[Simh] PDP-11 MARK

[Paul Koning]

> On Aug 1, 2018, at 9:06 PM, Johnny Billquist <bqt at softjar.se> wrote:
> 
> On 2018-08-02 02:26, dave porter wrote:
>> Not really a simh question, but this might be an appropriate
>> bunch of people.  What actually used MARK on PDP-11?
>> I think I recall that some Fortran system (F4P?) used it,
>> but that's a vague feeling at most.
>> Certainly as a Macro-11 kernel-mode weenie, I never
>> felt the need.
> 
> As far as I know, nothing used MARK.

I believe that is correct.

> It's ugly in that it requires that I-space and D-space overlap, at least for the stack, for it to work.

And you have to execute the stack, which is a weird thing for programs to do in any case.

> I once heard a rumor that DEC only came up with it to extend some copyright or patent a few more years, but I'm not sure there is much truth in that.

I've had some discussions around DEC with people who knew about MARK, but that particular rumor does not sound at all familiar.

> Anyway, it was an attempt at having a return mechanism for routines that cleaned up the stack, in a time where you did not have a frame pointer.

Yes.  But the cost (time and space) of pushing the right MARK onto the stack is likely comparable to the cost of simply cleaning up the stack explicitly, without any of the mess associated with that instruction.  

	paul