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From: "Paul A. Clayton" <paaronclayton@gmail.com>
Newsgroups: comp.arch
Subject: Re: What is an N-bit machine?
Date: Sun, 1 Dec 2024 11:23:26 -0500
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On 11/30/24 3:38 PM, Michael S wrote:
> On Sat, 30 Nov 2024 19:40:17 -0000 (UTC)
> John Levine <johnl@taugh.com> wrote:
[snip]
>> I get the impression that we will have 32 bit architectures for a very
>> long time, since they are smaller and cheaper to implement than 64 bit
>> and for a lot of embedded applications they are more than adequate.
>> Examples are ARM Cortex-R4 and -R5, high performance 32 bit realtime
>> chips.
>>
> 
> I agree with conclusions, but not with your examples.
> IMHO, the whole ARM Cortex-R series is solution looking for problem. It
> could be quite easily replaced by 64-bit A series cores.
> Now Cortex-M is completely different story. Here 64-bit cores would not
> be appropriate.

I wonder if a 32-bit version of AArch64 would have been
appropriate for the Cortex-R series. The area and power difference
might not have been sufficient to justify "backporting" AArch64 to
32-bit. In addition to the extra development costs, providing yet
another 32-bit architecture would have introduced marketing costs
not only to communicate the tradeoffs for the new products but
also to provide confidence that the M series will not be
abandoned.

Power and code density (and even cost) might not be _as_ important
for the R series as for the M series, so AArch64 might well have
been a reasonable fit.

I have never worked on embedded systems and never even closely
followed the market, but thinking about the tradeoffs can be fun.