Set Manual, Volume II: Privileged Architecture

3 Machine-Level ISA, Version 1.12

This chapter describes the machine-level operations available in machine-mode (M-mode), which is the highest privilege mode in a RISC-V system. M-mode is used for low-level access to a hardware platform and is the first mode entered at reset. M-mode can also be used to implement features that are too difficult or expensive to implement in hardware directly. The RISC-V machine-level ISA contains a common core that is extended depending on which other privilege levels are supported and other details of the hardware implementation.

3.1 Machine-Level CSRs

In addition to the machine-level CSRs described in this section, M-mode code can access all CSRs at lower privilege levels.

3.1.1 Machine ISA Register misa

The misa CSR is a WARL read-write register reporting the ISA supported by the hart. This register must be readable in any implementation, but a value of zero can be returned to indicate the misa register has not been implemented, requiring that CPU capabilities be determined through a separate non-standard mechanism.

Machine ISA register (misa).

The MXL (Machine XLEN) field encodes the native base integer ISA width as shown in Table [misabase]. The MXL field may be writable in implementations that support multiple base ISA widths. The effective XLEN in M-mode, MXLEN, is given by the setting of MXL, or has a fixed value if misa is zero. The MXL field is always set to the widest supported ISA variant at reset.

MXL XLEN
1 32
2 64
3 128

The misa CSR is MXLEN bits wide. If the value read from misa is nonzero, field MXL of that value always denotes the current MXLEN. If a write to misa causes MXLEN to change, the position of MXL moves to the most-significant two bits of misa at the new width.

The base width can be quickly ascertained using branches on the sign of the returned misa value, and possibly a shift left by one and a second branch on the sign. These checks can be written in assembly code without knowing the register width (XLEN) of the machine. The base width is given by XLEN = 2MXL + 4.

The base width can also be found if misa is zero, by placing the immediate 4 in a register then shifting the register left by 31 bits at a time. If zero after one shift, then the machine is RV32. If zero after two shifts, then the machine is RV64, else RV128.

The Extensions field encodes the presence of the standard extensions, with a single bit per letter of the alphabet (bit 0 encodes presence of extension “A” , bit 1 encodes presence of extension “B”, through to bit 25 which encodes “Z”). The “I” bit will be set for RV32I, RV64I, RV128I base ISAs, and the “E” bit will be set for RV32E. The Extensions field is a WARL field that can contain writable bits where the implementation allows the supported ISA to be modified. At reset, the Extensions field should contain the maximal set of supported extensions, and I should be selected over E if both are available.

The RV128I base ISA is not yet frozen, and while much of the remainder of this specification is expected to apply to RV128, this version of the document focuses only on RV32 and RV64.

The “U” and “S” bits will be set if there is support for user and supervisor modes respectively.

The “X” bit will be set if there are any non-standard extensions.

Bit Character Description
0 A Atomic extension
1 B Tentatively reserved for Bit-Manipulation extension
2 C Compressed extension
3 D Double-precision floating-point extension
4 E RV32E base ISA
5 F Single-precision floating-point extension
6 G Reserved
7 H Hypervisor extension
8 I RV32I/64I/128I base ISA
9 J Tentatively reserved for Dynamically Translated Languages extension
10 K Reserved
11 L Tentatively reserved for Decimal Floating-Point extension
12 M Integer Multiply/Divide extension
13 N User-level interrupts supported
14 O Reserved
15 P Tentatively reserved for Packed-SIMD extension
16 Q Quad-precision floating-point extension
17 R Reserved
18 S Supervisor mode implemented
19 T Tentatively reserved for Transactional Memory extension
20 U User mode implemented
21 V Tentatively reserved for Vector extension
22 W Reserved
23 X Non-standard extensions present
24 Y Reserved
25 Z Reserved

The misa CSR exposes a rudimentary catalog of CPU features to machine-mode code. More extensive information can be obtained in machine mode by probing other machine registers, and examining other ROM storage in the system as part of the boot process.

We require that lower privilege levels execute environment calls instead of reading CPU registers to determine features available at each privilege level. This enables virtualization layers to alter the ISA observed at any level, and supports a much richer command interface without burdening hardware designs.

The “E” bit is read-only. Unless misa is hardwired to zero, the “E” bit always reads as the complement of the “I” bit. An implementation that supports both RV32E and RV32I can select RV32E by clearing the “I” bit.

If an ISA feature x depends on an ISA feature y, then attempting to enable feature x but disable feature y results in both features being disabled. For example, setting “F”=0 and “D”=1 results in both “F” and “D” being cleared.

An implementation may impose additional constraints on the collective setting of two or more misa fields, in which case they function collectively as a single WARL field. An attempt to write an unsupported combination causes those bits to be set to some supported combination.

Writing misa may increase IALIGN, e.g., by disabling the “C” extension. If an instruction that would write misa increases IALIGN, and the subsequent instruction’s address is not IALIGN-bit aligned, the write to misa is suppressed, leaving misa unchanged.

3.1.2 Machine Vendor ID Register mvendorid

The mvendorid CSR is a 32-bit read-only register providing the JEDEC manufacturer ID of the provider of the core. This register must be readable in any implementation, but a value of 0 can be returned to indicate the field is not implemented or that this is a non-commercial implementation.

Vendor ID register (mvendorid).

JEDEC manufacturer IDs are ordinarily encoded as a sequence of one-byte continuation codes 0x7f, terminated by a one-byte ID not equal to 0x7f, with an odd parity bit in the most-significant bit of each byte. mvendorid encodes the number of one-byte continuation codes in the Bank field, and encodes the final byte in the Offset field, discarding the parity bit. For example, the JEDEC manufacturer ID 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x8a (twelve continuation codes followed by 0x8a) would be encoded in the mvendorid field as 0x60a.

Previously the vendor ID was to be a number allocated by the RISC-V Foundation, but this duplicates the work of JEDEC in maintaining a manufacturer ID standard. At time of writing, registering a manufacturer ID with JEDEC has a one-time cost of $500.

3.1.3 Machine Architecture ID Register marchid

The marchid CSR is an MXLEN-bit read-only register encoding the base microarchitecture of the hart. This register must be readable in any implementation, but a value of 0 can be returned to indicate the field is not implemented. The combination of mvendorid and marchid should uniquely identify the type of hart microarchitecture that is implemented.

Machine Architecture ID register (marchid).

Open-source project architecture IDs are allocated globally by the RISC-V Foundation, and have non-zero architecture IDs with a zero most-significant-bit (MSB). Commercial architecture IDs are allocated by each commercial vendor independently, but must have the MSB set and cannot contain zero in the remaining MXLEN-1 bits.

The intent is for the architecture ID to represent the microarchitecture associated with the repo around which development occurs rather than a particular organization. Commercial fabrications of open-source designs should (and might be required by the license to) retain the original architecture ID. This will aid in reducing fragmentation and tool support costs, as well as provide attribution. Open-source architecture IDs should be administered by the Foundation and should only be allocated to released, functioning open-source projects. Commercial architecture IDs can be managed independently by any registered vendor but are required to have IDs disjoint from the open-source architecture IDs (MSB set) to prevent collisions if a vendor wishes to use both closed-source and open-source microarchitectures.

The convention adopted within the following Implementation field can be used to segregate branches of the same architecture design, including by organization. The misa register also helps distinguish different variants of a design.

3.1.4 Machine Implementation ID Register mimpid

The mimpid CSR provides a unique encoding of the version of the processor implementation. This register must be readable in any implementation, but a value of 0 can be returned to indicate that the field is not implemented. The Implementation value should reflect the design of the RISC-V processor itself and not any surrounding system.

Machine Implementation ID register (mimpid).

The format of this field is left to the provider of the architecture source code, but will often be printed by standard tools as a hexadecimal string without any leading or trailing zeros, so the Implementation value can be left-justified (i.e., filled in from most-significant nibble down) with subfields aligned on nibble boundaries to ease human readability.

3.1.5 Hart ID Register mhartid

The mhartid CSR is an MXLEN-bit read-only register containing the integer ID of the hardware thread running the code. This register must be readable in any implementation. Hart IDs might not necessarily be numbered contiguously in a multiprocessor system, but at least one hart must have a hart ID of zero. Hart IDs must be unique within the execution environment.

Hart ID register (mhartid).

In certain cases, we must ensure exactly one hart runs some code (e.g., at reset), and so require one hart to have a known hart ID of zero.

For efficiency, system implementers should aim to reduce the magnitude of the largest hart ID used in a system.

3.1.6 Machine Status Registers (mstatus and mstatush)

The mstatus register is an MXLEN-bit read/write register formatted as shown in Figure 1.6 for RV64 and Figure 1.7 for RV32. The mstatus register keeps track of and controls the hart’s current operating state. A restricted view of mstatus appears as the sstatus register in the S-level ISA.

Machine-mode status register (mstatus) for RV64.
Machine-mode status register (mstatus) for RV32.

For RV32 only, mstatush is a 32-bit read/write register formatted as shown in Figure 1.8. Bits 30:4 of mstatush generally contain the same fields found in bits 62:36 of mstatus for RV64. Fields SD, SXL, and UXL do not exist in mstatush.

The mstatush register is not required to be implemented if every field would be hardwired to zero.

Additional machine-mode status register (mstatush) for RV32.

3.1.6.1 Privilege and Global Interrupt-Enable Stack in mstatus register

Global interrupt-enable bits, MIE and SIE, are provided for M-mode and S-mode respectively. These bits are primarily used to guarantee atomicity with respect to interrupt handlers in the current privilege mode.

The global x IE bits are located in the low-order bits of mstatus, allowing them to be atomically set or cleared with a single CSR instruction.

When a hart is executing in privilege mode x, interrupts are globally enabled when x IE=1 and globally disabled when x IE=0. Interrupts for lower-privilege modes, w<x, are always globally disabled regardless of the setting of any global w IE bit for the lower-privilege mode. Interrupts for higher-privilege modes, y>x, are always globally enabled regardless of the setting of the global y IE bit for the higher-privilege mode. Higher-privilege-level code can use separate per-interrupt enable bits to disable selected higher-privilege-mode interrupts before ceding control to a lower-privilege mode.

A higher-privilege mode y could disable all of its interrupts before ceding control to a lower-privilege mode but this would be unusual as it would leave only a synchronous trap, non-maskable interrupt, or reset as means to regain control of the hart.

To support nested traps, each privilege mode x that can respond to interrupts has a two-level stack of interrupt-enable bits and privilege modes. x PIE holds the value of the interrupt-enable bit active prior to the trap, and x PP holds the previous privilege mode. The x PP fields can only hold privilege modes up to x, so MPP is two bits wide and SPP is one bit wide. When a trap is taken from privilege mode y into privilege mode x, x PIE is set to the value of x IE; x IE is set to 0; and x PP is set to y.

