[TUHS] PDP-11 MARK (was: Early Unix function calls: expensive?)

Greg 'groggy' Lehey grog at lemis.com
Wed Jan 6 09:04:40 AEST 2016

On Tuesday,  5 January 2016 at 21:35:47 +0100, Johnny Billquist wrote:
> On 2016-01-05 18:43, Ronald Natalie<ron at ronnatalie.com> wrote:
>> Just never figured out how to make good use of the MARK instruction on the PDP-11.
> Not surprising. As others noted, few ever did. And apparently none
> responding actually do either.
> It *is* a stupid instruction in many ways. And it's not for multiple
> returns either.
> It's an odd way of handling stack cleanup without a frame pointer.

Thanks for the (omitted) explanation.  At first sight, the instruction
almost seems to make sense for functions with a variable number of
parameters, but of course there are simpler ways to do it.

I wonder if this is a case of "it sounded like a good idea at the
time" (when the instruction set was being designed), and it took a
while for people to realize that it wasn't of any use.

I recall a similar issue with the Siemens 306, their first stack-based
machine, round about 1975.  They had function call and return
instructions which manipulated the stack pointer, but it seemed that
nobody used them: they didn't work.  From memory, the call instruction
incremented the stack pointer and stored PC.  The return call
instruction decemented the stack pointer and loaded PC--from the wrong

I never used the 306 myself, but I often wondered if they modified one
of the instructions to DTRT.

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