• RISC-V Reset and NMI
• RISC-V Interrupts
  • Assertion and Service
  • Vectored or Direct
  • External Interrupts
  • Machine
  • Supervisor
  • User
  • Available for Platform Use
• RISC-V Synchronous Exceptions
  • Delegation to Lower Privilege Mode
• RISC-V Scratch Registers
• Interrupt Handlers In C

RISC-V Reset and NMI

These vectors are implementation defined.

• Reset
  • The reset process is:
    • Reset to 'm' mode.
    • mstatus.mie = 0, Disable all interrupts.
    • mstatus.mprv = 0, Select normal memory access privilege level.
    • misa = DEFAULT_MISA, enable all extensions.
    • mcause = 0 or Implementation defined RESET_MCAUSE_VALUES.
    • PC = Implementation defined RESET_VECTOR.
• Non Maskable Interrupt (NMI)
  • The NMI interrupt entry process is:
    • mepc = PC
    • mcause = 0 or Implementation defined NMI_MCAUSE_VALUES
    • PC = Implementation defined NMI_VECTOR

RISC-V Interrupts

These are the interrupts that are defined by the base ISA.

Assertion and Service

• mcause.Exception_Code (Where Interrupt==1) describes the possible interrupt sources.
• When the interrupt condition is met, a bit in the interrupt pending register (mip) is set.
• To service an interrupt the global interrupt enable (mstatus.mie ) and the per interrupt enable bit (mie) both need to be set.
• If multiple interrupts are pending and enabled simultaneously then the order they are serviced is determined by a fixed priority.
• Interrupts can't be pre-empted without the ISR writing to (mstatus.mie).

Vectored or Direct

Vectored or direct interrupts are both possible.

• mtvec.Mode, the lower 2 bits of the vector.
• When this is 0, it points to a single interrupt service routine (ISR).
• When this is 0, it points to the base of table of ISR entry points (4 bytes per entry).
• Regardless, the address of the ISR is loaded to the PC, so for vectored interrupts the entry should be a jump.

External Interrupts

The exception/interrupt handling seems to be designed around a few root interrupt sources, with external multiplexing of platform interrupts.

• The priority assignment is a simple fixed priority scheme. Within the priviledge level, the higher the bit the higher the priority.
• In the standard interrupts there is a sigle external interrupt bit ( mip.meip ). This can be used to service an external interrupt controller.
• Bits 16 and above of the mip/mie registers are platform specific. This can be used to bring platform interrupts in as root interrupt sources.

Machine

The entry procedure for a interrupt I into machine mode:

• mcause.interrupt = 1; mcause.exception_code = I
• mstatus.mpie = 1, save previous interrupt enable. (See ISR stack.)
• mstatus.mpp = Previous privilege mode (m, s or u).
• mstatus.mie = 0, interrupts are disabled unless the ISR re-writes this.
• mepc = Interrupted PC, save the return address.
• IF mtvec.Mode == Direct(0) THEN; PC = (mtvec & ~0xF)
• IF mtvec.Mode == Vectored(1) THEN; PC = (mtvec & ~0xF) + (I * 4)

The exit procedure for a machine interrupt when mret is executed.

• mstatus.mie = mstatus.mpie, restore interrupt enable
• Privilege mode = mstatus.mpp, restore privilege
• mstatus.mpp = mstatus.mpie = 0
• PC = mepc

Name	mip, mcause, Exception_Code	mtvec Mode==Vectored Base + Offset	Mode	Description	Condition
msi	3	0x0000000c	m	Machine Software Interrupt	mip.msip = Memory mapped control registers allow this bit to be written to assert a software interrupt. (It is used to provide a software interrupt from other harts).
mti	7	0x0000001c	m	Machine Timer Interrupt	mip.mtip = mtime >= mtimecmp (mtimecmp needs to be written to clear the pending bit).
mei	11	0x0000002c	m	Machine External Interrupt	mip.meip = External interrupt signal asserted
ssi	1	0x00000004	s	Supervisor Software Interrupt	mip.ssi = Implementation defined.
sti	5	0x00000014	s	Supervisor Timer Interrupt	mip.sti = Implementation defined.
sei	9	0x00000024	s	Supervisor External Interrupt	mip.sei = Implementation defined.
usi	0	0x00000000	u	User Software Interrupt	mip.usi = Implementation defined.
uti	4	0x00000010	u	User Timer Interrupt	mip.uti = Implementation defined.
uei	8	0x00000020	u	User External Interrupt	mip.uei = Implementation defined.
platform_defined	16+	0x00000040	m	Optional platform defined interrupt sources.	mip.platform_defined = Implementation defined.

