mbr: Make sure the MBR code starts with the byte 0x33

Apparently some BIOSes (including some Acer Travelmate machines)
require an MBR to start with 0x33; apparently Micro$oft MBRs start
with 33 C0, an alternate coding of the "xorw %ax,%ax" instruction.  As
such, follow suit to work on these braindead BIOSes.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

diff --git a/mbr/altmbr.S b/mbr/altmbr.S
index 794ab62..1b60905 100644 (file)
--- a/mbr/altmbr.S
+++ b/mbr/altmbr.S
@@ -49,8 +49,8 @@ bootsec:
        .text
        .globl  _start
 _start:
+       .byte   0x33, 0xc0      /* xorw %ax, %ax */
        cli
-       xorw    %ax, %ax
        movw    %ax, %ds
        movw    %ax, %ss
        movw    $stack, %sp

diff --git a/mbr/gptmbr.S b/mbr/gptmbr.S
index 8ed4cf4..ae0549f 100644 (file)
--- a/mbr/gptmbr.S
+++ b/mbr/gptmbr.S
@@ -55,8 +55,8 @@ bootsec:
        .text
        .globl  _start
 _start:
+       .byte   0x33, 0xc0      /* xorw %ax, %ax */
        cli
-       xorw    %ax, %ax
        movw    %ax, %ds
        movw    %ax, %ss
        movw    $stack, %sp

diff --git a/mbr/isohdpfx.S b/mbr/isohdpfx.S
index 0bf807e..2784fb8 100644 (file)
--- a/mbr/isohdpfx.S
+++ b/mbr/isohdpfx.S
@@ -65,9 +65,8 @@ bootsec:
        .text
        .globl  _start
 _start:
-
+       .byte   0x33, 0xed      /* xorw %bp, %bp */
        cli
-       xorw    %bp, %bp
        movw    %bp, %ss
        movw    $stack, %sp
        sti

diff --git a/mbr/mbr.S b/mbr/mbr.S
index 7caf4fc..b71cfb7 100644 (file)
--- a/mbr/mbr.S
+++ b/mbr/mbr.S
@@ -49,8 +49,8 @@ bootsec:
        .text
        .globl  _start
 _start:
+       .byte   0x33, 0xc0      /* xorw %ax, %ax */
        cli
-       xorw    %ax, %ax
        movw    %ax, %ds
        movw    %ax, %ss
        movw    $stack, %sp