Re: A20 Gate enable sequence (setup.S)

From: Richard B. Johnson (
Date: Fri Oct 13 2000 - 12:40:36 EST

On Fri, 13 Oct 2000, Alan Cox wrote:

> > Some boards (especially in the embedded PC area) implement only the
> > latter method (some don't even _have_ a keyboard controller).
> > With these boards, Linux fails to boot.
> Try the current 2.2.18pre. This has fast A20 too and I suspect will boot
> on your box. Its had no problems on 2.2.18pre reported against it so far. I've
> not done a 2.4 test patch but yours seems to be the sam ething

The AMD/Elan box snoops for the sequence sent to the keyboard
controller to enable A<20>. However, it also respects the port
0x92 control. Linux-2.2.17 doesn't have the 0x92 control so
it must have been added recently.

The AMD/Elan chip also comes to life after a hardware reset with
A<20> enabled. So, if you don't turn it OFF, there won't be any
problem during the protected-mode transition.

Dick Johnson

Penguin : Linux version 2.2.17 on an i686 machine (801.18 BogoMips).

"Memory is like gasoline. You use it up when you are running. Of
course you get it all back when you reboot..."; Actual explanation
obtained from the Micro$oft help desk.

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