[libre-riscv-dev] more compatible alternative to BE instructions on LE processor

[Luke Kenneth Casson Leighton]

On Wed, May 13, 2020 at 7:29 AM Lauri Kasanen <cand at gmx.com> wrote:

> The PowerISA 3.1 doc adds a 32-bit prefix (page 46 of the pdf (reader),
> page 22 of the official naming). I didn't go into details, but for such
> plans it'd be good to take into account.

ok that entire section covers a new (extended, 64-bit) instruction
format, including new extended immediates, vector-masks and so on.

it uses major opcode 0b0000001 to do so.

however unlike what we propose (and, crucially, propose to put behind
a mode-switch bit):

* the scheme critically depends on the instructions being chunked into
32-bit sections
* 48-bit and 16-bit instruction encodings are impossible
* we need far more bits (11 bits for C, 11 bits for SVP P48 and 27
bits for SVP P64) than can fit after *one* major opcode.

16 (prefix) minus 6 (major opcode) is 10.  that is not enough to fit
11 bits of SVP P48.

l.