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From: John Savard <quadibloc@servername.invalid>
Newsgroups: comp.arch
Subject: Re: "Mini" tags to reduce the number of op codes
Date: Sat, 06 Apr 2024 21:30:47 -0600
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On Fri, 5 Apr 2024 21:34:16 +0000, mitchalsup@aol.com (MitchAlsup1)
wrote:

>Early in My 66000 LLVM development Brian looked at the cost of having 
>only 1 FP OpCode set--and it did not look good--so we went back to the
>standard way of an OpCode for each FP size × calculation.

I do tend to agree.

However, a silly idea has now occurred to me.

256 bits can contain eight instructions that are 32 bits long.

Or they can also contain seven instructions that are 36 bits long,
with four bits left over.

So they could contain *nine* instructions that are 28 bits long, also
with four bits left over.

Thus, instead of having mode bits, one _could_ do the following:

Usually, have 28 bit instructions that are shorter because there's
only one opcode for each floating and integer operation. The first
four bits in a block give the lengths of data to be used.

But have one value for the first four bits in a block that indicates
36-bit instructions instead, which do include type information, so
that very occasional instructions for rarely-used types can be mixed
in which don't fill a whole block.

While that's a theoretical possibility, I don't view it as being
worthwhile in practice.

John Savard