Path: utzoo!attcan!uunet!ncrlnk!ncr-sd!hp-sdd!hplabs!decwrl!labrea!rutgers!rochester!pt.cs.cmu.edu!b.gp.cs.cmu.edu!Ralf.Brown@B.GP.CS.CMU.EDU
From: Ralf.Brown@B.GP.CS.CMU.EDU
Newsgroups: comp.sys.ibm.pc
Subject: Re: Kbd. LED Indicator Lights?
Message-ID: <23a131e9@ralf>
Date: 10 Dec 88 14:17:13 GMT
Sender: ralf@b.gp.cs.cmu.edu
Lines: 21
In-Reply-To: <15711@iuvax.cs.indiana.edu>

In article <15711@iuvax.cs.indiana.edu>, bkliewer@iuvax.cs.indiana.edu (Bradley Dyck Kliewer) writes:
}On IBM machines, the LED's toggle when the status bit in the BIOS data
}area changes.  I'm not sure of the exact address, but it's somewhere in
}segment 40h.  As for non-IBM machines and add-on keyboards, I don't think
}there are any standards.

They only toggle because the BIOS checks for changed bits each time the
keyboard sends a scan code, and updates the LEDs if necessary.  There is a
specific command that you send the keyboard controller at I/O port 64h which
sets the LEDs, but I can't find it off-hand.

So, if the program changes the status bits, but no keys are pressed, held
down, or released afterwards, the LEDs will not change.  Makes for a bit of an
unusual effect to have the NumLock LED change when you press Shift, for
example....

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