[libre-riscv-dev] Instruction sorta-prefixes for easier high-register access

[Luke Kenneth Casson Leighton]

On Sunday, January 27, 2019, Jacob Lifshay <programmerjake at gmail.com> wrote:

>
> > The brownfield encoding space may end up being used for future
> extensions:
> > xBitManip being the most likely candidate.
> >
> From what I saw last time I looked at the public info, they ended up using
> some more of funct7 similar to how the M extension does rather than using
> something that is different between OP and OP-32
>
>
Good to know.


> >
> > So where are we. 12 bits.
> >
> > * 5 bits vlpr5
> > * 1 for rd (or 2? issue with 32bit ops)
> > * 1 for rs (or 2?)
> >
> you forgot rs2. some ops (shift, mulhsu, sub, div, mod, etc.) are not
> commutative, so it'd be nice to have both scalar<<vector and
> vector<<scalar.


Ngggh! This is almost impossible.  There is just too much. Seriously
considering 64 bit space instead.


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