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From: dave_thompson_2@comcast.net
Newsgroups: comp.compilers
Subject: Re: Modern compilers for ye olde architectures
Date: Sun, 14 Nov 2021 15:04:37 -0500
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On Wed, 06 Oct 2021 07:56:59 GMT, anton@mips.complang.tuwien.ac.at
(Anton Ertl) wrote:
...
> As for the back-end, it seems to me that the major problem with the
> Z80 is that it does not have general-purpose registers; instead, many
> instructions deal with specific registers.  Many early architectures
> were like that, and assembly programmers could puzzle out good
> register assignments, but compilers were not particularly good at it.
> So eventually computer architects introduced machines with
> general-purpose registers like the PDP-11, the VAX, and the RISCs; ...

Eventually? PDP-11 was 7 years before Z-80, and 5 years before that
both PDP-6 and S/360 had 16 GPRs (& none 'wasted' as PC SP FP).

S/360 and PDP-11 did have floating-point registers separate, and at
least on the latter optional. (I believe there were 360 models listed
without FP, but heard that actual instances were about as rare as
PDP-6 without the 'option' for 0-17 to be registers instead of core.)
[Floating point was optional on the low end 360/22, /25, /30, and /40.
Considering what they were used for and how slow the FP was, e.g.,
on the /30 floating add was over 50us, multiply up to 400us, I expect
a lot of them skipped the floating point. Larger models all had it. -John]