Re: A20 Gate enable sequence (setup.S)

From: Robert Kaiser (
Date: Sun Oct 15 2000 - 02:25:47 EST

> The AMD/Elan box snoops for the sequence sent to the keyboard
> controller to enable A<20>.

Are you sure this is true of all boards using the AMD Elan ?
So far, I've dealt with three different AMD Elan boards and only
one of them (Jumptec DIMM-PC) seemed to have this feature. But even
on that one, Linux wouldn't boot out of the box because of A20 problems
(you had to reset the keyboard controller to make it work).

> However, it also respects the port
> 0x92 control.

The i386EX has port 0x92 built in (it does not have a built-in
keyboard controller ..), so port 0x92 should work on all 386EX
boards whereas the keyboard controller sequence may or may not work.

> 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.

Sure, but whether you CAN leave it on depends on the board's BIOS.
Some of them are "smart" and won't let you configure the initial
A20 state. Adding the port 0x92 method to setup.S as I'm suggesting
would be safe in all cases (assuming it won't break anything else).


Robert Kaiser email:
Am Pfaffenstein 14
D-55270 Klein-Winternheim / Germany

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