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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.arch
Subject: Re: Byte ordering (was: Whether something is RISC or not)
Date: Fri, 4 Oct 2024 11:23:50 +0200
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On 04/10/2024 00:17, Lawrence D'Oliveiro wrote:
> On Thu, 03 Oct 2024 09:39:03 GMT, Anton Ertl wrote:
> 
>> BTW, at least in my 32-bit PowerPC manual the claim is that PowerPC is a
>> 64-bit architecture, and that the manual describes only the 32-bit
>> subset.  Maybe the original Power was 32-bit.
> 
> I would say IBM designed 32-bit POWER/PowerPC as a cut-down 64-bit
> architecture, needing only a few gaps filled to make it fully 64-bit.
> 
> The PowerPC 601 was first shown publicly in 1993; I can’t remember when
> the fully 64-bit 620 came out, but it can’t have been long after.
> 

I don't remember the history well enough here.

> Motorola did a similar thing with the 68000 family: if you compare the
> original 68000 instruction set with the 68020, you will see the latter
> only needed to fill in a few gaps to become fully 32-bit.
> 

The m68k was always designed as a 32-bit ISA.  But the first 68000 
implementation used a 16-bit ALU and internal buses for size and cost 
reasons.  I would not describe the additional instructions in the 68020 
as "filling gaps to 32-bit", but merely a natural expansion of the ISA 
with a few more useful instructions.

> Compare this with the pain the x86 world went through, over a much longer
> time, to move to 32-bit.

The x86 started from 8-bit roots, and increased width over time, which 
is a very different path.

And much of the reason for it being a slow development is that the world 
was held back by MS's lack of progress in using new features.  The 80386 
was produced in 1986, but the MS world was firmly at 16-bit under it 
gained a bit of 32-bit features with Windows 95.  (Windows NT was 32-bit 
from 1993, and Win32s was from around the same time, but these were 
relatively small in the market.)