For lower privilege modes, any trap (synchronous or asynchronous) is usually taken at a higher privilege mode with interrupts disabled upon entry. The higher-level trap handler will either service the trap and return using the stacked information, or, if not returning immediately to the interrupted context, will save the privilege stack before re-enabling interrupts, so only one entry per stack is required.

An MRET or SRET instruction is used to return from a trap in M-mode or S-mode respectively. When executing an x RET instruction, supposing x PP holds the value y, x IE is set to x PIE; the privilege mode is changed to y; x PIE is set to 1; and x PP is set to U (or M if user-mode is not supported). If x PPM, x RET also sets MPRV=0.

x PP fields are WARL fields that can hold only privilege mode x and any implemented privilege mode lower than x. If privilege mode x is not implemented, then x PP must be hardwired to 0.

M-mode software can determine whether a privilege mode is implemented by writing that mode to MPP then reading it back.

If the machine provides only U and M modes, then only a single hardware storage bit is required to represent either 00 or 11 in MPP.

3.1.6.2 Base ISA Control in mstatus Register

For RV64 systems, the SXL and UXL fields are WARL fields that control the value of XLEN for S-mode and U-mode, respectively. The encoding of these fields is the same as the MXL field of misa, shown in Table [misabase]. The effective XLEN in S-mode and U-mode are termed SXLEN and UXLEN, respectively.

For RV32 systems, the SXL and UXL fields do not exist, and SXLEN=32 and UXLEN=32.

For RV64 systems, if S-mode is not supported, then SXL is hardwired to zero. Otherwise, it is a WARL field that encodes the current value of SXLEN. In particular, the implementation may hardwire SXL so that SXLEN=MXLEN.

For RV64 systems, if U-mode is not supported, then UXL is hardwired to zero. Otherwise, it is a WARL field that encodes the current value of UXLEN. In particular, the implementation may hardwire UXL so that UXLEN=MXLEN or UXLEN=SXLEN.

Whenever XLEN in any mode is set to a value less than the widest supported XLEN, all operations must ignore source operand register bits above the configured XLEN, and must sign-extend results to fill the entire widest supported XLEN in the destination register. Similarly, pc bits above XLEN are ignored, and when the pc is written, it is sign-extended to fill the widest supported XLEN.

We require that operations always fill the entire underlying hardware registers with defined values to avoid implementation-defined behavior.

To reduce hardware complexity, the architecture imposes no checks that lower-privilege modes have XLEN settings less than or equal to the next-higher privilege mode. In practice, such settings would almost always be a software bug, but machine operation is well-defined even in this case.

If MXLEN is changed from 32 to a wider width, each of mstatus fields SXL and UXL, if not hardwired to a forced value, gets the value corresponding to the widest supported width not wider than the new MXLEN.

3.1.6.3 Memory Privilege in mstatus Register

The MPRV (Modify PRiVilege) bit modifies the privilege level at which loads and stores execute in all privilege modes. When MPRV=0, loads and stores behave as normal, using the translation and protection mechanisms of the current privilege mode. When MPRV=1, load and store memory addresses are translated and protected, and endianness is applied, as though the current privilege mode were set to MPP. Instruction address-translation and protection are unaffected by the setting of MPRV. MPRV is hardwired to 0 if U-mode is not supported.

An MRET or SRET instruction that changes the privilege mode to a mode less privileged than M also sets MPRV=0.

The MXR (Make eXecutable Readable) bit modifies the privilege with which loads access virtual memory. When MXR=0, only loads from pages marked readable (R=1 in Figure [sv32pte]) will succeed. When MXR=1, loads from pages marked either readable or executable (R=1 or X=1) will succeed. MXR has no effect when page-based virtual memory is not in effect. MXR is hardwired to 0 if S-mode is not supported.

The MPRV and MXR mechanisms were conceived to improve the efficiency of M-mode routines that emulate missing hardware features, e.g., misaligned loads and stores. MPRV obviates the need to perform address translation in software. MXR allows instruction words to be loaded from pages marked execute-only.

The current privilege mode and the privilege mode specified by MPP might have different XLEN settings. When MPRV=1, load and store memory addresses are treated as though the current XLEN were set to MPP’s XLEN, following the rules in Section 1.1.6.2.

The SUM (permit Supervisor User Memory access) bit modifies the privilege with which S-mode loads and stores access virtual memory. When SUM=0, S-mode memory accesses to pages that are accessible by U-mode (U=1 in Figure [sv32pte]) will fault. When SUM=1, these accesses are permitted. SUM has no effect when page-based virtual memory is not in effect. Note that, while SUM is ordinarily ignored when not executing in S-mode, it is in effect when MPRV=1 and MPP=S. SUM is hardwired to 0 if S-mode is not supported.

The MXR and SUM mechanisms only affect the interpretation of permissions encoded in page-table entries. In particular, they have no impact on whether access-fault exceptions are raised due to PMAs or PMP.

3.1.6.4 Endianness Control in mstatus and mstatush Registers

The MBE, SBE, and UBE bits in mstatus and mstatush are WARL fields that control the endianness of memory accesses other than instruction fetches. Instruction fetches are always little-endian.

MBE controls whether non-instruction-fetch memory accesses made from M-mode (assuming mstatus.MPRV=0) are little-endian (MBE=0) or big-endian (MBE=1).

If S-mode is not supported, SBE is hardwired to 0. Otherwise, SBE controls whether explicit load and store memory accesses made from S-mode are little-endian (SBE=0) or big-endian (SBE=1).

If U-mode is not supported, UBE is hardwired to 0. Otherwise, UBE controls whether explicit load and store memory accesses made from U-mode are little-endian (UBE=0) or big-endian (UBE=1).

For implicit accesses to supervisor-level memory management data structures, such as page tables, endianness is always controlled by SBE. Since changing SBE alters the implementation’s interpretation of these data structures, if any such data structures remain in use across a change to SBE, M-mode software must follow such a change to SBE by executing an SFENCE.VMA instruction with rs1=x0 and rs2=x0.

Only in contrived scenarios will a given memory-management data structure be interpreted as both little-endian and big-endian. In practice, SBE will only be changed at runtime on world switches, in which case neither the old nor new memory-management data structure will be reinterpreted in a different endianness. In this case, no additional SFENCE.VMA is necessary, beyond what would ordinarily be required for a world switch.

If S-mode is supported, an implementation may hardwire SBE so that SBE=MBE. If U-mode is supported, an implementation may hardwire UBE so that UBE=MBE or UBE=SBE.

An implementation supports only little-endian memory accesses if fields MBE, SBE, and UBE are all hardwired to 0. An implementation supports only big-endian memory accesses (aside from instruction fetches) if MBE is hardwired to 1 and SBE and UBE are each hardwired to 1 when S-mode and U-mode are supported.

Volume I defines a hart’s address space as a circular sequence of 2XLEN bytes at consecutive addresses. The correspondence between addresses and byte locations is fixed and not affected by any endianness mode. Rather, the applicable endianness mode determines the order of mapping between memory bytes and a multibyte quantity (halfword, word, etc.).

Standard RISC-V ABIs are expected to be purely little-endian-only or big-endian-only, with no accommodation for mixing endianness. Nevertheless, endianness control has been defined so as to permit, for instance, an OS of one endianness to execute user-mode programs of the opposite endianness. Consideration has been given also to the possibility of nonstandard usages whereby software flips the endianness of memory accesses as needed.

RISC-V instructions are uniformly little-endian to decouple instruction encoding from the current endianness settings, for the benefit of both hardware and software. Otherwise, for instance, a RISC-V assembler or disassembler would always need to know the intended active endianness, despite that the endianness mode might change dynamically during execution. In contrast, by giving instructions a fixed endianness, it is sometimes possible for carefully written software to be endianness-agnostic even in binary form, much like position-independent code.

The choice to have instructions be only little-endian does have consequences, however, for RISC-V software that encodes or decodes machine instructions. In big-endian mode, such software must account for the fact that explicit loads and stores have endianness opposite that of instructions, for example by swapping byte order after loads and before stores.

3.1.6.5 Virtualization Support in mstatus Register

The TVM (Trap Virtual Memory) bit is a WARL field that supports intercepting supervisor virtual-memory management operations. When TVM=1, attempts to read or write the satp CSR or execute the SFENCE.VMA instruction while executing in S-mode will raise an illegal instruction exception. When TVM=0, these operations are permitted in S-mode. TVM is hard-wired to 0 when S-mode is not supported.

The TVM mechanism improves virtualization efficiency by permitting guest operating systems to execute in S-mode, rather than classically virtualizing them in U-mode. This approach obviates the need to trap accesses to most S-mode CSRs.

Trapping satp accesses and the SFENCE.VMA instruction provides the hooks necessary to lazily populate shadow page tables.

The TW (Timeout Wait) bit is a WARL field that supports intercepting the WFI instruction (see Section 1.2.3). When TW=0, the WFI instruction may execute in lower privilege modes when not prevented for some other reason. When TW=1, then if WFI is executed in any less-privileged mode, and it does not complete within an implementation-specific, bounded time limit, the WFI instruction causes an illegal instruction exception. The time limit may always be 0, in which case WFI always causes an illegal instruction exception in less-privileged modes when TW=1. TW is hard-wired to 0 when there are no modes less privileged than M.

Trapping the WFI instruction can trigger a world switch to another guest OS, rather than wastefully idling in the current guest.

When S-mode is implemented, then executing WFI in U-mode causes an illegal instruction exception, unless it completes within an implementation-specific, bounded time limit. A future revision of this specification might add a feature that allows S-mode to selectively permit WFI in U-mode. Such a feature would only be active when TW=0.

The TSR (Trap SRET) bit is a WARL field that supports intercepting the supervisor exception return instruction, SRET. When TSR=1, attempts to execute SRET while executing in S-mode will raise an illegal instruction exception. When TSR=0, this operation is permitted in S-mode. TSR is hard-wired to 0 when S-mode is not supported.

Trapping SRET is necessary to emulate the hypervisor extension (see Chapter [hypervisor]) on implementations that do not provide it.

3.1.6.6 Extension Context Status in mstatus Register

Supporting substantial extensions is one of the primary goals of RISC-V, and hence we define a standard interface to allow unchanged privileged-mode code, particularly a supervisor-level OS, to support arbitrary user-mode state extensions.

To date, the V extension is the only standard extension that defines additional state beyond the floating-point CSR and data registers.

