18 Feb
2025
18 Feb
'25
8:17 p.m.
On Tue, 18 Feb 2025 12:00:10 -0600
cctalk-request(a)classiccmp.org wrote:
On the more than "one stack pointer" in the
subject, it was a bit
arbitrary on the PDP-11 (or VAX) as the pre/post indexed indirect
addressing made every register a stack pointer. But this is where I
get hazy between DEC and 68K, and I did a lot more 68K. I'm pretty
sure you could do a move.l PC, An and you could certainly do an
indirect jmp (An), so effectively you could have multiple call stacks
if you wanted.
Almost, kinda-sorta. The JSR and RTS instructions are hard-wired to use
R6/SP, and there's nothing you can do about that. You *can* implement a
return off another "stack" by doing e.g. MOV @(Rn)+, PC as long as you
save the return address by hand, first - but this affects the flags,
unlike JSR/RTS.