[Info-vax] SOBGEQ vs SOBGTR

[Arne Vajhøj]

On 2/23/2024 10:14 PM, Lawrence D'Oliveiro wrote:
>          ... set up loop count in Rn ...
>          BR      9000$
> 1000$:
>          ... body of loop ...
> 9000$:  SOBGEQ  Rn, 1000$
> 
> Why branch and use SOBGEQ instead of SOBGTR? So that, if the loop
> count is initially zero, the branch falls right through without
> executing the body of the loop.

With the details provided then it will be pure
speculation.

     n = whatever
loop:
     if n <= 0 goto endloop
     ...
     n = n - 1
     goto loop
endloop:

has more instructions than:

     n = whatever
loop:
     ...
     n = n - 1
     if n > 0 goto loop

but the second has the problem of not handling the n = 0
case properly.

     n = whatever
     goto checkfirst
loop:
     ...
     n = n - 1
checkfirst:
     if n > 0 goto loop

looks like a hack to fix that problem.

Arne