The FS[1:0] WARL field and the XS[1:0] read-only field are used to reduce the cost of context save and restore by setting and tracking the current state of the floating-point unit and any other user-mode extensions respectively. The FS field encodes the status of the floating-point unit, including the CSR fcsr and floating-point data registers f0f31, while the XS field encodes the status of additional user-mode extensions and associated state. These fields can be checked by a context switch routine to quickly determine whether a state save or restore is required. If a save or restore is required, additional instructions and CSRs are typically required to effect and optimize the process.

The design anticipates that most context switches will not need to save/restore state in either or both of the floating-point unit or other extensions, so provides a fast check via the SD bit.

The FS and XS fields use the same status encoding as shown in Table [fsxsencoding], with the four possible status values being Off, Initial, Clean, and Dirty.

Status FS Meaning XS Meaning
0 Off All off
1 Initial None dirty or clean, some on
2 Clean None dirty, some clean
3 Dirty Some dirty

In systems that do not implement S-mode and do not have a floating-point unit, the FS field is hardwired to zero.

In systems without additional user extensions requiring new state, the XS field is hardwired to zero. Every additional extension with state provides a CSR field that encodes the equivalent of the XS states. The XS field represents a summary of all extensions’ status as shown in Table [fsxsencoding].

The XS field effectively reports the maximum status value across all user-extension status fields, though individual extensions can use a different encoding than XS.

The SD bit is a read-only bit that summarizes whether either the FS field or XS field signals the presence of some dirty state that will require saving extended user context to memory. If both XS and FS are hardwired to zero, then SD is also always zero.

When an extension’s status is set to Off, any instruction that attempts to read or write the corresponding state will cause an illegal instruction exception. When the status is Initial, the corresponding state should have an initial constant value. When the status is Clean, the corresponding state is potentially different from the initial value, but matches the last value stored on a context swap. When the status is Dirty, the corresponding state has potentially been modified since the last context save.

During a context save, the responsible privileged code need only write out the corresponding state if its status is Dirty, and can then reset the extension’s status to Clean. During a context restore, the context need only be loaded from memory if the status is Clean (it should never be Dirty at restore). If the status is Initial, the context must be set to an initial constant value on context restore to avoid a security hole, but this can be done without accessing memory. For example, the floating-point registers can all be initialized to the immediate value 0.

The FS and XS fields are read by the privileged code before saving the context. The FS field is set directly by privileged code when resuming a user context, while the XS field is set indirectly by writing to the status register of the individual extensions. The status fields will also be updated during execution of instructions, regardless of privilege mode.

Extensions to the user-mode ISA often include additional user-mode state, and this state can be considerably larger than the base integer registers. The extensions might only be used for some applications, or might only be needed for short phases within a single application. To improve performance, the user-mode extension can define additional instructions to allow user-mode software to return the unit to an initial state or even to turn off the unit.

For example, a coprocessor might require to be configured before use and can be “unconfigured” after use. The unconfigured state would be represented as the Initial state for context save. If the same application remains running between the unconfigure and the next configure (which would set status to Dirty), there is no need to actually reinitialize the state at the unconfigure instruction, as all state is local to the user process, i.e., the Initial state may only cause the coprocessor state to be initialized to a constant value at context restore, not at every unconfigure.

Executing a user-mode instruction to disable a unit and place it into the Off state will cause an illegal instruction exception to be raised if any subsequent instruction tries to use the unit before it is turned back on. A user-mode instruction to turn a unit on must also ensure the unit’s state is properly initialized, as the unit might have been used by another context meantime.

Changing the setting of FS has no effect on the contents of the floating-point register state. In particular, setting FS=Off does not destroy the state, nor does setting FS=Initial clear the contents. Other extensions might not preserve state when set to Off.

Implementations may choose to track the dirtiness of the floating-point register state imprecisely by reporting the state to be dirty even when it has not been modified. On some implementations, some instructions that do not mutate the floating-point state may cause the state to transition from Initial or Clean to Dirty. On other implementations, dirtiness might not be tracked at all, in which case the valid FS states are Off and Dirty, and an attempt to set FS to Initial or Clean causes it to be set to Dirty.

This definition of FS does not disallow setting FS to Dirty as a result of errant speculation. Some platforms may choose to disallow speculatively writing FS to close a potential side channel.

Table 1.9 shows all the possible state transitions for the FS or XS status bits. Note that the standard floating-point extensions do not support user-mode unconfigure or disable/enable instructions.

FS and XS state transitions.

Standard privileged instructions to initialize, save, and restore extension state are provided to insulate privileged code from details of the added extension state by treating the state as an opaque object.

Many coprocessor extensions are only used in limited contexts that allows software to safely unconfigure or even disable units when done. This reduces the context-switch overhead of large stateful coprocessors.

We separate out floating-point state from other extension state, as when a floating-point unit is present the floating-point registers are part of the standard calling convention, and so user-mode software cannot know when it is safe to disable the floating-point unit.

The XS field provides a summary of all added extension state, but additional microarchitectural bits might be maintained in the extension to further reduce context save and restore overhead.

The SD bit is read-only and is set when either the FS or XS bits encode a Dirty state (i.e., SD=((FS==11) OR (XS==11))). This allows privileged code to quickly determine when no additional context save is required beyond the integer register set and PC.

The floating-point unit state is always initialized, saved, and restored using standard instructions (F, D, and/or Q), and privileged code must be aware of FLEN to determine the appropriate space to reserve for each f register.

All privileged modes share a single copy of the FS and XS bits. In a system with more than one privileged mode, supervisor mode would normally use the FS and XS bits directly to record the status with respect to the supervisor-level saved context. Other more-privileged active modes must be more conservative in saving and restoring the extension state in their corresponding version of the context.

In any reasonable use case, the number of context switches between user and supervisor level should far outweigh the number of context switches to other privilege levels. Note that coprocessors should not require their context to be saved and restored to service asynchronous interrupts, unless the interrupt results in a user-level context swap.

3.1.7 Machine Trap-Vector Base-Address Register (mtvec)

The mtvec register is an MXLEN-bit WARL read/write register that holds trap vector configuration, consisting of a vector base address (BASE) and a vector mode (MODE).

Machine trap-vector base-address register (mtvec).

The mtvec register must always be implemented, but can contain a hardwired read-only value. If mtvec is writable, the set of values the register may hold can vary by implementation. The value in the BASE field must always be aligned on a 4-byte boundary, and the MODE setting may impose additional alignment constraints on the value in the BASE field.

We allow for considerable flexibility in implementation of the trap vector base address. On the one hand, we do not wish to burden low-end implementations with a large number of state bits, but on the other hand, we wish to allow flexibility for larger systems.

Value Name Description
0 Direct All exceptions set pc to BASE.
1 Vectored Asynchronous interrupts set pc to BASE+4×cause.
2 Reserved

The encoding of the MODE field is shown in Table [mtvec-mode]. When MODE=Direct, all traps into machine mode cause the pc to be set to the address in the BASE field. When MODE=Vectored, all synchronous exceptions into machine mode cause the pc to be set to the address in the BASE field, whereas interrupts cause the pc to be set to the address in the BASE field plus four times the interrupt cause number. For example, a machine-mode timer interrupt (see Table [mcauses]) causes the pc to be set to BASE+0x1c.

When vectored interrupts are enabled, interrupt cause 0, which corresponds to user-mode software interrupts, are vectored to the same location as synchronous exceptions. This ambiguity does not arise in practice, since user-mode software interrupts are either disabled or delegated to a less-privileged mode.

An implementation may have different alignment constraints for different modes. In particular, MODE=Vectored may have stricter alignment constraints than MODE=Direct.

Allowing coarser alignments in Vectored mode enables vectoring to be implemented without a hardware adder circuit.

Reset and NMI vector locations are given in a platform specification.

3.1.8 Machine Trap Delegation Registers (medeleg and mideleg)

By default, all traps at any privilege level are handled in machine mode, though a machine-mode handler can redirect traps back to the appropriate level with the MRET instruction (Section 1.2.2). To increase performance, implementations can provide individual read/write bits within medeleg and mideleg to indicate that certain exceptions and interrupts should be processed directly by a lower privilege level. The machine exception delegation register (medeleg) and machine interrupt delegation register ( mideleg) are MXLEN-bit read/write registers.

In systems with S-mode, the medeleg and mideleg registers must exist, and setting a bit in medeleg or mideleg will delegate the corresponding trap, when occurring in S-mode or U-mode, to the S-mode trap handler. In systems without S-mode, the medeleg and mideleg registers should not exist (unless the N extension for user-mode interrupts is implemented).

In versions 1.9.1 and earlier , these registers existed but were hardwired to zero in M-mode only, or M/U without N systems. There is no reason to require they return zero in those cases, as the misa register indicates whether they exist.

When a trap is delegated to S-mode, the scause register is written with the trap cause; the sepc register is written with the virtual address of the instruction that took the trap; the stval register is written with an exception-specific datum; the SPP field of mstatus is written with the active privilege mode at the time of the trap; the SPIE field of mstatus is written with the value of the SIE field at the time of the trap; and the SIE field of mstatus is cleared. The mcause, mepc, and mtval registers and the MPP and MPIE fields of mstatus are not written.

An implementation can choose to subset the delegatable traps, with the supported delegatable bits found by writing one to every bit location, then reading back the value in medeleg or mideleg to see which bit positions hold a one.

An implementation shall not hardwire any bits of medeleg to one, i.e., any synchronous trap that can be delegated must support not being delegated. Similarly, an implementation shall not hardwire to one any bits of mideleg corresponding to machine-level interrupts (but may do so for lower-level interrupts).

Version 1.11 and earlier prohibited hardwiring any bits of mideleg to one. Platform standards may always add such restrictions.

Traps never transition from a more-privileged mode to a less-privileged mode. For example, if M-mode has delegated illegal instruction exceptions to S-mode, and M-mode software later executes an illegal instruction, the trap is taken in M-mode, rather than being delegated to S-mode. By contrast, traps may be taken horizontally. Using the same example, if M-mode has delegated illegal instruction exceptions to S-mode, and S-mode software later executes an illegal instruction, the trap is taken in S-mode.

Delegated interrupts result in the interrupt being masked at the delegator privilege level. For example, if the supervisor timer interrupt (STI) is delegated to S-mode by setting mideleg[5], STIs will not be taken when executing in M-mode. By contrast, if mideleg[5] is clear, STIs can be taken in any mode and regardless of current mode will transfer control to M-mode.

Machine Exception Delegation Register medeleg.

medeleg has a bit position allocated for every synchronous exception shown in Table [mcauses], with the index of the bit position equal to the value returned in the mcause register (i.e., setting bit 8 allows user-mode environment calls to be delegated to a lower-privilege trap handler).

