[syslinux] [PATCH] gptmbr: Ctrl-key press forces load from first hard disk

[TJ]

---
 mbr/gptmbr.S |   16 ++++++++++++++--
 1 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/mbr/gptmbr.S b/mbr/gptmbr.S
index 385afe7..c161b44 100644
--- a/mbr/gptmbr.S
+++ b/mbr/gptmbr.S
@@ -59,6 +59,7 @@ _start:
 	movw	%ax, %ss
 	movw	$stack, %sp
 	movw	%sp, %si
+	movw	%sp, %bp	/* needed later if drive number is to be forced due to Ctrl key being pressed */
 	pushw	%es		/* 4(%bp) es:di -> $PnP header */
 	pushw	%di		/* 2(%bp) */
 	pushw	%dx		/* 0(%bp) = %dl -> drive number */
@@ -74,7 +75,18 @@ _start:
 	ljmpw	$0, $next
 
 next:
+	/* Provide a way for the user to force boot from first hard-disk by pressing a Ctrl key */
+get_shift_keys:	
+	movb	$0x02, %ah		/* function: get keyboard shift flags */
+	int		$0x16			/* read keys; shift flags returned in AL */
+	testb	$0x04, %al		/* control key pressed? */
+	jz		detect_bios_type
+force_first_hard_disk:
+	movb 	$0x80, -6(%bp)	/* offset from top-of-stack to drive number; force it to be first hard disk */
+
+detect_bios_type:
 	/* Check to see if we have EBIOS */
+	movw 	-6(%bp), %dx
 	pushw	%dx		/* drive number */
 	movb	$0x41, %ah	/* %al == 0 already */
 	movw	$0x55aa, %bx
@@ -210,7 +222,7 @@ boot:
 
 missing_os:
 	call	error
-	.ascii	"Operating system not bootable\r\n"
+	.ascii	"OS unbootable\r\n"
 
 saturate_stosl:
 	pushl	%eax
@@ -265,7 +277,7 @@ read_common:
 
 disk_error:
 	call	error
-	.ascii	"Disk error on boot\r\n"
+	.ascii	"Disk boot error\r\n"
 
 /*
  * Print error messages.  This is invoked with "call", with the
-- 
1.6.0.4