[TUHS] LSX swapping

Paul Ruizendaal pnr at planet.nl
Fri Jan 26 04:20:25 AEST 2018

>>> LSX has a maximum of three processes that are swapped in and out in a
>>> stack-like fashion. Only one process is ever in core.
> I'm having a hard time working out how this works. If process A is swapped
> out, and then B, B has to be swapped in before A can be? But only one process
> is ever in core at a time? To get A in, B has to be moved out? But then B
> would be the last one out, and would have to come in before A?
> Anyway, I don't understand why the OS could/would care which order processes
> we swapped in - unless it's something like the 'onion skin' memory allocation
> algorithm of CTSS (which also had only a single process resident at a time,
> IIRC), where, when a small process had to be swapped in, and a large one was
> already in, it only swapped out enough of the large one to make room for the
> small one. The process could recurse, hence the name.

The LSI-11 had 40KB of core. The lower 16KB held the LSX kernel, the upper 24K was
for the active process.

LSX has 3 process slots and 2 swap slots (on floppy!). Optionally, LSX could
be built with an additional background process ("BGOPTION"), but this does
not work very well. Process numbers and swap slots have a 1:1 relationship.

There is a global variable, cpid, with the process number of the current
process, initially zero. Upon boot, LSX will load a shell as process 0.
(There is no swapper process and no init in LSX).

The original LSI-11 kernel code is here:
I can refer to source lines in the repository for my TI990 port of it:

When a process forks, it writes the current core image out to the
corresponding swap slot (this swaps out the parent) and the core image
is repurposed as the new process (i.e. cpid is incremented and the
proc table filled in):

The child now runs and the parent remains suspended until the child completes.
Yes, this implies no pipes. The shell is modified to emulate pipes with files.

When the child completes, it stores its u area / exit code in its swap slot
(as it is running, the swap slot must be empty. Process 3 has a small swap slot
just for this). Next LSX decreases cpid and the parent process is reloaded from
its swap slot:

The actual swapping happens here:
The original PDP11 source has code to compress the empty space between _end and the
stack pointer, which I did not get to work properly.

LSX works well enough to run the standard V6 binaries unmodified. Some need to be
tweaked (e.g. table space in cpp) and the shell needed the pipe change. It can run
the V6 c compiler, but not with a wild card argument: that requires 4 stacked processes
(sh, glob, cc and cpp/c0/..) and LSX allows only 3.

Hope the above makes it a bit more clear.

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