Machine Interrupt Delegation Register mideleg.

mideleg holds trap delegation bits for individual interrupts, with the layout of bits matching those in the mip register (i.e., STIP interrupt delegation control is located in bit 5).

For exceptions that cannot occur in less privileged modes, the corresponding medeleg bits should be hardwired to zero. In particular, medeleg[11] is hardwired to zero.

3.1.9 Machine Interrupt Registers (mip and mie)

The mip register is an MXLEN-bit read/write register containing information on pending interrupts, while mie is the corresponding MXLEN-bit read/write register containing interrupt enable bits. Interrupt cause number i (as reported in CSR mcause, Section 1.1.16) corresponds with bit i in both mip and mie. Bits 15:0 are allocated to standard interrupt causes only, while bits 16 and above are available for platform or custom use.

Machine Interrupt-Pending Register (mip).
Machine Interrupt-Enable Register (mie).

An interrupt i will be taken if bit i is set in both mip and mie, and if interrupts are globally enabled. By default, M-mode interrupts are globally enabled if the hart’s current privilege mode is less than M, or if the current privilege mode is M and the MIE bit in the mstatus register is set. If bit i in mideleg is set, however, interrupts are considered to be globally enabled if the hart’s current privilege mode equals the delegated privilege mode and that mode’s interrupt enable bit (x IE in mstatus for mode x) is set, or if the current privilege mode is less than the delegated privilege mode.

Each individual bit in register mip may be writable or may be read-only. When bit i in mip is writable, a pending interrupt i can be cleared by writing 0 to this bit. If interrupt i can become pending but bit i in mip is read-only, the implementation must provide some other mechanism for clearing the pending interrupt.

A bit in mie must be writable if the corresponding interrupt can ever become pending. Bits of mie that are not writable must be hardwired to zero.

The standard portions (bits 15:0) of registers mip and mie are formatted as shown in Figures 1.15 and 1.16 respectively.

Standard portion (bits 15:0) of mip.
Standard portion (bits 15:0) of mie.

The machine-level interrupt registers handle a few root interrupt sources which are assigned a fixed service priority for simplicity, while separate external interrupt controllers can implement a more complex prioritization scheme over a much larger set of interrupts that are then muxed into the machine-level interrupt sources.

The non-maskable interrupt is not made visible via the mip register as its presence is implicitly known when executing the NMI trap handler.

Bits mip.MEIP and mie.MEIE are the interrupt-pending and interrupt-enable bits for machine-level external interrupts. MEIP is read-only in mip, and is set and cleared by a platform-specific interrupt controller.

Bits mip.MTIP and mie.MTIE are the interrupt-pending and interrupt-enable bits for machine timer interrupts. MTIP is read-only in mip, and is cleared by writing to the memory-mapped machine-mode timer compare register.

Bits mip.MSIP and mie.MSIE are the interrupt-pending and interrupt-enable bits for machine-level software interrupts. MSIP is read-only in mip, and is written by accesses to memory-mapped control registers, which are used by remote harts to provide machine-level interprocessor interrupts. A hart can write its own MSIP bit using the same memory-mapped control register.

If supervisor mode is not implemented, bits SEIP, STIP, and SSIP of mip and SEIE, STIE, and SSIE of mie are hardwired to zeros.

If supervisor mode is implemented, bits mip.SEIP and mie.SEIE are the interrupt-pending and interrupt-enable bits for supervisor-level external interrupts. SEIP is writable in mip, and may be written by M-mode software to indicate to S-mode that an external interrupt is pending. Additionally, the platform-level interrupt controller may generate supervisor-level external interrupts. Supervisor-level external interrupts are made pending based on the logical-OR of the software-writable SEIP bit and the signal from the external interrupt controller. When mip is read with a CSR instruction, the value of the SEIP bit returned in the rd destination register is the logical-OR of the software-writable bit and the interrupt signal from the interrupt controller. However, the value used in the read-modify-write sequence of a CSRRS or CSRRC instruction contains only the software-writable SEIP bit, ignoring the interrupt value from the external interrupt controller.

The SEIP field behavior is designed to allow a higher privilege layer to mimic external interrupts cleanly, without losing any real external interrupts. The behavior of the CSR instructions is slightly modified from regular CSR accesses as a result.

If supervisor mode is implemented, bits mip.STIP and mie.STIE are the interrupt-pending and interrupt-enable bits for supervisor-level timer interrupts. STIP is writable in mip, and may be written by M-mode software to deliver timer interrupts to S-mode.

If supervisor mode is implemented, bits mip.SSIP and mie.SSIE are the interrupt-pending and interrupt-enable bits for supervisor-level software interrupts. SSIP is writable in mip.

Interprocessor interrupts at supervisor level are implemented through implementation-specific mechanisms, e.g., via calls to an SEE, which might ultimately result in a machine-mode write to the receiving hart’s MSIP bit.

We allow a hart to directly write only its own SSIP bit, not those of other harts, as other harts might be virtualized and possibly descheduled by higher privilege levels. We rely on calls to the SEE to provide interprocessor interrupts for this reason. Machine-mode harts are not virtualized and can directly interrupt other harts by setting their MSIP bits, typically using uncached I/O writes to memory-mapped control registers depending on the platform specification.

Multiple simultaneous interrupts destined for different privilege modes are handled in decreasing order of destined privilege mode. Multiple simultaneous interrupts destined for the same privilege mode are handled in the following decreasing priority order: MEI, MSI, MTI, SEI, SSI, STI. Synchronous exceptions are of lower priority than all interrupts.

The machine-level interrupt fixed-priority ordering rules were developed with the following rationale.

Interrupts for higher privilege modes must be serviced before interrupts for lower privilege modes to support preemption.

The platform-specific machine-level interrupt sources in bits 16 and above have platform-specific priority, but are typically chosen to have the highest service priority to support very fast local vectored interrupts.

External interrupts are handled before internal (timer/software) interrupts as external interrupts are usually generated by devices that might require low interrupt service times.

Software interrupts are handled before internal timer interrupts, because internal timer interrupts are usually intended for time slicing, where time precision is less important, whereas software interrupts are used for inter-processor messaging. Software interrupts can be avoided when high-precision timing is required, or high-precision timer interrupts can be routed via a different interrupt path. Software interrupts are located in the lowest four bits of mip as these are often written by software, and this position allows the use of a single CSR instruction with a five-bit immediate.

Synchronous exceptions are given the lowest priority to minimize worst-case interrupt latency.

Restricted views of the mip and mie registers appear as the sip and sie registers for supervisor level. If an interrupt is delegated to S-mode by setting a bit in the mideleg register, it becomes visible in the sip register and is maskable using the sie register. Otherwise, the corresponding bits in sip and sie appear to be hardwired to zero.

3.1.10 Machine Timer Registers (mtime and mtimecmp)

Platforms provide a real-time counter, exposed as a memory-mapped machine-mode read-write register, mtime. mtime must run at constant frequency, and the platform must provide a mechanism for determining the timebase of mtime.

The mtime register has a 64-bit precision on all RV32 and RV64 systems. Platforms provide a 64-bit memory-mapped machine-mode timer compare register (mtimecmp), which causes a timer interrupt to be posted when the mtime register contains a value greater than or equal to the value in the mtimecmp register. The interrupt remains posted until mtimecmp becomes greater than mtime (typically as a result of writing mtimecmp). The interrupt will only be taken if interrupts are enabled and the MTIE bit is set in the mie register.

Machine time register (memory-mapped control register).
Machine time compare register (memory-mapped control register).

The timer facility is defined to use wall-clock time rather than a cycle counter to support modern processors that run with a highly variable clock frequency to save energy through dynamic voltage and frequency scaling.

Accurate real-time clocks (RTCs) are relatively expensive to provide (requiring a crystal or MEMS oscillator) and have to run even when the rest of system is powered down, and so there is usually only one in a system located in a different frequency/voltage domain from the processors. Hence, the RTC must be shared by all the harts in a system and accesses to the RTC will potentially incur the penalty of a voltage-level-shifter and clock-domain crossing. It is thus more natural to expose mtime as a memory-mapped register than as a CSR.

Lower privilege levels do not have their own timecmp registers. Instead, machine-mode software can implement any number of virtual timers on a hart by multiplexing the next timer interrupt into the mtimecmp register.

Simple fixed-frequency systems can use a single clock for both cycle counting and wall-clock time.

Writes to mtime and mtimecmp are guaranteed to be reflected in MTIP eventually, but not necessarily immediately.

A spurious timer interrupt might occur if an interrupt handler increments mtimecmp then immediately returns, because MTIP might not yet have fallen in the interim. All software should be written to assume this event is possible, but most software should assume this event is extremely unlikely. It is almost always more performant to incur an occasional spurious timer interrupt than to poll MTIP until it falls.

In RV32, memory-mapped writes to mtimecmp modify only one 32-bit part of the register. The following code sequence sets a 64-bit mtimecmp value without spuriously generating a timer interrupt due to the intermediate value of the comparand:

Sample code for setting the 64-bit time comparand in RV32, assuming a little-endian memory system and that the registers live in a strongly ordered I/O region. Storing -1 to the low-order bits of mtimecmp prevents mtimecmp from temporarily becoming smaller than the lesser of the old and new values.

3.1.11 Hardware Performance Monitor

M-mode includes a basic hardware performance-monitoring facility. The mcycle CSR counts the number of clock cycles executed by the processor core on which the hart is running. The minstret CSR counts the number of instructions the hart has retired. The mcycle and minstret registers have 64-bit precision on all RV32 and RV64 systems.

The counter registers have an arbitrary value after system reset, and can be written with a given value. Any CSR write takes effect after the writing instruction has otherwise completed. The mcycle CSR may be shared between harts on the same core, in which case writes to mcycle will be visible to those harts. The platform should provide a mechanism to indicate which harts share an mcycle CSR.

The hardware performance monitor includes 29 additional 64-bit event counters, mhpmcounter3mhpmcounter31. The event selector CSRs, mhpmevent3mhpmevent31, are MXLEN-bit WARL registers that control which event causes the corresponding counter to increment. The meaning of these events is defined by the platform, but event 0 is defined to mean “no event.” All counters should be implemented, but a legal implementation is to hard-wire both the counter and its corresponding event selector to 0.

Hardware performance monitor counters.

The mhpmcounters are WARL registers that support up to 64 bits of precision on RV32 and RV64.

A future revision of this specification will define a mechanism to generate an interrupt when a hardware performance monitor counter overflows.

On RV32 only, reads of the mcycle, minstret, and mhpmcountern CSRs return the low 32 bits, while reads of the mcycleh, minstreth, and mhpmcounternh CSRs return bits 63–32 of the corresponding counter.

Upper 32 bits of hardware performance monitor counters, RV32 only.

