Re: Booting big kernels on IBM ThinkPad 770X

Alan Cox (
Wed, 2 Dec 1998 22:32:09 +0000 (GMT)

> According to the Technical Reference Manual there is a special
> control port that, among other things, controls the ability of setting
> the A20-gate in real mode, which is initially disabled. It is bit 1 of
> the System Control Port A (0x92).
> I inserted a call to the following routine in setup.S just before setting A20
> and now my bzImage boots!
> !
> ! enables a20-handling on ThinkPads while in real mode
> !
> ThinkPad_SCP_A:
> in al,#0x92 ! read "System Control Port A"
> or al,#0x02 ! Set "Alternate Gate A20" - Bit
> out #0x92,al ! write "System Control Port A"
> ret
> Now, I'm not an assembler programmer at all. Maybe someone could take a closer
> look into that.

That would make some sense. Thats the A20 active bit on the IBM PS/2 as well. (Hey
MCA hacking finally produces something useful 8)) . It might also be generally ok
since the great Ralf Brown interrupt list documents no other users


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