Path: utzoo!utgpu!watmath!mks!wheels
From: wheels@mks.UUCP (Gerry Wheeler)
Newsgroups: comp.os.minix
Subject: Re: PS/2 level-triggered interrupts
Summary: makes sense
Message-ID: <644@mks.UUCP>
Date: 18 Jan 89 22:24:10 GMT
References: <1907@ast.cs.vu.nl>
Organization: Mortice Kern Systems, Waterloo, Ont.
Lines: 26

In article <1907@ast.cs.vu.nl>, ast@cs.vu.nl (Andy Tanenbaum) writes:
> 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.
> The floppy disk
> controller must be given some sort of command to tell it to negate the
> interrupt.

While I'm not familiar with the PS/2 family, that sounds similar to
other computers I have used, and is really the essence of using level
sensitive interrupts.  If you have several IO boards connected to a
single interrupt line, perhaps several serial ports, there may be more
that one asserting the interrupt at one time.  You get the interrupt,
poll the devices to find who caused it, service the first device found,
it releases the interrupt, you send EOI, and get another interrupt to
remind you there is someone else still awaiting service.

In theory, this seems far superior to edge sensitive interrupts, because
in that case you cannot have more than one device on an interrupt.  In
practice, when it can't be made to work, the superiority may not be
appreciated.  :-)

-- 
     Gerry Wheeler                           Phone: (519)884-2251
Mortice Kern Systems Inc.               UUCP: uunet!watmath!mks!wheels
   35 King St. North                             BIX: join mks
Waterloo, Ontario  N2J 2W9                  CompuServe: 73260,1043