[Libre-soc-dev] results of phone call about simple-v prefix
programmerjake at gmail.com
Fri Nov 27 23:05:48 GMT 2020
On Fri, Nov 27, 2020, 14:41 Jacob Lifshay <programmerjake at gmail.com> wrote:
> we mostly resolved the issue of encodings for primary opcodes 0, 1, and 5
> potentially being decoded as valid but unintended instructions on old power
> processors, we will need to double check that they trap on old processors.
Turns out that primary opcode 0 is used for implementation-specific
opcodes. primary opcode 1 and 5 are defined to cause illegal instruction
traps for the encodings we want to use. We could potentially move to using
primary opcode 6 for 32/48-bit instructions -- 1/8 is currently used (new
in v3.1), the rest causes illegal instruction traps. This would reduce the
space for svp32 somewhat, svp48 would have just as much space as before.
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