Re: Decrement And Branch

On Tue, 13 Aug 2024 22:00:12 +0000, Lawrence D'Oliveiro wrote:

On Tue, 13 Aug 2024 09:00:25 -0000 (UTC), I wrote:
>

However, the good old VAX (in
its usual kitchen-sink fashion) had a whole set of variations, including
one that decremented down to -1 instead of zero. And the Motorola 68000
family only had the decrement down to -1 version.

>
VAX example of how to use SOBGEQ instead of SOBGTR:
>
        movl    «loop count», Rn
        br      bottom_of_loop
top_of_loop:
        .... body of loop ...
bottom_of_loop:
        sobgeq  Rn, top_of_loop
>
Like I said, I wondered why this sort of thing wasn’t more common ...

Perhaps the RISC mantra has permeated the minds of ISA designers.
Mark Horowitz:   Decode should be as simple as possible.
Albert Einstein: Everything should be as simple as possible,
                 but no simpler.
One of the above got it right...