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

[bugzilla-daemon@libre-riscv.org]

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

--- Comment #5 from Jacob Lifshay <programmerjake@gmail.com> ---
(In reply to Luke Kenneth Casson Leighton from comment #4)
> so, um... it's just easier to use BE instruction format and "cheat" a
> little, because the MSBs of the 32-bit instruction, containing the major
> opcode,
> end up in the 1st byte:
> 
> byte 0:
> 31 30 29 28 27 26 25 24
> | major opcode  | rest-of-32-bit-instruction...
> 
> byte 1:
> 23 22 21 20 19 18 17 16
> more of 32-bit-instruction
> 
> see https://libre-riscv.org/openpower/:
> 
> if we take over 2 opcodes for each of C, SVP P48, SVP P64 and VBLOCK
> then we have a workaround.

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)

>  SVP P32 can potentially be "paged".
> 
> it's a bit of a pain that there's so few bits, due to the way that POWER
> was never designed for this type of thing originally.
> 
> the way VLE works is, you actually have an entire new memory page which
> is allocated a "format".  that means that mixing 16-bit VLE and 32-bit
> VLE is basically impossible, and you have to jump between two completely
> different memory pages repeatedly just to get access to regular
> instructions which are not available in VLE 16-bit.

VLE requiring separate pages seems like a total mess that we should not
emulate.

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