3.1.12 Counter-Enable Registers ([m|s]counteren)

The counter-enable registers mcounteren and scounteren are 32-bit registers that control the availability of the hardware performance-monitoring counters to the next-lowest privileged mode.

Counter-enable registers (mcounteren and scounteren).

The settings in these registers only control accessibility. The act of reading or writing these registers does not affect the underlying counters, which continue to increment even when not accessible.

When the CY, TM, IR, or HPMn bit in the mcounteren register is clear, attempts to read the cycle, time, instret, or hpmcountern register while executing in S-mode or U-mode will cause an illegal instruction exception. When one of these bits is set, access to the corresponding register is permitted in the next implemented privilege mode (S-mode if implemented, otherwise U-mode).

If S-mode is implemented, the same bit positions in the scounteren register analogously control access to these registers while executing in U-mode. If S-mode is permitted to access a counter register and the corresponding bit is set in scounteren, then U-mode is also permitted to access that register.

Registers mcounteren and scounteren are WARL registers that must be implemented if U-mode and S-mode are implemented. Any of the bits may contain a hardwired value of zero, indicating reads to the corresponding counter will cause an illegal instruction exception when executing in a less-privileged mode.

The counter-enable bits support two common use cases with minimal hardware. For systems that do not need high-performance timers and counters, machine-mode software can trap accesses and implement all features in software. For systems that need high-performance timers and counters but are not concerned with obfuscating the underlying hardware counters, the counters can be directly exposed to lower privilege modes.

The cycle, instret, and hpmcountern CSRs are read-only shadows of mcycle, minstret, and mhpmcounter n, respectively. The time CSR is a read-only shadow of the memory-mapped mtime register. Analogously, on RV32I the cycleh, instreth and hpmcountern CSRs are read-only shadows of mcycleh, minstreth and mhpmcounternh, respectively. On RV32I the timeh CSR is a read-only shadow of the upper 32 bits of the memory-mapped mtime register, while time shadows only the lower 32 bits of mtime.

Implementations can convert reads of the time and timeh CSRs into loads to the memory-mapped mtime register, or emulate this functionality in M-mode software.

3.1.13 Machine Counter-Inhibit CSR (mcountinhibit)

Counter-inhibit register mcountinhibit.

The counter-inhibit register mcountinhibit is a 32-bit WARL register that controls which of the hardware performance-monitoring counters increment. The settings in this register only control whether the counters increment; their accessibility is not affected by the setting of this register.

When the CY, IR, or HPMn bit in the mcountinhibit register is clear, the cycle, instret, or hpmcountern register increments as usual. When the CY, IR, or HPMn bit is set, the corresponding counter does not increment.

The mcycle CSR may be shared between harts on the same core, in which case the mcountinhibit.CY field is also shared between those harts, and so writes to mcountinhibit.CY will be visible to those harts.

If the mcountinhibit register is not implemented, the implementation behaves as though the register were set to zero.

When the cycle and instret counters are not needed, it is desirable to conditionally inhibit them to reduce energy consumption. Providing a single CSR to inhibit all counters also allows the counters to be atomically sampled.

Because the time counter can be shared between multiple cores, it cannot be inhibited with the mcountinhibit mechanism.

3.1.14 Machine Scratch Register (mscratch)

The mscratch register is an MXLEN-bit read/write register dedicated for use by machine mode. Typically, it is used to hold a pointer to a machine-mode hart-local context space and swapped with a user register upon entry to an M-mode trap handler.

Machine-mode scratch register.

The MIPS ISA allocated two user registers (k0/k1) for use by the operating system. Although the MIPS scheme provides a fast and simple implementation, it also reduces available user registers, and does not scale to further privilege levels, or nested traps. It can also require both registers are cleared before returning to user level to avoid a potential security hole and to provide deterministic debugging behavior.

The RISC-V user ISA was designed to support many possible privileged system environments and so we did not want to infect the user-level ISA with any OS-dependent features. The RISC-V CSR swap instructions can quickly save/restore values to the mscratch register. Unlike the MIPS design, the OS can rely on holding a value in the mscratch register while the user context is running.

3.1.15 Machine Exception Program Counter (mepc)

mepc is an MXLEN-bit read/write register formatted as shown in Figure 1.21. The low bit of mepc (mepc[0]) is always zero. On implementations that support only IALIGN=32, the two low bits (mepc[1:0]) are always zero.

If an implementation allows IALIGN to be either 16 or 32 (by changing CSR misa, for example), then, whenever IALIGN=32, bit mepc[1] is masked on reads so that it appears to be 0. This masking occurs also for the implicit read by the MRET instruction. Though masked, mepc[1] remains writable when IALIGN=32.

mepc is a WARL register that must be able to hold all valid virtual addresses. It need not be capable of holding all possible invalid addresses. Implementations may convert some invalid address patterns into other invalid addresses prior to writing them to mepc.

When address translation is not in effect, virtual addresses and physical addresses are equal. Hence, the set of addresses mepc must be able to represent includes the set of physical addresses that can be used as a valid pc or effective address.

When a trap is taken into M-mode, mepc is written with the virtual address of the instruction that was interrupted or that encountered the exception. Otherwise, mepc is never written by the implementation, though it may be explicitly written by software.

Machine exception program counter register.

3.1.16 Machine Cause Register (mcause)

The mcause register is an MXLEN-bit read-write register formatted as shown in Figure 1.22. When a trap is taken into M-mode, mcause is written with a code indicating the event that caused the trap. Otherwise, mcause is never written by the implementation, though it may be explicitly written by software.

The Interrupt bit in the mcause register is set if the trap was caused by an interrupt. The Exception Code field contains a code identifying the last exception. Table [mcauses] lists the possible machine-level exception codes. The Exception Code is a WLRL field, so is only guaranteed to hold supported exception codes.

Machine Cause register mcause.
Interrupt Exception Code Description
1 0 Reserved
1 1 Supervisor software interrupt
1 2 Reserved
1 3 Machine software interrupt
1 4 Reserved
1 5 Supervisor timer interrupt
1 6 Reserved
1 7 Machine timer interrupt
1 8 Reserved
1 9 Supervisor external interrupt
1 10 Reserved
1 11 Machine external interrupt
1 12–15 Reserved
1 16 Available for platform use
0 0 Instruction address misaligned
0 1 Instruction access fault
0 2 Illegal instruction
0 3 Breakpoint
0 4 Load address misaligned
0 5 Load access fault
0 6 Store/AMO address misaligned
0 7 Store/AMO access fault
0 8 Environment call from U-mode
0 9 Environment call from S-mode
0 10 Reserved
0 11 Environment call from M-mode
0 12 Instruction page fault
0 13 Load page fault
0 14 Reserved
0 15 Store/AMO page fault
0 16–23 Reserved
0 24–31 Available for custom use
0 32–47 Reserved
0 48–63 Available for custom use
0 64 Reserved

Interrupts can be separated from other traps with a single branch on the sign of the mcause register value. A shift left can remove the interrupt bit and scale the exception codes to index into a trap vector table.

We do not distinguish privileged instruction exceptions from illegal opcode exceptions. This simplifies the architecture and also hides details of which higher-privilege instructions are supported by an implementation. The privilege level servicing the trap can implement a policy on whether these need to be distinguished, and if so, whether a given opcode should be treated as illegal or privileged.

If an instruction raises multiple synchronous exceptions, the decreasing priority order of Table [exception-priority] indicates which exception is taken and reported in mcause. The priority of any custom synchronous exceptions is implementation-defined.

Priority Exception Code Description
Highest 3 Instruction address breakpoint
12 Instruction page fault
1 Instruction access fault
2 Illegal instruction
0 Instruction address misaligned
8, 9, 11 Environment call
3 Environment break
3 Load/Store/AMO address breakpoint
6 Store/AMO address misaligned
4 Load address misaligned
15 Store/AMO page fault
13 Load page fault
7 Store/AMO access fault
Lowest 5 Load access fault

Instruction address breakpoints have the same cause value as, but different priority than, data address breakpoints (a.k.a. watchpoints) and environment break exceptions (which are raised by the EBREAK instruction).

Instruction address misaligned exceptions are raised by control-flow instructions with misaligned targets, rather than by the act of fetching an instruction. Therefore, these exceptions have lower priority than other instruction address exceptions.

3.1.17 Machine Trap Value (mtval) Register

The mtval register is an MXLEN-bit read-write register formatted as shown in Figure 1.23. When a trap is taken into M-mode, mtval is either set to zero or written with exception-specific information to assist software in handling the trap. Otherwise, mtval is never written by the implementation, though it may be explicitly written by software. The hardware platform will specify which exceptions must set mtval informatively and which may unconditionally set it to zero.

When a hardware breakpoint is triggered, or an address-misaligned, access-fault, or page-fault exception occurs on an instruction fetch, load, or store, mtval is written with the faulting virtual address. On an illegal instruction trap, mtval may be written with the first XLEN or ILEN bits of the faulting instruction as described below. For other traps, mtval is set to zero, but a future standard may redefine mtval’s setting for other traps.

The mtval register replaces the mbadaddr register in the previous specification. In addition to providing bad addresses, the register can now provide the bad instruction that triggered an illegal instruction trap (and may in future be used to return other information). Returning the instruction bits accelerates instruction emulation and also removes some races that might be present when trying to emulate illegal instructions.

When page-based virtual memory is enabled, mtval is written with the faulting virtual address, even for physical-memory access-fault exceptions. This design reduces datapath cost for most implementations, particularly those with hardware page-table walkers.

Machine Trap Value register.

For misaligned loads and stores that cause access-fault or page-fault exceptions, mtval will contain the virtual address of the portion of the access that caused the fault. For instruction access-fault or page-fault exceptions on systems with variable-length instructions, mtval will contain the virtual address of the portion of the instruction that caused the fault while mepc will point to the beginning of the instruction.

The mtval register can optionally also be used to return the faulting instruction bits on an illegal instruction exception ( mepc points to the faulting instruction in memory).

If this feature is not provided, then mtval is set to zero on an illegal instruction fault.

If this feature is provided, after an illegal instruction trap, mtval will contain the shortest of:

the actual faulting instruction

the first ILEN bits of the faulting instruction

the first XLEN bits of the faulting instruction

The value loaded into mtval is right-justified and all unused upper bits are cleared to zero.

Capturing the faulting instruction in mtval reduces the overhead of instruction emulation, potentially avoiding several partial instruction loads if the instruction is misaligned, and likely data cache misses or slow uncached accesses when loads are used to fetch the instruction into a data register. There is also a problem of atomicity if another agent is manipulating the instruction memory, as might occur in a dynamic translation system.

