[Libre-soc-bugs] [Bug 238] POWER Compressed Formal Standard writeup

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=238

--- Comment #71 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Luke Kenneth Casson Leighton from comment #65)

> you make a start on an encoding in ikiwiki, start at the top level with how
> the FSM would decode (identify) modes, how 16/32 sequence together,
> cut/paste the ASCII layouts if it helps.

i made a start in the wiki page, can you confirm it looks reasonable?

| byte 0 | byte 1 | byte 2 | byte 3 |
| v3.0B standard 32 bit instruction |
| EXT000 | 16 bit          | 16...  |
| .. bit | 8nop   | v3.0b stand...  |
| .. ard 32 bit   | EXT000 | 16...  |
| .. bit | 16 bit          | 8nop   |
| v3.0B standard 32 bit instruction |

if it does, the next phases would be:

* make a preliminary assessment of branch in/out viability
* confirm FSM encoding (is LSB of PC really enough?)
* guestimate opcode and register allocation (without necessarily doing a full
encoding)
* write throwaway python program that estimates compression ratio from objdump
raw parsing
* finally do full opcode allocation
* rerun objdump compression ratio estimates

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