[cctalk] Re: Classic computers with more than one stack pointer, but not FORTH machines.

[Paul Koning via cctalk]

> On Feb 17, 2025, at 9:02 AM, Frank Leonhardt via cctalk 
> <cctalk@classiccmp.org> wrote:
> 
> On 17/02/2025 05:12, ben via cctalk wrote:
>> 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. 

Indeed, but the subroutine call instructions were hardwired to auto-decrement 
on a particular register, conventionally called SP (register 6 in the PDP-11).  
So while you could create data stacks that grow in either direction, with a 
pointer of your choice, you could only do calls via SP and only downward.

        paul