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From: Thomas Koenig <tkoenig@netcologne.de>
Newsgroups: comp.arch
Subject: Re: "Mini" tags to reduce the number of op codes
Date: Wed, 3 Apr 2024 20:02:25 -0000 (UTC)
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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_formats
where 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.