Slow printing in 386/ix

Kurt Gollhardt kdg at nirvo.uucp
Mon Sep 4 02:16:45 AEST 1989

In article <1989Sep1.004003.6311 at esegue.uucp> johnl at esegue.UUCP (John Levine) writes:
>I gather that there is some aspect of the 8259 interrupt controller chip so
>that if an interrupt request comes and goes away, as happens with the
>unlatched printer request line, the 8259 interrupts anyway on level 7.
>(That was reputed to be the excuse for not latching the printer request so
>they could avoid a 50 cent flip-flop on the card.)  Did the network card
>make that fail, or is it more likely that having two devices on the same
>interrupt line made the printer unable to request the interrupt in the first
>place?  Perhaps someone with more 8259 combat experience would know.

If an interrupt request goes away after the 8259 has interrupted the CPU,
when the CPU asks the 8259 for the vector, it has to supply one anyway, so
it gives a 7 (arbitrary).  But it's possible, by checking status registers
of the 8259, to determine whether this is a real interrupt 7 or not.  In
UNIX, this is done, and fake interrupt 7s are discarded.

In your case, I would say that the network card must have been actively
driving the interrupt line to a false state, preventing the interrupt from
ever getting to the 8259.  This is not always what happens when two cards
are on the same interrupt line, but it's one of the possible scenarios;
depends on how both cards are designed.

In general, you can't arbitrarily share interrupts on the AT bus.  First,
as your case illustrates, many cards are not designed to expect this to
occur, and "take over" the interrupt line assuming nothing else will use it.
Second, the 8259(s) on these machines are used in a mode (edge-triggered)
which is not compatible with sharing interrupts; if you attempt to, and the
two (or more) interrupt requests ever overlap, only the first one will be

On the other hand, MCA was specifically designed to allow interrupt sharing.
The 8259s are used in level-triggered mode and the way boards must interface
to the request lines are specified so as to allow sharing to work.
Kurt Gollhardt                      \   Nirvonics, Inc. -- Plainfield, NJ
Kurt.Gollhardt at nirvo.uucp           /\     Software Design and Consulting
...!rutgers!nirvo!Kurt.Gollhardt   /  \
     "It's all about people; not you and me or him and her, but *us*."

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