[PATCH] Reduce boot header size with 1 byte

[PATCH] Reduce boot header size with 1 byte

[xyproto]

From: Alexander F. Rødseth <xyproto@archlinux.org>

Only ah needs to be set to 0 before calling interrupt 0x16 for waiting
for a keypress.

This patch changes the line that uses xor so that it only zeroes "ah" instead of "ax".
This saves a byte.

Signed-off-by: Alexander F. Rødseth <xyproto@archlinux.org>
---
 arch/x86/boot/header.S | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
index 850b8762e889..905cb96f43d4 100644
--- a/arch/x86/boot/header.S
+++ b/arch/x86/boot/header.S
@@ -71,14 +71,15 @@ msg_loop:
 	jmp	msg_loop
 
 bs_die:
-	# Allow the user to press a key, then reboot
-	xorw	%ax, %ax
+	# Allow the user to press a key
+	xorb	%ah, %ah
 	int	$0x16
+
+	# Boostrap by reading sector 1, head 0, track 0 from drive
 	int	$0x19
 
-	# int 0x19 should never return.  In case it does anyway,
-	# invoke the BIOS reset code...
-	ljmp	$0xf000,$0xfff0
+	# If boostrap somehow did not work, invoke the BIOS reset code
+	ljmp	$0xf000, $0xfff0
 
 #ifdef CONFIG_EFI_STUB
 	.org	0x3c
-- 
2.19.0

Re: [PATCH] Reduce boot header size with 1 byte

[Randy Dunlap]

On 9/27/18 6:41 AM, xyproto@archlinux.org wrote:
> From: Alexander F. Rødseth <xyproto@archlinux.org>
> 
> Only ah needs to be set to 0 before calling interrupt 0x16 for waiting
> for a keypress.
> 
> This patch changes the line that uses xor so that it only zeroes "ah" instead of "ax".
> This saves a byte.
> 
> Signed-off-by: Alexander F. Rødseth <xyproto@archlinux.org>

Hi,

You should Cc: the X86 maintainers on this patch.
Otherwise it will just be lost in the ether.

thanks.

> ---
>  arch/x86/boot/header.S | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
> index 850b8762e889..905cb96f43d4 100644
> --- a/arch/x86/boot/header.S
> +++ b/arch/x86/boot/header.S
> @@ -71,14 +71,15 @@ msg_loop:
>  	jmp	msg_loop
>  
>  bs_die:
> -	# Allow the user to press a key, then reboot
> -	xorw	%ax, %ax
> +	# Allow the user to press a key
> +	xorb	%ah, %ah
>  	int	$0x16
> +
> +	# Boostrap by reading sector 1, head 0, track 0 from drive
>  	int	$0x19
>  
> -	# int 0x19 should never return.  In case it does anyway,
> -	# invoke the BIOS reset code...
> -	ljmp	$0xf000,$0xfff0
> +	# If boostrap somehow did not work, invoke the BIOS reset code
> +	ljmp	$0xf000, $0xfff0
>  
>  #ifdef CONFIG_EFI_STUB
>  	.org	0x3c
> 


-- 
~Randy

Re: [PATCH] Reduce boot header size with 1 byte

[Brian Gerst]

On Thu, Sep 27, 2018 at 9:42 AM <xyproto@archlinux.org> wrote:
>
> From: Alexander F. Rødseth <xyproto@archlinux.org>
>
> Only ah needs to be set to 0 before calling interrupt 0x16 for waiting
> for a keypress.
>
> This patch changes the line that uses xor so that it only zeroes "ah" instead of "ax".
> This saves a byte.
>
> Signed-off-by: Alexander F. Rødseth <xyproto@archlinux.org>
> ---
>  arch/x86/boot/header.S | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
> index 850b8762e889..905cb96f43d4 100644
> --- a/arch/x86/boot/header.S
> +++ b/arch/x86/boot/header.S
> @@ -71,14 +71,15 @@ msg_loop:
>         jmp     msg_loop
>
>  bs_die:
> -       # Allow the user to press a key, then reboot
> -       xorw    %ax, %ax
> +       # Allow the user to press a key
> +       xorb    %ah, %ah

Because this is code running in 16-bit real mode, xorw does not need a
16-bit prefix and only uses 2 bytes.  You save nothing by using xorb.

--
Brian Gerst