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: Request for advice on PS/2 problems
Message-ID: <1944@ast.cs.vu.nl>
Date: 22 Jan 89 22:32:09 GMT
Sender: ast@cs.vu.nl
Reply-To: ast@cs.vu.nl (Andy Tanenbaum)
Organization: VU Informatica, Amsterdam
Lines: 47

Steve Ackerman has reported getting MINIX to boot on the PS/2 Model 80.
Good work!  However, we are not really done yet.  I have to figure out
how to deal with level-triggered interrupts in a reasonable way.  This
message solicits suggestions.  Please post them, since 10K heads are
better than 1.

MINIX, like all other message passing operating systems, does not do much
in its interrupt routines.  It just sends a message and returns from the
interrupt.  This means that after a floppy disk interrupt, the interrupt
line would normally still be asserted.  However, to shut off the 8259A
interrupt controller, on line 1888, we order it to shut up.  On the PS/2
this doesn't work.  We have to actually talk to the device.  It is
probably unwise to do this inside the interupt() function.  This means
we have to move line 1888 to each device driver and do it there.  Unpleasant
but possible.

However, things are worse than this.  If we delete line 1888 and move it
to the device drivers, when the interrupt routine is finished, we will get
another interrupt.  To prevent this, Steve diddled the 8259A's interrupt
mask around line 1888.  Double ugly.

My question is: does anybody see a better and cleaner way to do this?
I don't especially want the code around line 1888 having to figure out
which of the 8259A's to diddle, and which bit, etc.  For the floppy it
may not be too bad, but for serial I/O, networking, etc, it will make
things more complicated and error prone in a sensitive area.  On the other
hand, I don't want a major redesign of things.  I think the original
design is fairly simple and don't want to muck it up because of the way
the PS/2 does things.) I'd also like to minimize the amount of code of the sort

if (ps)				/* Models 25 - 80 */
if (mca) ...			/* microchannel architectures only */
if (ps && mca == FALSE) ...	/* Models 25 and 30 */
etc.

Hardware types, get out your thinking caps.

Another point, I am still trying to figure out exactly how to tell if 
a machine is (1) PS/2 and (2) Microchannel.  Several people have pointed
out that INT 15 is supposed to tell you, but I can't find that in my
(PC) BIOS manual.  Maybe it is something new.  Could some kind soul please
write a little piece of code that can be added to klib88.s making the
call to int15, and then write the C code for using this call to set the
variables ps and mca in such a way that it works on PC, XT, AT, and PS/2
(preferably without including a complete expert system in there).  Thanks.

Andy Tanenbaum (ast@cs.vu.nl)