[Simh] PDP-11 MARK

[Johnny Billquist]

On 2018-08-02 15:12, Paul Koning wrote:
> 
> 
>> 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.

Yes.

>> 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.

Like I said, I'm not sure I put much worth into it.
I don't recall the exact conspiracy theory behind it, but essentially 
went along the path of this extension allowed DEC to put a new patent 
in, which extended the protection of the architecture for another few 
years. But it might also have been more generally related to the EIS. 
Can't remember for sure, and I have no recollection where I read it.

>> 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.

The one advantage of MARK is that you don't have to know how much stuff 
have been put on the stack in the local routine before leaving. All that 
is implicitly cleaned up through the use of the MARK.

But it's definitely not worth it, since you have to sacrifice quite a 
lot of potential separate I-space for it, along with forcing you to use 
R5 a bit special as well.

   Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol