Re: [OFFTOPIC] A20 handler problem

H. Peter Anvin (hpa@zytor.com)
17 Jul 2000 08:43:40 -0700

Followup to: <Pine.LNX.4.10.10007171737050.2830-100000@orbita.don.sitek.net>
By author: Andrey Panin <pazke@orbita.don.sitek.net>
In newsgroup: linux.dev.kernel
>
> Hi,
>
> most(all?) modern PC chipsets support so called system
> control port (0x92) which provides A20 line handling
> along with other features.
>
> AFAIK bit 1 of 0x92 port control A20 line:
> 0 - A20 disable / forced 0 (real mode)
> 1 - A20 address line enable
>
> But I didn't try it, you warned :)).
>

On most chips, you need to tweak both the 0x92 port and the keyboard
controller if you want to disable A20. There are some funny latencies
going on, too, since this is usually in external logic.

The source code to SYSLINUX contains A20 enable/disable code. Beyond
this I suggest taking this to comp.lang.asm.x86.

-hpa

-- 
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."
http://www.zytor.com/~hpa/puzzle.txt


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