Major overhaul of our master boot record.

We now use EDD calls if the disk supports it, so we can boot partitions
that start above the 8 GB CHS limit.  Of itself, this change does not
remove the current 8 GB limit for OpenBSD/i386.

Much testing nick@; thanks.

ok weingart@, deraadt@.

1 files changed, 412 insertions, 200 deletions

diff --git a/sys/arch/i386/stand/mbr/mbr.S b/sys/arch/i386/stand/mbr/mbr.S
index 6829c7b47e9..4b9e8532fa7 100644
--- a/sys/arch/i386/stand/mbr/mbr.S
+++ b/sys/arch/i386/stand/mbr/mbr.S
@@ -1,7 +1,8 @@
-/*	$OpenBSD: mbr.S,v 1.18 2003/06/03 20:22:12 mickey Exp $	*/
+/*	$OpenBSD: mbr.S,v 1.19 2004/01/05 00:08:23 tom Exp $	*/
 
 /*
  * Copyright (c) 1997 Michael Shalayeff and Tobias Weingartner
+ * Copyright (c) 2003 Tom Cosgrove <tom.cosgrove@arches-consulting.com>
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -40,13 +41,36 @@
 #include <machine/asm.h>
 #include <assym.h>
 
-#define data32	.byte 0x66
-#define addr32	.byte 0x67
-
-#define BOOTBIOS	0x7c0	/* segment where we are loaded */
-#define BOOTRELOC	0x7a0	/* segment where to relocate */
+/*
+ * Memory layout:
+ *
+ * 0x07C00 -> 0x07DFF	BIOS loads us here	(at  31k)
+ * 0x07E00 -> 0x17BFC	our stack		(to  95k)
+ *
+ * 0x07A00 -> 0x07BFF	we relocate to here	(at  30k5)
+ *
+ * 0x07C00 -> 0x07DFF	we load PBR here	(at  31k)
+ *
+ * The BIOS loads us at physical address 0x07C00.  We use a long jmp to
+ * normalise our address to seg:offset 07C0:0000.  We then relocate to
+ * 0x07A00, seg:offset 07A0:0000.
+ *
+ * We use a long jmp to normalise our address to seg:offset 07A0:0000
+ * We set the stack to start at 07C0:FFFC (grows down on i386)
+ * We load the partition boot record (PBR) /boot at seg:offset 4000:0000
+ */
+#define BOOTSEG		0x7c0	/* segment where we are loaded */
+#define BOOTRELOCSEG	0x7a0	/* segment where we relocate to */
+#define BOOTSTACKOFF	0xfffc	/* stack starts here, grows down */
 #define PARTSZ		16	/* each partition table entry is 16 bytes */
 
+#define CHAR_LBA_READ	'.'
+#define CHAR_CHS_READ	';'
+#define CHAR_CHS_FORCE	'!'
+#define CHAR_SHIFT_SEEN	0x07	/* Use BEL */
+
+#define MBR_FLAGS_FORCE_CHS	0x0001
+
 #ifdef DEBUG
 #define CHAR_S		'S'	/* started */
 #define CHAR_R		'R'	/* relocated */
@@ -54,293 +78,482 @@
 #define CHAR_B		'B'	/* loading boot */
 #define CHAR_G		'G'	/* jumping to boot */
 
-#define DBGMSG(msg)		\
-	movb    $msg, %al;	\
-	/* call    Lchr */;	\
-	.byte	0xe8;		\
-	.word	Lchr - . - 2
+#define DBGMSG(c)	movb	$c, %al;	call	Lchr
 #else /* !DEBUG */
-#define DBGMSG(msg)
+#define DBGMSG(c)
 #endif /* !DEBUG */
-#define	puts(s)			\
-	data32;			\
-	movl	$s, %esi;	\
-	/* call Lmessage */;	\
-	.byte	0xe8;		\
-	.word	Lmessage - . - 2
+
+/* Clobbers %al - maybe more */
+#define	putc(c)		movb	$c, %al;	call	Lchr
+
+/* Clobbers %esi - maybe more */
+#define	puts(s)		movw	$s, %si;	call	Lmessage
+
 
 	.text
+	.code16
 
 	.globl	start
 start:
 	/* Adjust %cs to be right */
-	data32
-	ljmp 	$BOOTBIOS, $1f
+	ljmp 	$BOOTSEG, $1f
 1:
 	/* Set up stack */
-	movl	%cs, %eax
-	cli
-	mov	%ax, %ss
-	data32
-	movl	$0xfffc, %esp
-	sti
+	movw	%cs, %ax
+
+	/*
+	 * We don't need to disable and re-enable interrupts around the
+	 * the load of ss and sp.
+	 *
+	 * From 80386 Programmer's Reference Manual:
+	 * "A MOV into SS inhibits all interrupts until after the execution
+	 * of the next instruction (which is presumably a MOV into eSP)"
+	 *
+	 * According to Hamarsoft's 86BUGS list (which is distributed with
+	 * Ralph Brown's Interrupt List), some early 8086/88 processors
+	 * failed to disable interrupts following a load into a segment
+	 * register, but this was fixed with later steppings.
+	 *
+	 * Accordingly, this code will fail on very early 8086/88s, but
+	 * nick@ will just have to live with it.  Others will note that
+	 * we require an 80386 (or compatible) or above processor, anyway.
+	 */
+	/* cli */
+	movw	%ax, %ss
+	movw	$BOOTSTACKOFF, %sp
+	/* sti */			/* XXX not necessary; see above */
 
