[libre-riscv-dev] uniform instruction format

[Jacob Lifshay]

On Mon, Jun 17, 2019, 01:21 Luke Kenneth Casson Leighton <lkcl at lkcl.net>
wrote:

> ---
> crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68
>
> On Mon, Jun 17, 2019 at 8:55 AM Jacob Lifshay <programmerjake at gmail.com>
> wrote:
> >
> > On Sun, Jun 16, 2019 at 11:24 PM Luke Kenneth Casson Leighton
> > <lkcl at lkcl.net> wrote:
> > >
> > > jacob, you missed some questions, which are important to resolve.
> going
> > > through the pseudo-code, and also through the RV spec, the format of
> > > RV48-I-type seems wrong.  the pseudo-code says that it should match
> exactly
> > > with RV32-I-Type, which it does not.
> > >
> > > also can you clarify if the other potential error that i spotted is or
> is
> > > not correct?
> > the P48FI encoding is correct, the fmt field is the lower 2 bits of
> > funct7. The underlying format is actually I-type, but the entire
> > immediate field is used for opcode.
> > for an example, see the encoding of fsqrt.s on page 66 of Volume I:
> > RISC-V Unprivileged ISA V20181221-Public-Review-draft.
> > bit fields:
> > 31:27 -- funct5 (more opcode)
> > 26:25 -- fmt (set to S which is 00)
> > 24:20 -- rs2 (0 for fsqrt.s, which is what makes it I-type instead of
> R-type)
> > 19:15 -- rs1
> > 14:12 -- rm
> > 11:7 -- rd
> > 6:0 -- opcode
>
> turning the horizontal table entry for P48FI-type around:
>
> 31:27   | funct7[6:2] <---- this should be funct5
>
it's funct7, since it comes from the same positions as funct7 in R-type.
fmt is just the 2 lowest bits in funct7.

> 26:25:  fmt
> 24:20  funct5 <---- this is the one that's not correctly listed

that would be called rs2, but there is no rs2, so I picked func5. feel free
to rename.

> 19:15 rs1[4:0]
> 14:12 rm
> 11:7 rd[4:0]
> 6:0 opcode
>
> once that's corrected, it's back to looking just like RV32-I-Type,
> just with the special option "rs2=0" to specify FSQRT.
>
I-type doesn't have an rs2 field, so I guess fsqrt.s is a strange hybrid
between R-type and I-type, hence why I called it FI-type.

>
> btw i enabled rst on the libre-riscv.org ikiwiki and put the document
> here:
> https://libre-riscv.org/simple_v_extension/sv_prefix_proposal/

was wondering where it was.

>
>
> l.
>
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