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From: Brett <ggtgp@yahoo.com>
Newsgroups: comp.arch
Subject: Re: My 66000 and High word facility
Date: Mon, 19 Aug 2024 23:35:54 -0000 (UTC)
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MitchAlsup1 <mitchalsup@aol.com> wrote:
> On Mon, 19 Aug 2024 18:52:39 +0000, Brett wrote:
> 
>> MitchAlsup1 <mitchalsup@aol.com> wrote:
>>> On Sun, 11 Aug 2024 0:46:09 +0000, Brett wrote:
>>> 
>>> 
>>> The thing is that one you go down the GBOoO route, your lack of
>>> registers
>>> "namable in ASM" ceases to become a performance degrader. With renaming
>>> one can have R7 in use 40 times in a 100 instruction deep execution
>>> window.
>> 
>> If this was true we would have 16 or even 8 visible registers, and all
>> would be fine. x86 does mostly fine with 16, of course x84 had fab and
>> cubic dollar advantages that dwarfed the register limit.
> 
> Careful, here::
> 
> x86 has LD-OPs and LD-OP-STs which makes the 16 register file feel more
> like it has 20-22 registers. Do not underestimate this phenomenon. The
> gain from 16-32 registers is only 3%-ish so one would estimate that 22
> registers would have already gained 1/2 of all of what is possible.
> 
>> 64 separate registers was a bridge to far, but it was an interesting
>> exercise before it crashed and burned due to the bits being not quite
>> available. So close, yet so far. I could not make it work.
> 
> We remain hobbled by the definition of Byte containing exactly 8-bits.
> It is this which drives the 16-bit and 32-bit instruction sizes; and
> it is this which drives the sizes of constants used by the instruction
> stream.
> 
> 64 registers makes PERFECT sense in a 36-bit (or 72-bit) architecture.
> But we must all face facts::
> a) Little Endian Won
> b) 8-bit Bytes Won
> c) longer operands are composed of multiple bytes mostly powers of 2.
> d) otherwise it is merely an academic exercise.
> 

If you pack 7 instructions in 8 long words that gives you an extra nibble,
4 bits.
You can do lots of four operand dual operations, which may get you back the
code density lost, while improving performance.

3 instructions packed in 4 longs gives 64 registers plus four operand dual
instructions.