A requirement is that the entire instruction (or at least the first XLEN bits) are fetched into mtval before taking the trap. This should not constrain implementations, which would typically fetch the entire instruction before attempting to decode the instruction, and avoids complicating software handlers.

A value of zero in mtval signifies either that the feature is not supported, or an illegal zero instruction was fetched. A load from the instruction memory pointed to by mepc can be used to distinguish these two cases (or alternatively, the system configuration information can be interrogated to install the appropriate trap handling before runtime).

If the hardware platform specifies that no exceptions set mtval to a nonzero value, then it may be hardwired to zero. Otherwise, mtval is a WARL register that must be able to hold all valid virtual addresses and the value 0. It need not be capable of holding all possible invalid addresses. Implementations may convert some invalid address patterns into other invalid addresses prior to writing them to mtval. If the feature to return the faulting instruction bits is implemented, mtval must also be able to hold all values less than 2N, where N is the smaller of XLEN and ILEN.

3.2 Machine-Mode Privileged Instructions

3.2.1 Environment Call and Breakpoint

image

The ECALL instruction is used to make a request to the supporting execution environment. When executed in U-mode, S-mode, or M-mode, it generates an environment-call-from-U-mode exception, environment-call-from-S-mode exception, or environment-call-from-M-mode exception, respectively, and performs no other operation.

ECALL generates a different exception for each originating privilege mode so that environment call exceptions can be selectively delegated. A typical use case for Unix-like operating systems is to delegate to S-mode the environment-call-from-U-mode exception but not the others.

The EBREAK instruction is used by debuggers to cause control to be transferred back to a debugging environment. It generates a breakpoint exception and performs no other operation.

As described in the “C” Standard Extension for Compressed Instructions in Volume I of this manual, the C.EBREAK instruction performs the same operation as the EBREAK instruction.

ECALL and EBREAK cause the receiving privilege mode’s epc register to be set to the address of the ECALL or EBREAK instruction itself, not the address of the following instruction. As ECALL and EBREAK cause synchronous exceptions, they are not considered to retire, and should not increment the minstret CSR.

3.2.2 Trap-Return Instructions

Instructions to return from trap are encoded under the PRIV minor opcode.

image

To return after handling a trap, there are separate trap return instructions per privilege level, MRET and SRET. MRET is always provided. SRET must be provided if supervisor mode is supported, and should raise an illegal instruction exception otherwise. SRET should also raise an illegal instruction exception when TSR=1 in mstatus, as described in Section 1.1.6.5. An x RET instruction can be executed in privilege mode x or higher, where executing a lower-privilege x RET instruction will pop the relevant lower-privilege interrupt enable and privilege mode stack. In addition to manipulating the privilege stack as described in Section 1.1.6.1, x RET sets the pc to the value stored in the xepc register.

Previously, there was only a single ERET instruction (which was also earlier known as SRET). To support the addition of user-level interrupts, we needed to add a separate URET instruction to continue to allow classic virtualization of OS code using the ERET instruction. It then became more orthogonal to support a different x RET instruction per privilege level.

If the A extension is supported, the x RET instruction is allowed to clear any outstanding LR address reservation but is not required to. Trap handlers should explicitly clear the reservation if required (e.g., by using a dummy SC) before executing the x RET.

If x RET instructions always cleared LR reservations, it would be impossible to single-step through LR/SC sequences using a debugger.

3.2.3 Wait for Interrupt

The Wait for Interrupt instruction (WFI) provides a hint to the implementation that the current hart can be stalled until an interrupt might need servicing. Execution of the WFI instruction can also be used to inform the hardware platform that suitable interrupts should preferentially be routed to this hart. WFI is available in all privileged modes, and optionally available to U-mode. This instruction may raise an illegal instruction exception when TW=1 in mstatus, as described in Section 1.1.6.5.

image

If an enabled interrupt is present or later becomes present while the hart is stalled, the interrupt exception will be taken on the following instruction, i.e., execution resumes in the trap handler and mepc = pc + 4.

The following instruction takes the interrupt exception and trap, so that a simple return from the trap handler will execute code after the WFI instruction.

The purpose of the WFI instruction is to provide a hint to the implementation, and so a legal implementation is to simply implement WFI as a NOP.

If the implementation does not stall the hart on execution of the instruction, then the interrupt will be taken on some instruction in the idle loop containing the WFI, and on a simple return from the handler, the idle loop will resume execution.

The WFI instruction can also be executed when interrupts are disabled. The operation of WFI must be unaffected by the global interrupt bits in mstatus (MIE and SIE) and the delegation register mideleg (i.e., the hart must resume if a locally enabled interrupt becomes pending, even if it has been delegated to a less-privileged mode), but should honor the individual interrupt enables (e.g, MTIE) (i.e., implementations should avoid resuming the hart if the interrupt is pending but not individually enabled). WFI is also required to resume execution for locally enabled interrupts pending at any privilege level, regardless of the global interrupt enable at each privilege level.

If the event that causes the hart to resume execution does not cause an interrupt to be taken, execution will resume at pc + 4, and software must determine what action to take, including looping back to repeat the WFI if there was no actionable event.

By allowing wakeup when interrupts are disabled, an alternate entry point to an interrupt handler can be called that does not require saving the current context, as the current context can be saved or discarded before the WFI is executed.

As implementations are free to implement WFI as a NOP, software must explicitly check for any relevant pending but disabled interrupts in the code following an WFI, and should loop back to the WFI if no suitable interrupt was detected. The mip, sip, or uip registers can be interrogated to determine the presence of any interrupt in machine, supervisor, or user mode respectively.

The operation of WFI is unaffected by the delegation register settings.

WFI is defined so that an implementation can trap into a higher privilege mode, either immediately on encountering the WFI or after some interval to initiate a machine-mode transition to a lower power state, for example.

The same “wait-for-event” template might be used for possible future extensions that wait on memory locations changing, or message arrival.

3.3 Reset

Upon reset, a hart’s privilege mode is set to M. The mstatus fields MIE and MPRV are reset to 0. If little-endian memory accesses are supported, the mstatus/mstatush field MBE is reset to 0. The misa register is reset to enable the maximal set of supported extensions and widest MXLEN, as described in Section 1.1.1. The pc is set to an implementation-defined reset vector. The mcause register is set to a value indicating the cause of the reset. Writable PMP registers’ A and L fields are set to 0. All other hart state is .

The mcause values after reset have implementation-specific interpretation, but the value 0 should be returned on implementations that do not distinguish different reset conditions. Implementations that distinguish different reset conditions should only use 0 to indicate the most complete reset (e.g., hard reset).

Some designs may have multiple causes of reset (e.g., power-on reset, external hard reset, brownout detected, watchdog timer elapse, sleep-mode wakeup), which machine-mode software and debuggers may wish to distinguish.

mcause reset values may alias mcause values following synchronous exceptions. There should be no ambiguity in this overlap, since on reset the pc is typically set to a different value than on other traps.

3.4 Non-Maskable Interrupts

Non-maskable interrupts (NMIs) are only used for hardware error conditions, and cause an immediate jump to an implementation-defined NMI vector running in M-mode regardless of the state of a hart’s interrupt enable bits. The mepc register is written with the address of the next instruction to be executed at the time the NMI was taken, and mcause is set to a value indicating the source of the NMI. The NMI can thus overwrite state in an active machine-mode interrupt handler.

The values written to mcause on an NMI are implementation-defined, but a value of 0 is reserved to mean “unknown cause” and implementations that do not distinguish sources of NMIs via the mcause register should return 0.

Unlike resets, NMIs do not reset processor state, enabling diagnosis, reporting, and possible containment of the hardware error.

3.5 Physical Memory Attributes

The physical memory map for a complete system includes various address ranges, some corresponding to memory regions, some to memory-mapped control registers, and some to empty holes in the address space. Some memory regions might not support reads, writes, or execution; some might not support subword or subblock accesses; some might not support atomic operations; and some might not support cache coherence or might have different memory models. Similarly, memory-mapped control registers vary in their supported access widths, support for atomic operations, and whether read and write accesses have associated side effects. In RISC-V systems, these properties and capabilities of each region of the machine’s physical address space are termed physical memory attributes (PMAs). This section describes RISC-V PMA terminology and how RISC-V systems implement and check PMAs.

PMAs are inherent properties of the underlying hardware and rarely change during system operation. Unlike physical memory protection values described in Section 1.6, PMAs do not vary by execution context. The PMAs of some memory regions are fixed at chip design time—for example, for an on-chip ROM. Others are fixed at board design time, depending, for example, on which other chips are connected to off-chip buses. Off-chip buses might also support devices that could be changed on every power cycle (cold pluggable) or dynamically while the system is running (hot pluggable). Some devices might be configurable at run time to support different uses that imply different PMAs—for example, an on-chip scratchpad RAM might be cached privately by one core in one end-application, or accessed as a shared non-cached memory in another end-application.

Most systems will require that at least some PMAs are dynamically checked in hardware later in the execution pipeline after the physical address is known, as some operations will not be supported at all physical memory addresses, and some operations require knowing the current setting of a configurable PMA attribute. While many other architectures specify some PMAs in the virtual memory page tables and use the TLB to inform the pipeline of these properties, this approach injects platform-specific information into a virtualized layer and can cause system errors unless attributes are correctly initialized in each page-table entry for each physical memory region. In addition, the available page sizes might not be optimal for specifying attributes in the physical memory space, leading to address-space fragmentation and inefficient use of expensive TLB entries.

For RISC-V, we separate out specification and checking of PMAs into a separate hardware structure, the PMA checker. In many cases, the attributes are known at system design time for each physical address region, and can be hardwired into the PMA checker. Where the attributes are run-time configurable, platform-specific memory-mapped control registers can be provided to specify these attributes at a granularity appropriate to each region on the platform (e.g., for an on-chip SRAM that can be flexibly divided between cacheable and uncacheable uses). PMAs are checked for any access to physical memory, including accesses that have undergone virtual to physical memory translation. To aid in system debugging, we strongly recommend that, where possible, RISC-V processors precisely trap physical memory accesses that fail PMA checks. Precisely trapped PMA violations manifest as instruction, load, or store access-fault exceptions, distinct from virtual-memory page-fault exceptions. Precise PMA traps might not always be possible, for example, when probing a legacy bus architecture that uses access failures as part of the discovery mechanism. In this case, error responses from slave devices will be reported as imprecise bus-error interrupts.

PMAs must also be readable by software to correctly access certain devices or to correctly configure other hardware components that access memory, such as DMA engines. As PMAs are tightly tied to a given physical platform’s organization, many details are inherently platform-specific, as is the means by which software can learn the PMA values for a platform. Some devices, particularly legacy buses, do not support discovery of PMAs and so will give error responses or time out if an unsupported access is attempted. Typically, platform-specific machine-mode code will extract PMAs and ultimately present this information to higher-level less-privileged software using some standard representation.

