[libre-riscv-dev] more compatible alternative to BE instructions on LE processor
Luke Kenneth Casson Leighton
lkcl at lkcl.net
Wed May 13 09:09:08 BST 2020
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
* 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.
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