[Libre-soc-dev] setvl immediate field definition broken

[Jacob Lifshay]

On Fri, Mar 12, 2021 at 4:31 PM Luke Kenneth Casson Leighton
<lkcl at lkcl.net> wrote:
>
> On Saturday, March 13, 2021, Jacob Lifshay <programmerjake at gmail.com> wrote:
>
> >
> > I suggest redefining the immediate field to be bits 16:23 inclusive,
> > and setting bits 16 or 17 causes an illegal exception. fields.text
> > will need to be modified too.
>
>
> numbers in MSB0 format are a damn nuisance.
>
> can i suggest just using all bits then saying "values greater than 63 are
> reserved"?

Sounds good, as long as we explicitly mention that reserved values
*must* trap, since otherwise we end up with the OpenPower bad default
of ignoring reserved valued. Also, we should disambiguate between
values of the bits in the field and values of the assembly text level
field, since they are off-by-one.
>
> it also makes it clear what the bits are for (larger VL, later)

Yup, though I'm planning on using at least some of them for enabling
increased register-file size, allowing easy backwards compatibility.
In any case, non-zero bits are reserved for the future.

I'll let you or whoever gets to it first to make the changes, I'm done
for the day.

Jacob