[libre-riscv-dev] [isa-dev] Re: FP reciprocal sqrt extension proposal

Jacob Lifshay programmerjake at gmail.com
Sun Aug 4 06:55:29 BST 2019

see table in

On Sat, Aug 3, 2019, 22:33 lkcl <luke.leighton at gmail.com> wrote:

> On Thursday, July 11, 2019 at 11:45:38 AM UTC+1, Jacob Lifshay wrote:
>> For the encoding, I think using an encoding similar to both the
>> fsqrt.* and fdiv.* encodings is a good idea, since frsqrt is similar
>> to both fdiv and fsqrt; Therefore, as an initial proposal, I think
>> using a funct7 value of 0111100 and the rest of the instruction
>> identical to fsqrt is a good idea, since, as far as I'm aware, that
>> doesn't conflict with anything currently.
> ah! rats, i just started looking at the RV32/64G instruction set listings,
> chap 25 V20190621-draft, and hadn't noticed that FSQRT.S is
> funct7=0b0101100 where rs2=00000, FSQRT.D is funct7=0b0101101 and FSQRT.Q
> is funct7=0b0101111
> so correspondingly:
> FRSQRT.S would need to be funct7=0b011100
> FRSQRT.D would need to be funct7=0b011101
> FRSQRT.Q would need to be funct7=0b011111
> also: where would FSQRT.H go, and likewise FRSQRT.H?  *checks table 11.3*
> section 11.6.... ok so those would be:
> FRSQRT.H would need to be funct7=0b011110 and
> FSQRT.H would need to be funct7=0b0101110
> so the first 2 bits of funct7 select S/D/H/Q, the other 5 select the
> "function type".
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