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From: mitchalsup@aol.com (MitchAlsup1)
Newsgroups: comp.arch
Subject: Re: Banked register files
Date: Tue, 20 Aug 2024 01:19:10 +0000
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On Mon, 19 Aug 2024 23:23:11 +0000, Brett wrote:

> MitchAlsup1 <mitchalsup@aol.com> wrote:
>> On Mon, 19 Aug 2024 21:46:07 +0000, Brett wrote:
>>
>>> Banked register files, a mental exercise at expanding the register file.
>>>
>>> With three operand RISC you have you have three 5 bit register
>>> specifiers using 15 bits.
>>>
>>> If instead you have eight banks of eight registers you have a 3 bit bank
>>> specifier and three 3 bit register specifiers for 12 bits.
>>>
>>> Now the banks need to talk to each other and so you would add a bit to
>>> each register specifier to tell whether it uses the bank or the base
>>> registers, for 72 registers total, not 64. So a 3 bit bank specifier
>>> and three 4 bit register specifiers for 15 bits, the same as a 32
>>> register RISC chip.
>>
>> This covers 100% of instructions that smell like::
>>
>>     ADD    R17,R17,R25
>>
>> but covers 000% of the instructions that smell like::
>>
>>     ADD    R7,R17,R25
>>
>> I strongly suspect that it covers less than 50% of the 3-operand
>> instruction uses.
>
> My description was bad, let’s do a MC 68000 version, base registers are
> addressing mostly, and the banks are integer/float.

There is a reason this style fell out of fashion.
Not enough address registers at the same time one had not enough
data registers--whereas a flat 16-entry file had enough for either.

There is a reason CRAY-2 staging memory was not copied, too; and
it is mainly the same reason.


> The compiler can handle this easily, simple dependency grouping and if
> you
> need more than 8 registers you use the No base flag to total to the base
> registers. So you have two chains that total in two banks and both write
> to
> the base registers where the last total of the two chains are added.
>
> This saves a lot of no bank bits, only the result needs a no bank
> override.

Yes, 68K did have pretty good code density--{{Now if 68020 had NOT
gone a blown up the addressing modes...}}
>
> Simple code only uses base registers, or base plus one bank.
> Call and return parameters are in the base registers, spilling to a bank
> if you need more.
>
> Since only the result needs an override, you can do 4 banks of 16
> registers.

Dream on.