linux-kernel - [PATCH] Reduce boot header size with 1 byte

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Message-Id: <20180927134117.15371-1-xyproto@archlinux.org>
Date:   Thu, 27 Sep 2018 15:41:17 +0200
From:   xyproto@...hlinux.org
To:     linux-kernel@...r.kernel.org
Cc:     Alexander F. Rødseth <xyproto@...hlinux.org>
Subject: [PATCH] Reduce boot header size with 1 byte

From: Alexander F. Rødseth <xyproto@...hlinux.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@...hlinux.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