Sujet : Re: "Mini" tags to reduce the number of op codes
De : tkoenig (at) *nospam* netcologne.de (Thomas Koenig)
Groupes : comp.archDate : 03. Apr 2024, 21:02:25
Autres entêtes
Organisation : A noiseless patient Spider
Message-ID : <uukckh$4g83$1@dont-email.me>
References : 1
User-Agent : slrn/1.0.3 (Linux)
Stephen Fuld <
sfuld@alumni.cmu.edu.invalid> schrieb:
[saving opcodes]
The idea is to add 32 bits to the processor state, one per register
(though probably not physically part of the register file) as a tag. If
set, the bit indicates that the corresponding register contains a
floating-point value. Clear indicates not floating point (integer,
address, etc.).
I don't think this would save a lot of opcode space, which
is the important thing.
A typical RISC design has a six-bit major opcode.
Having three registers takes away fifteen bits, leaving
eleven, which is far more than anybody would ever want as
minor opdoce for arithmetic instructions. Compare with
https://en.wikipedia.org/wiki/DEC_Alpha#Instruction_formatswhere DEC actually left out three bits because they did not
need them.
What is _really_ eating up opcode space are many- (usually 16-) bit
constants in the instructions.