 	/* Set up data segment */
-	mov	%ax, %ds
+	movw	%ax, %ds
 	DBGMSG(CHAR_S)
 
-	/* Relocate 512 bytes so we can load PBS here  */
-	data32
-	movl	$BOOTRELOC, %eax
-	movl	%eax, %es
-	data32
-	xorl	%esi, %esi
-	data32
-	xorl	%edi, %edi
-	data32
-	movl	$0x200, %ecx
+	/*
+	 * On the PC architecture, the boot record (originally on a floppy
+	 * disk) is loaded at 0000:7C00 (hex) and execution starts at the
+	 * beginning.
+	 *
+	 * When hard disk support was added, a scheme to partition disks into
+	 * four separate partitions was used, to allow multiple operating
+	 * systems to be installed on the one disk.  The boot sectors of the
+	 * operating systems on each partition would of course expect to be
+	 * loaded at 0000:7C00.
+	 *
+	 * The first sector of the hard disk is the master boot record (MBR).
+	 * It is this which defines the partitions and says which one is
+	 * bootable.  Of course, the BIOS loads the MBR at 0000:7C00, the
+	 * same location where the MBR needs to load the partition boot
+	 * record (PBR, called biosboot in OpenBSD).
+	 *
+	 * Therefore, the MBR needs to relocate itself before loading the PBR.
+	 *
+	 * Make it so.
+	 */
+	movw	$BOOTRELOCSEG, %ax
+	movw	%ax, %es
+	xorw	%si, %si
+	xorw	%di, %di
+	movw	$0x200, %cx		/* Bytes in MBR, relocate it all */
 	cld
 	rep
 	movsb
 
 	/* Jump to relocated self */
-	data32
-	ljmp $BOOTRELOC, $reloc
+	ljmp $BOOTRELOCSEG, $reloc
 reloc:
 	DBGMSG(CHAR_R)
 
 	/* Set up %es and %ds */
-	pushl	%ds
-	popl	%es	/* next boot is at the same place as we were loaded */
-	pushl	%cs
-	popl	%ds	/* and %ds is at the %cs */
+	pushw	%ds
+	popw	%es	/* next boot is at the same place as we were loaded */
+	pushw	%cs
+	popw	%ds	/* and %ds is at the %cs */
 
 #ifdef SERIAL
 	/* Initialize the serial port to 9600 baud, 8N1.
-	 * Do we need to do this?  Most things at this level
-	 * do not know or care (on a PC) where the output is
-	 * happening to go.  I think if we are headless,
-	 * /boot should figure (as it does now) that out.
-	 *
-	 * If there is a problem with this stage of the boot
-	 * process, connect up a monitor and kbd, and see what
-	 * is going on.  Left here for the time being.
-	 *
-	 * --Toby.
 	 */
-	xorl	%eax, %eax
+	xorw	%ax, %ax
 	movb	$0xe3, %ax
-	data32
-	movl	$SERIAL, %dx
+	movw	$SERIAL, %dx
 	int	$0x14
 #endif
 
-	/* bootstrap passes us drive number in %dl
-	 *
-	 * XXX - This is not always true.  We currently
-	 * check if %dl points to a HD, and if not we
-	 * complain, and set it to point to the first
-	 * HDD.  Note, this is not 100% correct, since
-	 * there is a possibility that you boot of of
-	 * HD #2, and still get (%dl & 0x80) == 0x00,
-	 * these type of systems will loose.  I don't
-	 * know of any like this, but I've come to the
-	 * conclusion, that if it can exist, it will,
-	 * someplace in the PC world.  If anyone knows
-	 * how to fix this, speak up!
+	/*
+	 * If the SHIFT key is held down on entry, force CHS read
+	 */
+
+	/*
+	 * BIOS call "INT 0x16 Get Keyboard Shift Flags
+	 *	Call with	%ah = 0x02
+	 *	Return:
+	 *			%al = shift flags
+	 *			%ah - undefined by many BIOSes
+	 */
+	movb	$0x02, %ah
+	int	$0x16
+	testb	$0x3, %al	/* Either shift key down? */
+	jz	no_shift
+
+	putc(CHAR_SHIFT_SEEN)	/* Signal that shift key was seen */
+
+	orb	$MBR_FLAGS_FORCE_CHS, flags
+
+no_shift:
+	/* BIOS passes us drive number in %dl
 	 *
-	 * Toby - Thu Jul 31 21:01:00 CDT 1997
+	 * XXX - This is not always true.  We currently check if %dl
+	 * points to a HD, and if not we complain, and set it to point
+	 * to the first HDD.  Note, this is not 100% correct, since
+	 * there is a possibility that you boot of of HD #2, and still
+	 * get (%dl & 0x80) == 0x00, these type of systems will lose.
 	 */
 	testb	$0x80, %dl
-	jnz	1f
+	jnz	drive_ok
 
