Path: utzoo!attcan!uunet!mcvax!hp4nl!botter!star.cs.vu.nl!ast@cs.vu.nl
From: ast@cs.vu.nl (Andy Tanenbaum)
Newsgroups: comp.os.minix
Subject: PS/2 level-triggered interrupts
Message-ID: <1907@ast.cs.vu.nl>
Date: 16 Jan 89 22:32:40 GMT
Sender: ast@cs.vu.nl
Reply-To: ast@cs.vu.nl (Andy Tanenbaum)
Organization: VU Informatica, Amsterdam
Lines: 18


I sort of take back what I said earlier about the 8259A.  I went and found a
data sheet and tried to make some sense out of it.  I am not a programmable
interrupt controller wizard, so take this with a metric ton of salt.

The data sheet says: "The interrupt request must be removed before the EOI
command is issued or the CPU interrupt is enabled to prevent a second
interrupt from occurring."

I suspect what they are trying to say is that if the EOI is issued when the
level-triggered interrupt is still asserted, you get another interrupt.
Thus, there is no way to have the 8259A fix the problem.  The floppy disk
controller must be given some sort of command to tell it to negate the
interrupt.  Furthermore, every other device (clock, hard disk, network, 
etc) that can generate interrupts has the same problem.

I wonder if somebody over in comp.sys.ibm can help.  I'll post a query there.
Andy Tanenbaum (ast@cs.vu.nl)