[TUHS] PDP-11/70 SPL

[jnc@mercury.lcs.mit.edu (Noel Chiappa)]

    > From: Johnny Billquist

    > this also means that you must be at SPL 7 before any of this

Yes, I assumed that (since it wouldn't make sense otherwise :-).

    > In general, I would say that this is not the way I would write code, but
    > ... 2.11BSD do an SPL 0 followed by WAIT.

Right; even if one does something like have every interrupt set a flag (which
is cleared while interrupts are disabled), and check that after lowering the
priority, but before doing the WAIT, there's _still_ a window between that
check, and the WAIT (although I guess it's less likely to be hit, since the
interrupt request would have to be posted _in that window_, not be hanging
around waiting to be serviced).

The only way (that I can work out) to atomically lower the priority and wait
is to do an RTI with the PC on the stack pointing to the WAIT instruction, but
I'm not sure even that is guaranteed to work.

I guess it all depends on the CPU implementation: does it check for pending
interrupts before each instruction, or only at the end of each instuction, or
what? If before, and there's an interrupt pending, it will go off before the
WAIT is executed. Although I suppose if it's at the end, it would do the check
at the end of the RTI, and do the interrupt then.

And whether it's at the end, or the beginning, WAIT itself must be special
cased, to check for pending interrupts during the execution (which can take an
indeterminate amount of time).

    > 2.11BSD will work potentially with a slight degration if the SPL do not
    > block interrupts.  It will still work fine, as you will, at a minimum,
    > get an interrupt at the next clock tick, which will wake it up. But it
    > might possibly be sitting in a WAIT slightly longer than required.

Yes, exactly.

    > RSX in fact just loops after the WAIT. If an interrupt should cause the
    > system to be able to do something more productive, it will not return to
    > the idle loop. So yes, it also detects in the interrupt exit processing,
    > that it was/is in the idle loop.

Does it detect if it was _before_ the WAIT instruction? I would assume it does,
but I don't know anything sbout RSX.

    > But at least processor and chip manuals do not say that SPL will block
    > interrupts.

Yes, I looked too, in a variety of places (PDP-11 Architecture, including in
the 'model differences' table, 11/73 Tech Manual, etc). Crickets...

    Noel