Re: [Qemu-trivial] [PATCH] SGABIOS: fix wrong video attrs for int 10h, ah==13h

[Laurent Vivier]

On 07/03/2017 02:30, herbierobinson wrote:
> Fix Bug #1670509: wrong video attrs for int 10h, ah==13h
> The subroutine set_cursor_position is trashing %bx.
> Unfortunately, %bl contains the video attribute for write_string.
> 
> The fix saves %bx in the function prolog and restores it in
> the epilog.  Also, remove save and restore in set_current_cursor,
> because set_cursor_position does it.
> 
> Signed-off-by: herbierobinson <address@hidden>
> ---
>  sgabios.S | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/sgabios.S b/sgabios.S
> index 275d063..da5acc2 100644
> --- a/sgabios.S
> +++ b/sgabios.S
> @@ -1728,6 +1728,7 @@ write_string_tail:
>  
>  set_cursor_position:
>    pushw %ax
> +  pushw %bx
>    pushw %ds
>    pushw $BDA_SEG
>    popw %ds                             /* ds = 0x40 */
> @@ -1738,6 +1739,7 @@ set_cursor_position:
>    movw %ax, %bx                        /* bx = cursor save offset */
>    movw %dx, (%bx)                      /* save new cursor value */
>    popw %ds
> +  popw %bx
>    popw %ax
>    ret
>  
> @@ -1752,12 +1754,10 @@ set_cursor_position:
>  
>  set_current_cursor:
>    pushw %ds
> -  pushw %bx
>    pushw $BDA_SEG
>    popw %ds                             /* ds = 0x40 */
>    movb BDA_ACTIVE_PAGE, %bh

Well, we are trashing %bh here, I think the push/pop %bx is needed.
[comment says "all registers preserved except flags"]

>    call set_cursor_position
> -  popw %bx
>    popw %ds
>    ret
>  
>