[SeaBIOS] [RFC] [PATCH 1/2] Implement support for transitioning from 16 bit to 32 bit

H. Peter Anvin hpa at zytor.com
Mon Nov 22 23:09:23 CET 2010

On 11/22/2010 09:08 AM, Stefan Berger wrote:
> +
> +        // enable a20
> +        inb $PORT_A20, %al
> +        orb $A20_ENABLE_BIT, %al
> +        outb %al, $PORT_A20
> +

Let me *strongly* recommend isolate A20M handling in a single module for
the following reasons:

1. it is necessary to handle INT 15h, AX=2400/2401h;
2. different platforms have different requirements (port 92h is not
universally supported) and even if it is the amount of time until it is
active varies;
3. some newer CPUs (e.g. Core i3/5/7) don't even have A20M so there is
absolutely no reason to waste time poking it at all;
4. testing for A20M active may be advisable for speed purposes.


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