Where platforms support dynamic reconfiguration of PMAs, an interface will be provided to set the attributes by passing requests to a machine-mode driver that can correctly reconfigure the platform. For example, switching cacheability attributes on some memory regions might involve platform-specific operations, such as cache flushes, that are available only to machine-mode.

3.5.1 Main Memory versus I/O versus Empty Regions

The most important characterization of a given memory address range is whether it holds regular main memory, or I/O devices, or is empty. Regular main memory is required to have a number of properties, specified below, whereas I/O devices can have a much broader range of attributes. Memory regions that do not fit into regular main memory, for example, device scratchpad RAMs, are categorized as I/O regions. Empty regions are also classified as I/O regions but with attributes specifying that no accesses are supported.

3.5.2 Supported Access Type PMAs

Access types specify which access widths, from 8-bit byte to long multi-word burst, are supported, and also whether misaligned accesses are supported for each access width.

Although software running on a RISC-V hart cannot directly generate bursts to memory, software might have to program DMA engines to access I/O devices and might therefore need to know which access sizes are supported.

Main memory regions always support read and write of all access widths required by the attached devices, and can specify whether instruction fetch is supported.

Some platforms might mandate that all of main memory support instruction fetch. Other platforms might prohibit instruction fetch from some main memory regions.

In some cases, the design of a processor or device accessing main memory might support other widths, but must be able to function with the types supported by the main memory.

I/O regions can specify which combinations of read, write, or execute accesses to which data widths are supported.

For systems with page-based virtual memory, I/O and memory regions can specify which combinations of hardware page-table reads and hardware page-table writes are supported.

Unix-like operating systems generally require that all of cacheable main memory supports page-table walks.

3.5.3 Atomicity PMAs

Atomicity PMAs describes which atomic instructions are supported in this address region. Support for atomic instructions is divided into two categories: LR/SC and AMOs.

Some platforms might mandate that all of cacheable main memory support all atomic operations required by the attached processors.

3.5.3.1 AMO PMA

Within AMOs, there are four levels of support: AMONone, AMOSwap, AMOLogical, and AMOArithmetic. AMONone indicates that no AMO operations are supported. AMOSwap indicates that only amoswap instructions are supported in this address range. AMOLogical indicates that swap instructions plus all the logical AMOs (amoand, amoor, amoxor) are supported. AMOArithmetic indicates that all RISC-V AMOs are supported. For each level of support, naturally aligned AMOs of a given width are supported if the underlying memory region supports reads and writes of that width. Main memory and I/O regions may only support a subset or none of the processor-supported atomic operations.

AMO Class Supported Operations
AMONone None
AMOSwap amoswap
AMOLogical above + amoand, amoor, amoxor
AMOArithmetic above + amoadd, amomin, amomax, amominu, amomaxu

We recommend providing at least AMOLogical support for I/O regions where possible.

3.5.3.2 Reservability PMA

For LR/SC, there are three levels of support indicating combinations of the reservability and eventuality properties: RsrvNone, RsrvNonEventual, and RsrvEventual. RsrvNone indicates that no LR/SC operations are supported (the location is non-reservable). RsrvNonEventual indicates that the operations are supported (the location is reservable), but without the eventual success guarantee described in the unprivileged ISA specification. RsrvEventual indicates that the operations are supported and provide the eventual success guarantee.

We recommend providing RsrvEventual support for main memory regions where possible. Most I/O regions will not support LR/SC accesses, as these are most conveniently built on top of a cache-coherence scheme, but some may support RsrvNonEventual or RsrvEventual.

When LR/SC is used for memory locations marked RsrvNonEventual, software should provide alternative fall-back mechanisms used when lack of progress is detected.

3.5.3.3 Alignment

Memory regions that support aligned LR/SC or aligned AMOs might also support misaligned LR/SC or misaligned AMOs for some addresses and access widths. If, for a given address and access width, a misaligned LR/SC or AMO generates an address-misaligned exception, then all loads, stores, LRs/SCs, and AMOs using that address and access width must generate address-misaligned exceptions.

The standard “A” extension does not support misaligned AMOs or LR/SC pairs. Support for misaligned AMOs is provided by the standard “Zam” extension. Support for misaligned LR/SC sequences is not currently standardized, so LR and SC to misaligned addresses must raise an exception.

Mandating that misaligned loads and stores raise address-misaligned exceptions wherever misaligned AMOs raise address-misaligned exceptions permits the emulation of misaligned AMOs in an M-mode trap handler. The handler guarantees atomicity by acquiring a global mutex and emulating the access within the critical section. Provided that the handler for misaligned loads and stores uses the same mutex, all accesses to a given address that use the same word size will be mutually atomic.

Implementations may raise access-fault exceptions instead of address-misaligned exceptions for some misaligned accesses, indicating the instruction should not be emulated by a trap handler. If, for a given address and access width, all misaligned LRs/SCs and AMOs generate access-fault exceptions, then regular misaligned loads and stores using the same address and access width are not required to execute atomically.

3.5.4 Memory-Ordering PMAs

Regions of the address space are classified as either main memory or I/O for the purposes of ordering by the FENCE instruction and atomic-instruction ordering bits.

Accesses by one hart to main memory regions are observable not only by other harts but also by other devices with the capability to initiate requests in the main memory system (e.g., DMA engines). Main memory regions always have either the RVWMO or RVTSO memory model.

Accesses by one hart to an I/O region are observable not only by other harts and bus mastering devices but also by targeted slave I/O devices, and I/O regions may be accessed with either relaxed or strong ordering. Accesses to an I/O region with relaxed ordering are generally observed by other harts and bus mastering devices in a manner similar to the ordering of accesses to an RVWMO memory region, as discussed in Section A.4.2 in Volume I of this specification. By contrast, accesses to an I/O region with strong ordering are generally observed by other harts and bus mastering devices in program order.

Each strongly ordered I/O region specifies a numbered ordering channel, which is a mechanism by which ordering guarantees can be provided between different I/O regions. Channel 0 is used to indicate point-to-point strong ordering only, where only accesses by the hart to the single associated I/O region are strongly ordered.

Channel 1 is used to provide global strong ordering across all I/O regions. Any accesses by a hart to any I/O region associated with channel 1 can only be observed to have occurred in program order by all other harts and I/O devices, including relative to accesses made by that hart to relaxed I/O regions or strongly ordered I/O regions with different channel numbers. In other words, any access to a region in channel 1 is equivalent to executing a fence io,io instruction before and after the instruction.

Other larger channel numbers provide program ordering to accesses by that hart across any regions with the same channel number.

Systems might support dynamic configuration of ordering properties on each memory region.

Strong ordering can be used to improve compatibility with legacy device driver code, or to enable increased performance compared to insertion of explicit ordering instructions when the implementation is known to not reorder accesses.

Local strong ordering (channel 0) is the default form of strong ordering as it is often straightforward to provide if there is only a single in-order communication path between the hart and the I/O device.

Generally, different strongly ordered I/O regions can share the same ordering channel without additional ordering hardware if they share the same interconnect path and the path does not reorder requests.

3.5.5 Coherence and Cacheability PMAs

Coherence is a property defined for a single physical address, and indicates that writes to that address by one agent will eventually be made visible to other agents in the system. Coherence is not to be confused with the memory consistency model of a system, which defines what values a memory read can return given the previous history of reads and writes to the entire memory system. In RISC-V platforms, the use of hardware-incoherent regions is discouraged due to software complexity, performance, and energy impacts.

The cacheability of a memory region should not affect the software view of the region except for differences reflected in other PMAs, such as main memory versus I/O classification, memory ordering, supported accesses and atomic operations, and coherence. For this reason, we treat cacheability as a platform-level setting managed by machine-mode software only.

Where a platform supports configurable cacheability settings for a memory region, a platform-specific machine-mode routine will change the settings and flush caches if necessary, so the system is only incoherent during the transition between cacheability settings. This transitory state should not be visible to lower privilege levels.

We categorize RISC-V caches into three types: master-private, shared, and slave-private. Master-private caches are attached to a single master agent, i.e., one that issues read/write requests to the memory system. Shared caches are located between masters and slaves and may be hierarchically organized. Slave-private caches do not impact coherence, as they are local to a single slave and do not affect other PMAs at a master, so are not considered further here. We use private cache to mean a master-private cache in the following section, unless explicitly stated otherwise.

Coherence is straightforward to provide for a shared memory region that is not cached by any agent. The PMA for such a region would simply indicate it should not be cached in a private or shared cache.

Coherence is also straightforward for read-only regions, which can be safely cached by multiple agents without requiring a cache-coherence scheme. The PMA for this region would indicate that it can be cached, but that writes are not supported.

Some read-write regions might only be accessed by a single agent, in which case they can be cached privately by that agent without requiring a coherence scheme. The PMA for such regions would indicate they can be cached. The data can also be cached in a shared cache, as other agents should not access the region.

If an agent can cache a read-write region that is accessible by other agents, whether caching or non-caching, a cache-coherence scheme is required to avoid use of stale values. In regions lacking hardware cache coherence (hardware-incoherent regions), cache coherence can be implemented entirely in software, but software coherence schemes are notoriously difficult to implement correctly and often have severe performance impacts due to the need for conservative software-directed cache-flushing. Hardware cache-coherence schemes require more complex hardware and can impact performance due to the cache-coherence probes, but are otherwise invisible to software.

For each hardware cache-coherent region, the PMA would indicate that the region is coherent and which hardware coherence controller to use if the system has multiple coherence controllers. For some systems, the coherence controller might be an outer-level shared cache, which might itself access further outer-level cache-coherence controllers hierarchically.

Most memory regions within a platform will be coherent to software, because they will be fixed as either uncached, read-only, hardware cache-coherent, or only accessed by one agent.

3.5.6 Idempotency PMAs

Idempotency PMAs describe whether reads and writes to an address region are idempotent. Main memory regions are assumed to be idempotent. For I/O regions, idempotency on reads and writes can be specified separately (e.g., reads are idempotent but writes are not). If accesses are non-idempotent, i.e., there is potentially a side effect on any read or write access, then speculative or redundant accesses must be avoided.

For the purposes of defining the idempotency PMAs, changes in observed memory ordering created by redundant accesses are not considered a side effect.

While hardware should always be designed to avoid speculative or redundant accesses to memory regions marked as non-idempotent, it is also necessary to ensure software or compiler optimizations do not generate spurious accesses to non-idempotent memory regions.

