14 “Q” Standard Extension for Quad-Precision Floating-Point, Version 2.0
This chapter describes the Q standard extension for 128-bit binary floating-point instructions compliant with the IEEE 754-2008 arithmetic standard. The 128-bit or quad-precision binary floating-point instruction subset is named “Q”, and requires RV64IFD. The floating-point registers are now extended to hold either a single, double, or quad-precision floating-point value (FLEN=128). The NaN-boxing scheme described in Section [nanboxing] is now extended recursively to allow a single-precision value to be NaN-boxed inside a double-precision value which is itself NaN-boxed inside a quad-precision value.
14.1 Quad-Precision Load and Store Instructions
New 128-bit variants of LOAD-FP and STORE-FP instructions are added, encoded with a new value for the funct3 width field.
FLQ and FSQ are only guaranteed to execute atomically if the effective address is naturally aligned and XLEN=128.
14.2 Quad-Precision Computational Instructions
A new supported format is added to the format field of most instructions, as shown in Table [tab:fpextfmt].
| fmt field | Mnemonic | Meaning |
|---|---|---|
| 00 | S | 32-bit single-precision |
| 01 | D | 64-bit double-precision |
| 10 | - | reserved |
| 11 | Q | 128-bit quad-precision |
The quad-precision floating-point computational instructions are defined analogously to their double-precision counterparts, but operate on quad-precision operands and produce quad-precision results.
14.3 Quad-Precision Convert and Move Instructions
New floating-point to floating-point conversion instructions FCVT.S.Q, FCVT.Q.S, FCVT.D.Q, FCVT.Q.D are added.
Floating-point to floating-point sign-injection instructions, FSGNJ.Q, FSGNJN.Q, and FSGNJX.Q are defined analogously to the double-precision sign-injection instruction.
FMV.X.Q and FMV.Q.X instructions are not provided, so quad-precision bit patterns must be moved to the integer registers via memory.
14.4 Quad-Precision Floating-Point Compare Instructions
Floating-point compare instructions perform the specified comparison (equal, less than, or less than or equal) between floating-point registers rs1 and rs2 and record the Boolean result in integer register rd.
14.5 Quad-Precision Floating-Point Classify Instruction
The quad-precision floating-point classify instruction, FCLASS.Q, is defined analogously to its double-precision counterpart, but operates on quad-precision operands.
RV128 supports FMV.X.Q and FMV.Q.X in the Q extension.