[Libre-soc-isa] [Bug 933] prefix-code (like huffman code) decode/encode instructions

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=933

--- Comment #17 from Jacob Lifshay <programmerjake at gmail.com> ---
(In reply to Luke Kenneth Casson Leighton from comment #14)
> +    | PO    |  RT  |   RA  |   RB  | RC    | XO |once|
> 
> please keep to short names, i have told you this many times.

once is short compared to BHRBE which is a field.

> 
> +56,ALU,OP_PCDEC,RA,RB,RC,RT,NONE,CR0,0
> +        elif regs == ['RA', 'RB', 'RC', 'RT', '', 'CR0']:  # pcdec
> 
> you cannot use the same bit for Rc as for the immediate.

PowerISA already does that in many places -- one example is the EX immediate
bit is in the same place as Rc.

The key is that pcdec always writes CR, it is not optional based on a Rc
immediate, therefore Rc is unused and can be used for something else.
> 
> you also cannot use a different-width XO in the same Form
> so a different Form will be needed.

It uses the exact same XO width as every other VA2 form.

That said, PowerISA has many instances of different XO widths for the same
form...

XX2-form is probably the worst example (look in the v3.1B pdf, not fields.txt):
it splits XO into two fields:
XX2-form (1.6.1.21):
| ... |21 |25 |26 |29 |30|31|
| ... | XO            |BX|TX|
| ... |XO |dc |XO |dm |BX|TX|

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