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From: Stephen Fuld <sfuld@alumni.cmu.edu.invalid>
Newsgroups: comp.arch
Subject: Re: "Mini" tags to reduce the number of op codes
Date: Tue, 16 Apr 2024 15:08:58 -0700
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On 4/3/2024 1:02 PM, Thomas Koenig wrote:
> 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.

I think that is probably true for 32 bit instructions, but what about 16 
bit?

-- 
  - Stephen Fuld
(e-mail address disguised to prevent spam)