[TUHS] Article on 'not meant to understand this'

Noel Chiappa jnc at mercury.lcs.mit.edu
Tue Jan 17 03:16:27 AEST 2017

    > From: Angelo Papenhoff

    > The problem is that the function which did the savu was not necessarily
    > the same as the function that does the retu, so after retu the function
    > could have the call stack of a different function. As dmr explained,
    > this worked with the PDP-11 compiler but not with the interdata
    > compiler.

To put it slightly differently, in PDP-11 C all stack frames look identical,
but this is not true of other machines/compilers. So if routine A called
savu(), and routine B called aretu(), when the call to aretu() returned,
procedure B is still running, but on procedure A's stack frame. So on machines
where A's stack frame looks different from B's, hilarity ensues.

(Note that aretu() was significantly different from retu() - the latter
switched to a different process/stack, whereas aretu() did a 'non-local goto'
[technically, switched to a different stack frame on the current stack] in the
current process.)

    > Note that Lions doesn't explain this either, he assumed that the
    > difficulty was with with u_rsav and u_ssav .. (he probably wasn't that
    > wrong though, it really is confusing, but it's just not what the comment
    > refers to)

Right. There are actually _three_ sets of saved stack info:

	int	u_rsav[2];	/* save r5,r6 when exchanging stacks */
	int	u_qsav[2];	/* label variable for quits and interrupts */
	int	u_ssav[2];	/* label variable for swapping */

and it was the interaction among the three of them that I found very hard to
understand - hence my (incorrect) memory that the 'you are not' comment
actually referred to that, not the savu/aretu stuff!

Calls to retu(), the primitive to switch stacks/processes, _always_ use
rsav. The others are for 'non-local gotos' inside a process.

Think of qsav as a poor man's exception handler for process software
interrupts. When a process is sleeping on some event, when it is interrupted,
rather than the sleep() call returning, it wakes up returning from the
procedure that did the savu(qsav). (That last is because sleep() - which is
the procedure that's running when the call to aretu(qsav) returns - does a
return immediately after restoring the stack to the frame saved in qsav.)

And I've forgotten exactly how ssav worked - IIRC it was something to do with
how when a process is swapped out, since that can happen in a number of
ways/places, the stack can contains calls to various things like expand(),
etc; when it's swapped back in, the simplest thing to do is to just throw that
all away and have it go back to where it was just before it was decided to
swap it out.


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