[Libre-soc-dev] compressed instructions state requirements

[Luke Kenneth Casson Leighton]

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crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68

On Wed, Nov 25, 2020 at 8:07 PM Jacob Lifshay <programmerjake at gmail.com> wrote:
>
> On Wed, Nov 25, 2020, 06:07 Luke Kenneth Casson Leighton <lkcl at lkcl.net>
> wrote:
>
> > On Tue, Nov 24, 2020 at 9:07 PM Jacob Lifshay <programmerjake at gmail.com>
> > wrote:
> >
> > > My idea of how 16-bit instructions work is that they should be usable
> > > anywhere (like RVC), no special pages needed. The extra info is
> > > conceptually part of the PC (or some decode status register).
> >
> > right.  "some decode status register".  let's walk through it.  let me
> > know (explicitly) if you agree or disagree with the answers.
> >
> > Q: how often will that status register need to be set/reset?
> > A: on every call into a standard PPC64LE ABI v3.0B formatted function
> >
>
> Actually, on every instruction. Just like PC is changed by every
> instruction.


> Q: how do you return *to* the enhanced mode?
> >
>
> All the time -- whenever you want to encode a 16-bit instruction.
>
> A: on *entry* to every function encoded in enhanced-variable-encoding
> > mode, instructions are required to set the new mode
> > Q: what is the cost of setting a status register?
> >
>
> 1 16-bit instruction -- it is set by every instruction executed. It is not
> intended to be normally written through a SPR.

rright.  clearly this is so simple that even a child of 4 should be
able to do it.  https://www.youtube.com/watch?v=V2rX-1rTuhE

not having access to a 4 year old, i'm going to have to pretend to be
one myself, and go back to coloured pens, stick back paper and sharp
scissors.

give me a mo to do a video...

l.