isohybrid: revert to a stack format compatible with previous versionsyslinux-3.81-pre14

Revert the isohybrid handover protocol so that it has a stack format
compatible with the previous versions; that way we can also revert the
magic number to a compatible one.

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

3 files changed, 32 insertions, 34 deletions

diff --git a/core/isolinux.asm b/core/isolinux.asm
index b85ecd36..104de14a 100644
--- a/core/isolinux.asm
+++ b/core/isolinux.asm
@@ -224,35 +224,32 @@ bi_end:
 		; Custom entry point for the hybrid-mode disk.
 		; The following values will have been pushed onto the
 		; entry stack:
+		;	- partition offset (qword)
 		;	- ES
 		;	- DI
 		;	- DX (including drive number)
-		;	- partition offset (qword)
-		;	- EBIOS flag
-		;	- CBIOS Sectors
 		;	- CBIOS Heads
+		;	- CBIOS Sectors
+		;	- EBIOS flag
 		;       (top of stack)
 		;
 %ifndef DEBUG_MESSAGES
 _hybrid_signature:
-		dd 0x0defe3f7
+	       dd 0x7078c0fb			; An arbitrary number...
+
 _start_hybrid:
-		pop word [cs:bsHeads]
+		pop cx				; EBIOS flag
 		pop word [cs:bsSecPerTrack]
-
-		pop ax
-		mov si,bios_cbios
-		and ax,ax
-		jz .cbios
-		mov si,bios_ebios
-.cbios:
-
-		pop dword [cs:bsHidden]
-		pop dword [cs:bsHidden+4]
-
+		pop word [cs:bsHeads]
 		pop dx
 		pop di
 		pop es
+		pop dword [cs:bsHidden]
+		pop dword [cs:bsHidden+4]
+
+		mov si,bios_cbios
+		jcxz _start_common
+		mov si,bios_ebios
 		jmp _start_common
 %endif
 
diff --git a/mbr/isohdpfx.S b/mbr/isohdpfx.S
index 53e1ed6b..31a792e8 100644
--- a/mbr/isohdpfx.S
+++ b/mbr/isohdpfx.S
@@ -39,15 +39,15 @@
 	.code16
 	.text
 
-HYBRID_MAGIC			= 0x0defe3f7
+HYBRID_MAGIC			= 0x7078c0fb
 isolinux_hybrid_signature	= 0x7c00+64
 isolinux_start_hybrid		= 0x7c00+64+4
 
 	.globl	bootsec
 /* Important: the top 6 words on the stack are passed to isolinux.bin */
 stack		= 0x7c00
-driveno		= (stack-6)
-partoffset	= (stack-14)
+partoffset	= (stack-8)
+driveno		= (stack-14)
 ebios_flag	= (stack-16)
 sectors		= (stack-18)
 heads		= (stack-20)
@@ -70,17 +70,19 @@ _start:
 	xorw	%bx, %bx
 	movw	%bx, %ds
 	movw	%bx, %ss
-	movw	$stack, %sp
-	pushw	%es			/* -4: es:di -> $PnP header */
+	movw	$partoffset, %sp
+	pushw	%es			/* -12: es:di -> $PnP header */
 	pushw	%di
 	movw	%bx, %es
 	sti
 	cld
 
 	ADJUST_DRIVE
-	pushw	%dx			/* -6: dl -> drive number */
+	pushw	%dx			/* -14: dl -> drive number */
 
 	/* Check to see if we have a partition table entry */
+	xorl	%ebx, %ebx
+	xorl	%ecx, %ecx
 #ifdef PARTITION_SUPPORT
 	andw	%si, %si		/* %si == 0 -> no partition data */
 	jz	1f
@@ -90,22 +92,21 @@ _start:
 	jne	2f
 	cmpb	$0xee, 4(%si)		/* EFI partition type? */
 	jne	2f
+	
 	/* We have GPT partition information */
-	pushl	(36+16)(%si)		/* -10: partoffset_hi */
-	pushl	(32+16)(%si)		/* -14: partoffset_lo */
-	jmp	3f
-2:
+	movl	(36+16)(%si), %ecx
+	movl	(32+16)(%si), %ebx
+	jmp	1f
+
 	/* We have non-GPT partition information */
-	pushl	$0			/* -10: partoffset_hi */
-	pushl	8(%si)			/* -14: partoffset_lo */
-	jmp	3f
+2:
+	movl	8(%si), %ecx
 #endif
 1:
 	/* We have no partition information */
-	pushl	$0			/* -10: partoffset_hi */
-	pushl	$0			/* -14: partoffset_lo */
-3:
-	
+	movl	%ecx, (partoffset)	/* -4: partoffset_hi */
+	movl	%ebx, (partoffset+4)	/* -0: partoffset_lo */
+
 	/* Copy down to 0:0x600 */
 	movw	$0x7c00, %si
 	movw	$_start, %di
diff --git a/utils/isohybrid.in b/utils/isohybrid.in
index b3a502bc..cfb92ed6 100644
--- a/utils/isohybrid.in
+++ b/utils/isohybrid.in
@@ -165,7 +165,7 @@ if ($de_boot != 0x88 || $de_media != 0 ||
 # Now $de_lba should contain the CD sector number for isolinux.bin
 seek(FILE, $de_lba*2048+0x40, SEEK_SET) or die "$0: $file: $!\n";
 read(FILE, $ibsig, 4);
-if ($ibsig ne "\xf7\xe3\xef\x0d") {
+if ($ibsig ne "\xfb\xc0\x78\x70") {
     die "$0: $file: bootloader is missing current (3.81+) isolinux.bin\n".
 	"        hybrid signature; Note that isolinux-debug.bin does not\n".
 	"        support hybrid booting.\n";