Re: writing to POST leds?

Leonard N. Zubkoff (
Sun, 8 Sep 1996 18:59:53 -0700

Date: Sun, 8 Sep 1996 17:53:25 -0500 (CDT)
From: Edward Welbon <>

I have a POST (Power On Self Test) display card that registers and
displays the POST values on LEDs. I have removed the useless clock speed
LEDs with POST codes instead since in addition to the BIOS, the kernel
etc. writes to them. The address is IO port 80. Here is the dumb
question, since I doubt that the meanings of the values are documented
anywhere, how might I find places where the code references them. As you
might guess, I am not entirely clear on how to write to the LEDs. Is
there any standard way?

I have one of these also. It's useful for debugging device drivers when
printk's perturb the environment enough to make a bug disappear. All you need
to do is output a value to port 0x80. However, the SLOW_DOWN_IO code in
asm/io.h writes random values to port 0x80, which tends to obscure the values
written by users.