Re: [PATCH v1 12/14] x86/boot: tweak a20.c for better code generation

[H. Peter Anvin]

On January 24, 2026 3:07:41 PM PST, David Laight <david.laight.linux@xxxxxxxxx> wrote:
>On Fri, 23 Jan 2026 20:24:55 -0800
>"H. Peter Anvin" <hpa@xxxxxxxxx> wrote:
>
>> On 2026-01-23 19:00, Maciej W. Rozycki wrote:
>> > On Wed, 21 Jan 2026, David Laight wrote:
>> >   
>> >> No loops needed.  
>> > 
>> >  A loop is needed because there can be a considerable delay from issuing 
>> > the I/O request to flip the A20 gate till the circuitry responding.  This 
>> > is particularly true with the command issued to the 8042 device, which is 
>> > a microcontroller running its own firmware that needs it time to process 
>> > an incoming request to drive one of the microcontroller's GPIOs.  There 
>> > was a reason for port 0x92 circuitry later added to the PC architecture 
>> > with the IBM PS/2 being called the "fast A20 gate".
>> >   
>> 
>> Indeed. I thought I had responded to this already but I hadn't, apparently.
>> 
>> Note that the "long" delay is 2^21 loops! That number wasn't taken out of the
>> air, either; we found machines that actually needed that many iterations.
>
>Ok, so you need a loop because it might take ages for the value read from
>0x1000200 to change.
>But there is no need to keep changing the value.
>The comments in the code don't really stress that.
>
>> In the case where A20 is enabled already, the loop terminates on either the
>> first or second iteration (the second iteration is when the value at 0x1000200
>> is exactly 1 higher than the value at 0x200.
>> 
>> Modern machines (Nehalem+) already have A20 enabled, and most machines of the
>> i686+ generation implement int 0x15 function 0x2401.
>
>I know some of the history.
>And just read some more of the gory details...
>
>A20 being disabled is there to make a 286 compatible with the older 8086 PCs
>and any software that relied on address wrapping (rather than using it to get
>an extra ~64kB in real mode).
>That would be for dos and win 3.11...
>
>The only 8088 and 286 cpu I used were on IO cards.
>
>> 
>> 	-hpa
>> 
>

No, there is a reason to keep changing the value: you have no idea what is currently stored in that memory, *and you have no way of knowing*.

Whatever value you write might purely accidentally be the value that already is stored at that memory location.