Supervisor

Supervisor-level interrupts require support of the s mode privilege. The mideleg register needs to set by m mode to enable s mode to take the interrupts.

Supervisor mode CSRs matching machine mode CSRs are used to implement supervisor interrupts and exceptions. Some are a restricted view to the machine mode CSRs, with dedicated bits for supervisor status, while others are replicated to dedicated registers.

Restricted View CSRs	sstatus, sip, sie
Replicated CSRs	scause, sepc, stvec, sscratch

The entry procedure for a interrupt I into supervisor mode (Assuming mideleg[I] is set):

• scause.interrupt = 1; scause.exception_code = I
• sstatus.spie = 1, save previous interrupt enable. (See ISR stack.)
• sstatus.spp = Previous privilege mode (s or u).
• sstatus.sie = 0, interrupts are disabled unless the ISR re-writes this.
• sepc = Interrupted PC, save the return address.
• IF stvec.mode == Direct THEN; PC = (stvec.Base & ~0xF)
• IF stvec.mode == Vectored THEN; PC = (stvec.Base & ~0xF) + (I * 4)

The exit procedure for a supervisor interrupt when sret is executed.

• sstatus.mie = sstatus.mpie, restore interrupt enable
• Privilege mode = sstatus.spp, restore privilege
• sstatus.spp = sstatus.spie = 0
• PC = sepc

Supervisor interrupts assuming mideleg.ssi, mideleg.sti, mideleg.sei are set:

Name	sip, scause, Exception_Code	stvec Mode==Vectored Base + Offset	Mode	Description	Condition
ssi	1	0x00000004	s	Supervisor Software Interrupt	sip.ssip = Implementation dependent, CSR is written by software on the local hart to assert a software interrupt.
sti	5	0x00000014	s	Supervisor Timer Interrupt	sip.stip = Implementation dependent, Written my 'm' mode timer interrupt handler, an SEE call needs to be made to clear this).
sei	9	0x00000024	s	Supervisor External Interrupt	sip.seip = Implementation dependent, External interrupt signal asserted

User

User-level interrupts are optional for implementation. They rely on the n extension. This specification is not yet complete.

User interrupts assuming sideleg.usi, sideleg.uti, sideleg.uei are set:

Name	uip, ucause, Exception_Code	utvec Mode==Vectored Base + Offset	Mode	Description	Condition
usi	0	0x00000000	u	User Software Interrupt
uti	4	0x00000010	u	User Timer Interrupt
uei	8	0x00000020	u	User External Interrupt

Available for Platform Use

mcause.exception_code values greater than 16 are available for platform use, allowing use if bits 16..mxlen-1 in mip and mie. The platform can define a fixed priority scheme. These can be used to implement a fast vectored interrupt scheme.

RISC-V Synchronous Exceptions

These are the exceptions that are defined by the base ISA.

• Synchronous Exceptions are always caused by software execution. e.g. Faults, errors and environment calls.
• Mcause.Exception_Code (Where Interrupt==0) describes the possible interrupt sources.
• Simultaneous exceptions are serviced according to a fixed priority.

The entry procedure for an exception E into machine mode:

• mcause.interrupt = 0; mcause.exception_code = E
• mstatus.mpie = mstatus.mie, save previous interrupt enable
• mstatus.mpp = Previous privilege mode (m, s or u)
• mstatus.mie = 0
• Privilege mode = m
• mtval = Exception specific information related to the cause.
• mepc = PC, Instruction that caused trap.
• PC = (mtvec & ~0xF) (Exceptions are not vectored)

The exit procedure using mret is the same as for interrupts.

As for interrupts exception E can be taken in supervisor or user mode if available and configured via medeleg or sedeleg. The exception needs to be generated in the same or lower privilege level.

Delegation to Lower Privilege Mode

There are two methods to delegate exception E to lower privilege modes:

1. Enter in 'm' mode. Write mstatus.mpp = lower privilege mode. Execute mret
2. Configure medeleg[E]. The exception E will be taken in s mode when it occurs in s mode or lower privilege. (NOT when it occurs in m mode.)

RISC-V Scratch Registers

Scratch CSRs are available for each privilege trap level.

• mscratch
• sscratch
• uscratch

Interrupt Handlers In C

For GCC RISC-V Function Attributes are used to mark a function as an interrupt handler. These are also supported by LLVM

void f (void) __attribute__ ((interrupt ("user")));

As a parameter user, supervisor, and machine can be specified as the type. (To select the correct return instruction).