[libre-riscv-dev] [Bug 264] ISA switch needs to be a privileged operation

[bugzilla-daemon at libre-riscv.org]

http://bugs.libre-riscv.org/show_bug.cgi?id=264

--- Comment #8 from Jacob Lifshay <programmerjake at gmail.com> ---
(In reply to Luke Kenneth Casson Leighton from comment #6)
> (In reply to Jacob Lifshay from comment #5)
> 
> > If we use the same 2 C opcodes for compressed pairs, then we can end up with
> > 13.5 bits available per instruction, rather than just 11, which allows us to
> > define about 5.656 (!) times as many compressed instructions.
> > 
> > 13.5 = (32 - 6 + 1) / 2
> > 11 = 16 - 6 + 1
> > 5.656... = 2 ^ (13.5 - 11)
> 
> other ideas along this theme include saying, in that initial 32-bit space,
> is to specify how many of the next instructions are to be encoded as C.
> 
> i'd recommend to reserve the option to do this, as it involves storing state
> (the countdown timer) whereas just having 2 C opcodes as pairs is dead
> simple, as long as you accept that the PC encodes the state information
> about the fact that if the PC is on a 2-byte boundary you're in the middle
> of the pair.
> 
> this allows the PC to store the state if there is a trap.

Except, like I mentioned before, using the PC to store that info makes it
ambiguous between executing the second instruction in a pair and executing a
non-paired instruction that happens to start at a 2-byte boundary because there
was a preceding 48-bit instruction. So, we'll either have to not have any
48-bit instructions, not trap in the middle of a pair, or not have paired
instructions.

Switching instruction memory to always be in BE mode is not a good solution
since that is not compatible with standard Power code that doesn't know about
the endian switch.

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