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From: ralerche@lindy.stanford.edu (Robert A. Lerche)
Newsgroups: comp.sys.ibm.pc.misc,alt.msdos.programmer,comp.os.msdos.programmer
Subject: Re: Writing my own print spooler
Message-ID: <ralerche.662708668@lindy>
Date: 1 Jan 91 05:44:28 GMT
References: <CHAS.90Dec27202731@tira.uchicago.edu> <uV5wu2w163w@ozonebbs.UUCP> <1990Dec30.160422.27898@xrtll.uucp> <1990Dec31.090454.13869@ecst.csuchico.edu>
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It is true that the 8259A interrupt controller will generate IRQ7 as its
"default interrupt", but this is a symptom of a failing interrupt system
(e.g., two devices fighting over the same IRQ line).  In normal system
operation, default interrupts should not occur.

It is also true that some clones (e.g., early Olivetti models) do not
correctly implement IRQ7 on their parallel ports.  Oh well.  IBM machines,
of course, do it right.

My suggestion is to implement IRQ 7 processing, but also hook the timer
interrupt and, if a long timeout with no "ack" occurs, use the timer
interrupt to restart output.  Mainframes do this -- it's referred to as
"missing interrupt processing."