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RISC-V Instruction Set Manual, Volume I: RISC-V User-Level ISA , 20181106-Base-Ratification 2018/11/06

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.

FLQ and FSQ do not modify the bits being transferred; in particular, the payloads of non-canonical NaNs are preserved.

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 H 16-bit half-precision
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 in RV32 or RV64, so quad-precision bit patterns must be moved to the integer registers via memory.

RV128 will support FMV.X.Q and FMV.Q.X in the Q extension.

14.4 Quad-Precision Floating-Point Compare Instructions

The quad-precision floating-point compare instructions are defined analogously to their double-precision counterparts, but operate on quad-precision operands.


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.