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Re: [PATCH] x86emul: Fix AAM emulation AH output
- To: Teddy Astie <teddy.astie@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 24 Sep 2026 14:00:46 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monné <roger@xxxxxxxxxxxxxx>
- Delivery-date: Thu, 24 Sep 2026 13:00:57 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 24/09/2026 12:59 pm, Teddy Astie wrote:
> AAM is defined in APM as
>
> AH = (AL/10d)
> AL = (AL mod 10d)
>
> However, due to the order of operations, we're incorrectly computing it as
>
> AL = (AL mod 10d)
> AH = (AL/10d) = ((originalAL mod 10d)/10d) = 0
>
> Fix it by reordering the operations to match what's defined in APM
> and avoid the unexpected dependency between the 2 operations.
>
> Fixes: 8993572b9c16 ("x86emul: fold/eliminate some local variables")
> Signed-off-by: Teddy Astie <teddy.astie@xxxxxxxxxx>
Oops. Yes, looking at the code, it was previously:
movzbl -0x88(%rbp),%eax
xor %edx,%edx
div %ecx
mov %dl,-0x88(%rbp)
mov %edx,%eax
xor %edx,%edx
div %ecx
mov %al,-0x87(%rbp)
and with this fix:
movzbl -0x88(%rbp),%eax
xor %edx,%edx
div %ecx
mov %al,-0x87(%rbp)
mov %dl,-0x88(%rbp)
Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxx>
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