Path: utzoo!utgpu!water!watmath!clyde!rutgers!husc6!linus!mbunix!jcmorris
From: jcmorris@mitre-bedford.ARPA (Joseph C. Morris)
Newsgroups: comp.os.minix
Subject: Re: How do you turn on the AT's LEDs?
Message-ID: <22877@linus.UUCP>
Date: 27 Jan 88 20:56:53 GMT
References: <1832@botter.cs.vu.nl> <4240@eagle.ukc.ac.uk> <1836@botter.cs.vu.nl>
Sender: news@linus.UUCP
Reply-To: jcmorris@mbunix (Morris)
Organization: The MITRE Corporation, McLean, VA.
Lines: 26

In article <1836@botter.cs.vu.nl> ast@cs.vu.nl (Andy Tanenbaum) writes:
>As everyone with MINIX on an AT has observed, hitting caps lock does not turn
>on the LED.

That depends on the flavor of your keyboard.  The replacement PC keyboards
for the pre-AT machines (translation: the machines for which IBM doesn't
provide LED's) all toggle the lights based on keystrokes in the keyboard,
and I wouldn't be surprised to find that several of the AT clones took the
same el cheapo approach.

BIOS in the true-Blue AT's includes about two pages of code listing to
turn the LED's on and off, so it's not a trivial exercise.  The actual
hardware code isn't complex; the fun comes because you have to disable the
keyboard (after ensuring that it isn't busy with some previous function),
send the necessary bit pattern, re-enable the keyboard, mess with the
interrupt controller, go out for a beer, and finally set switches to
stop the next process from doing anything to the keyboard interface until
the keyboard processor acknowledges that it has accepted your order to
change the LED display.

There's no assurance that the various clone manufacturers have implemented
anything like this design.  Since relatively few programs have a need for
messing with this interface (MINIX, for example) it probably wasn't seen 
as a real issue when the boxes were designed.

Joe Morris