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From: anton@mips.complang.tuwien.ac.at (Anton Ertl)
Newsgroups: comp.arch
Subject: Re: Why VAX Was the Ultimate CISC and Not RISC
Date: Wed, 12 Mar 2025 09:14:00 GMT
Organization: Institut fuer Computersprachen, Technische Universitaet Wien
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Lawrence D'Oliveiro <ldo@nz.invalid> writes:
>On Wed, 12 Mar 2025 00:26:50 -0000 (UTC), John Levine wrote:
>
>> Next was PDP=11 where MOV R1,R2 copies R1 into R2.
>
>What about CMP (compare) versus SUB (subtract)? CMP does the subtract 
>without updating the destination operand, only setting the condition 
>codes. But are the operands the same way around as SUB (i.e. backwards for 
>comparison purposes) or are they flipped?

In AT&T syntax for IA-32 and AMD64, the operands are the same way
round as for SUB (i.e., flipped compared to Intel syntax).  This leads
to counterintuitive combinations of compare and branches.  E.g., "cmp
%rax,%rbx; jlt ..." jumps if %rax is greater than %rbx.

For the PDP-11, where the mnemonics etc. were created with that order
in mind, the mnemonics of the flags-using instructions could be named
appropriately.  I.e., after "cmp x,y" or "sub x,y", "blt" could branch
if x<y.  I have not checked if that's the case for the PDP-11, though.

- anton
-- 
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
  Mitch Alsup, <c17fcd89-f024-40e7-a594-88a85ac10d20o@googlegroups.com>