17 Feb
2025
17 Feb
'25
9:06 a.m.
On 2/17/25 08:02, Frank Leonhardt via cctalk wrote:
On 17/02/2025 05:12, ben via cctalk wrote:
Yes, on the PDP11, any register could be used as an
autoincrement/decrement indirect pointer. So, the use of R6
was merely a convention, baked into the macros in the
assembler. R7 was firmly fixed in the CPU logic as the PC,
however.
Jon
Did any classic computers have a subroutine call
as
(S++)=PC, PC=(EFA)
as well as the standard call (--S)=PC,PC=(EFA) ?
One could have a virtual stack machine, using helper
functions without having to deal with return addresses on
the stack.
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.