Non-idempotent regions might not support misaligned accesses. Misaligned accesses to such regions should raise access-fault exceptions rather than address-misaligned exceptions, indicating that software should not emulate the misaligned access using multiple smaller accesses, which could cause unexpected side effects.

3.6 Physical Memory Protection

To support secure processing and contain faults, it is desirable to limit the physical addresses accessible by software running on a hart. An optional physical memory protection (PMP) unit provides per-hart machine-mode control registers to allow physical memory access privileges (read, write, execute) to be specified for each physical memory region. The PMP values are checked in parallel with the PMA checks described in Section 1.5.

The granularity of PMP access control settings are platform-specific, but the standard PMP encoding supports regions as small as four bytes. Certain regions’ privileges can be hardwired—for example, some regions might only ever be visible in machine mode but in no lower-privilege layers.

Platforms vary widely in demands for physical memory protection, and some platforms may provide other PMP structures in addition to or instead of the scheme described in this section.

PMP checks are applied to all accesses whose effective privilege mode is S or U, including instruction fetches in S and U mode, data accesses in S and U mode when the MPRV bit in the mstatus register is clear, and data accesses in any mode when the MPRV bit in mstatus is set and the MPP field in mstatus contains S or U. PMP checks are also applied to page-table accesses for virtual-address translation, for which the effective privilege mode is S. Optionally, PMP checks may additionally apply to M-mode accesses, in which case the PMP registers themselves are locked, so that even M-mode software cannot change them without a system reset. In effect, PMP can grant permissions to S and U modes, which by default have none, and can revoke permissions from M-mode, which by default has full permissions.

PMP violations are always trapped precisely at the processor.

3.6.1 Physical Memory Protection CSRs

PMP entries are described by an 8-bit configuration register and one MXLEN-bit address register. Some PMP settings additionally use the address register associated with the preceding PMP entry. Up to 16 PMP entries are supported. If any PMP entries are implemented, then all PMP CSRs must be implemented, but all PMP CSR fields are WARL and may be hardwired to zero. PMP CSRs are only accessible to M-mode.

The PMP configuration registers are densely packed into CSRs to minimize context-switch time. For RV32, four CSRs, pmpcfg0pmpcfg3, hold the configurations pmp0cfgpmp15cfg for the 16 PMP entries, as shown in Figure 1.24. For RV64, pmpcfg0 and pmpcfg2 hold the configurations for the 16 PMP entries, as shown in Figure 1.25; pmpcfg1 and pmpcfg3 are illegal.

RV64 systems use pmpcfg2, rather than pmpcfg1, to hold configurations for PMP entries 8–15. This design reduces the cost of supporting multiple MXLEN values, since the configurations for PMP entries 8–11 appear in pmpcfg2[31:0] for both RV32 and RV64.

RV32 PMP configuration CSR layout.
RV64 PMP configuration CSR layout.

The PMP address registers are CSRs named pmpaddr0pmpaddr15. Each PMP address register encodes bits 33–2 of a 34-bit physical address for RV32, as shown in Figure 1.26. For RV64, each PMP address register encodes bits 55–2 of a 56-bit physical address, as shown in Figure 1.27. Not all physical address bits may be implemented, and so the pmpaddr registers are WARL.

The Sv32 page-based virtual-memory scheme described in Section [sec:sv32] supports 34-bit physical addresses for RV32, so the PMP scheme must support addresses wider than XLEN for RV32. The Sv39 and Sv48 page-based virtual-memory schemes described in Sections [sec:sv39] and [sec:sv48] support a 56-bit physical address space, so the RV64 PMP address registers impose the same limit.

PMP address register format, RV32.
PMP address register format, RV64.

Figure 1.28 shows the layout of a PMP configuration register. The R, W, and X bits, when set, indicate that the PMP entry permits read, write, and instruction execution, respectively. When one of these bits is clear, the corresponding access type is denied. The combination R=0 and W=1 is reserved for future use. The remaining two fields, A and L, are described in the following sections.

PMP configuration register format.

Attempting to fetch an instruction from a PMP region that does not have execute permissions raises an instruction access-fault exception. Attempting to execute a load or load-reserved instruction whose effective address lies within a PMP region without read permissions raises a load access-fault exception. Attempting to execute a store, store-conditional (regardless of success), or AMO instruction whose effective address lies within a PMP region without write permissions raises a store access-fault exception.

If MXLEN is changed, the contents of the pmpxcfg fields are preserved, but appear in the pmpcfgy CSR prescribed by the new setting of MXLEN. For example, when MXLEN is changed from 64 to 32, pmp4cfg moves from pmpcfg0[39:32] to pmpcfg1[7:0]. The pmpaddr CSRs follow the usual CSR width modulation rules described in Section [sec:csrwidthmodulation].

Address Matching

The A field in a PMP entry’s configuration register encodes the address-matching mode of the associated PMP address register. The encoding of this field is shown in Table [pmpcfg-a]. When A=0, this PMP entry is disabled and matches no addresses. Two other address-matching modes are supported: naturally aligned power-of-2 regions (NAPOT), including the special case of naturally aligned four-byte regions (NA4); and the top boundary of an arbitrary range (TOR). These modes support four-byte granularity.

A Name Description
0 OFF Null region (disabled)
1 TOR Top of range
2 NA4 Naturally aligned four-byte region
3 NAPOT Naturally aligned power-of-two region, 8 bytes

NAPOT ranges make use of the low-order bits of the associated address register to encode the size of the range, as shown in Table 1.29.

NAPOT range encoding in PMP address and configuration registers.

If TOR is selected, the associated address register forms the top of the address range, and the preceding PMP address register forms the bottom of the address range. If PMP entry i’s A field is set to TOR, the entry matches any address y such that ${\tt pmpaddr}_{i-1}\leq y < {\tt pmpaddr}_i$. If PMP entry 0’s A field is set to TOR, zero is used for the lower bound, and so it matches any address $y < {\tt pmpaddr}_0$.

Although the PMP mechanism supports regions as small as four bytes, platforms may specify coarser PMP regions. In general, the PMP grain is 2G + 2 bytes and must be the same across all PMP regions. When G ≥ 1, the NA4 mode is not selectable. When G ≥ 2 and ${\tt pmpcfg}_i$.A[1] is set, i.e. the mode is NAPOT, then bits ${\tt pmpaddr}_i$[G-2:0] read as all ones. When G ≥ 1 and ${\tt pmpcfg}_i$.A[1] is clear, i.e. the mode is OFF or TOR, then bits ${\tt pmpaddr}_i$[G-1:0] read as all zeros. Bits ${\tt pmpaddr}_i$[G-1:0] do not affect the TOR address-matching logic. Although changing ${\tt pmpcfg}_i$.A[1] affects the value read from ${\tt pmpaddr}_i$, it does not affect the underlying value stored in that register—in particular, ${\tt pmpaddr}_i$[G-1] retains its original value when ${\tt pmpcfg}_i$.A is changed from NAPOT to TOR/OFF then back to NAPOT.

Software may determine the PMP granularity by writing zero to pmp0cfg, then writing all ones to pmpaddr0, then reading back pmpaddr0. If G is the index of the least-significant bit set, the PMP granularity is 2G + 2 bytes.

If the current XLEN is greater than MXLEN, the PMP address registers are zero-extended from MXLEN to XLEN bits for the purposes of address matching.

Locking and Privilege Mode

The L bit indicates that the PMP entry is locked, i.e., writes to the configuration register and associated address registers are ignored. Locked PMP entries may only be unlocked with a system reset. If PMP entry i is locked, writes to pmpicfg and pmpaddri are ignored. Additionally, if pmpicfg.A is set to TOR, writes to pmpaddri-1 are ignored.

Setting the L bit locks the PMP entry even when the A field is set to OFF.

In addition to locking the PMP entry, the L bit indicates whether the R/W/X permissions are enforced on M-mode accesses. When the L bit is set, these permissions are enforced for all privilege modes. When the L bit is clear, any M-mode access matching the PMP entry will succeed; the R/W/X permissions apply only to S and U modes.

Priority and Matching Logic

PMP entries are statically prioritized. The lowest-numbered PMP entry that matches any byte of an access determines whether that access succeeds or fails. The matching PMP entry must match all bytes of an access, or the access fails, irrespective of the L, R, W, and X bits. For example, if a PMP entry is configured to match the four-byte range 0xC0xF, then an 8-byte access to the range 0x80xF will fail, assuming that PMP entry is the highest-priority entry that matches those addresses.

If a PMP entry matches all bytes of an access, then the L, R, W, and X bits determine whether the access succeeds or fails. If the L bit is clear and the privilege mode of the access is M, the access succeeds. Otherwise, if the L bit is set or the privilege mode of the access is S or U, then the access succeeds only if the R, W, or X bit corresponding to the access type is set.

If no PMP entry matches an M-mode access, the access succeeds. If no PMP entry matches an S-mode or U-mode access, but at least one PMP entry is implemented, the access fails.

If at least one PMP entry is implemented, but all PMP entries’ A fields are set to OFF, then all S-mode and U-mode memory accesses will fail.

Failed accesses generate an instruction, load, or store access-fault exception. Note that a single instruction may generate multiple accesses, which may not be mutually atomic. An access-fault exception is generated if at least one access generated by an instruction fails, though other accesses generated by that instruction may succeed with visible side effects. Notably, instructions that reference virtual memory are decomposed into multiple accesses.

On some implementations, misaligned loads, stores, and instruction fetches may also be decomposed into multiple accesses, some of which may succeed before an access-fault exception occurs. In particular, a portion of a misaligned store that passes the PMP check may become visible, even if another portion fails the PMP check. The same behavior may manifest for floating-point stores wider than XLEN bits (e.g., the FSD instruction in RV32D), even when the store address is naturally aligned.

3.6.2 Physical Memory Protection and Paging

The Physical Memory Protection mechanism is designed to compose with the page-based virtual memory systems described in Chapter [supervisor]. When paging is enabled, instructions that access virtual memory may result in multiple physical-memory accesses, including implicit references to the page tables. The PMP checks apply to all of these accesses. The effective privilege mode for implicit page-table accesses is S.

Implementations with virtual memory are permitted to perform address translations speculatively and earlier than required by an explicit virtual-memory access. The PMP settings for the resulting physical address may be checked at any point between the address translation and the explicit virtual-memory access. Hence, when the PMP settings are modified in a manner that affects either the physical memory that holds the page tables or the physical memory to which the page tables point, M-mode software must synchronize the PMP settings with the virtual memory system. This is accomplished by executing an SFENCE.VMA instruction with rs1=x0 and rs2=x0, after the PMP CSRs are written.

If page-based virtual memory is not implemented, or when it is disabled, memory accesses check the PMP settings synchronously, so no fence is needed.