Path: utzoo!attcan!uunet!tektronix!percival!bucket!leonard
From: leonard@bucket.UUCP (Leonard Erickson)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Kbd. LED Indicator Lights?
Message-ID: <1170@bucket.UUCP>
Date: 16 Dec 88 05:16:35 GMT
References: <23a131e9@ralf> <15736@iuvax.cs.indiana.edu>
Reply-To: leonard@bucket.UUCP (Leonard Erickson)
Organization: Rick's Home Grown Unix; Portland, OR
Lines: 29

In article <15736@iuvax.cs.indiana.edu> bkliewer@iuvax.UUCP (Bradley Dyck Kliewer) writes:
<In article <23a131e9@ralf> Ralf.Brown@B.GP.CS.CMU.EDU writes:
<>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... 
<
<Not on IBM machines.  Toggling the bits in the BIOS data area immediately
<update the LED's (I just tried it to make sure I remembered correctly).
<I have always assumed some type of interrupt generates the change, but it's
<not the keyboard interrupt.  Could it be the timer interrupt? 
<
<As for the command at port 64h, EDh is Set/Reset Mode Indicators.  The next
<byte sets the LED's bit 2=Caps Lock, bit 1=Num Lock, bit 0=Scroll Lock.

Sorry, but the question was talking about *XT* class machines. Not even IBM XT
type machines will update keyboard LEDs (except maybe on PS/2 machines). This
should come as no surprise because there weren't any LEDs on keyboards back
then so the keyboards didn't expect to be told what to do with them...

-- 
Leonard Erickson		...!tektronix!reed!percival!bucket!leonard
CIS: [70465,203]
"I used to be a hacker. Now I'm a 'microcomputer specialist'.
You know... I'd rather be a hacker."