 	/* MBR on floppy or old BIOS
-	 * Note: MBR (this code) should never
-	 * be on a floppy.  It does not belong
-	 * there, so %dl should never be 0x00.
+	 * Note: MBR (this code) should never be on a floppy.  It does
+	 * not belong there, so %dl should never be 0x00.
 	 *
-	 * Here we simply complain (should we?),
-	 * and then hardcode the boot drive to
-	 * 0x80.
+	 * Here we simply complain (should we?), and then hardcode the
+	 * boot drive to 0x80.
 	 */
-	puts(fdmbr)
+	puts(efdmbr)
 
-	/* If we are passed bogus data, set it to HD #1.
-	 * We should load the value from a hard coded
-	 * location in this sector.  Maybe I'll write
-	 * that next, since my machines seem to be one
-	 * of the weird ones...
+	/* If we are passed bogus data, set it to HD #1
 	 */
 	movb	$0x80, %dl
 
-	/* Do we need to check our signature?  The BIOS will
-	 * check it for us, I doubt there is a need for us to
-	 * do the same thing over again.  If we fail here,
-	 * something terrible is wrong.  However, I doubt we
-	 * can recover anyways.  The message might be nice
-	 * for the (l)user though.
-	 */
-1:	xor	%bx, %bx
-	# cmpw	$DOSMBR_SIGNATURE, (%bx)
-	.byte	0x81, 0xbf
-	.word	signature
-	.word	DOSMBR_SIGNATURE
-	je	sigok
-	puts(esig)
-
-	/* find the first active partition
-	 * Note: this should be the only active
-	 * partition.  We currently don't check
-	 * for that, but we really should.  If
-	 * and when I feel up to it, I'll add
-	 * that code.
+drive_ok:
+	/* Find the first active partition.
+	 * Note: this should be the only active partition.  We currently
+	 * don't check for that.
 	 */
-sigok:
-	data32
-	movl	$pt, %esi
-	data32
-	movl	$NDOSPART, %ecx
-1:
+	movw	$pt, %si
+
+	movw	$NDOSPART, %cx
+find_active:
 	DBGMSG(CHAR_L)
-	# movb	(%si), %al
-	.byte	0x8a, 0x44, 0x00
+	movb	(%si), %al
+
 	cmpb	$DOSACTIVE, %al
 	je	found
-	data32
-	addl	$PARTSZ, %esi
-	loop	1b
+
+	addw	$PARTSZ, %si
+	loop	find_active
 
 	/* No bootable partition */
 no_part:
-	puts(noboot)
+	movw	$enoboot, %si
+
 err_stop:
+	call	Lmessage
+
+stay_stopped:
 	cli
 	hlt
 	/* Just to make sure */
-	jmp	err_stop
+	jmp	stay_stopped
 
-	/* Found bootable partition */
 found:
+	/*
+	 * Found bootable partition
+	 */
+
 	DBGMSG(CHAR_B)
-	pushl	%eax
-	/* Save drive and partition */
-	movl	%edx, %eax
-	andl	$0x0F, %eax
-	orl	$0x30, %eax
-	#movb	%al, adrive
-	.byte	0xA2
-	.word	adrive
-
-	movl	%ecx, %eax
-	decl	%eax
-	xorl	$0x03, %eax
-	andl	$0x0F, %eax
-	orl	$0x30, %eax
-	#movb	%al, aprtn
-	.byte	0xA2
-	.word	aprtn
-
-	popl	%eax
 
-	/* Load values from active partition table entry */
-	# movb	1(%si), %dh	# head
-	.byte   0x8a, 0x74, 0x01
-	# movw	2(%si), %cx	# sect, cyl
-	.byte   0x8b, 0x4c, 0x02
-	# movb	4(%si), %al	# partition type
-	.byte   0x8a, 0x44, 0x04
+	/* Store the drive number (from %dl) in decimal */
+	movb	%dl, %al
+	andb	$0x0F, %al
+	addb	$'0', %al
+	movb	%al, drive_num
 
-/*
-# BIOS call "INT 0x13 Function 0x2" to read sectors from disk into memory
-#       Call with       %ah = 0x2
-#                       %al = number of sectors
-#                       %ch = cylinder
-#                       %cl = sector
-#                       %dh = head
-#                       %dl = drive (0x80 for hard disk, 0x0 for floppy disk)
-#                       %es:%bx = segment:offset of buffer
-#       Return:
-#                       %al = 0x0 on success; err code on failure
-*/
-	data32
-	movl	$0x200 | 1, %eax	/* number of blocks */
-	xor	%bx, %bx		/* put it at %es:0 */
+	/*
+	 * Store the partition number, in decimal.
+	 *
+	 * We started with cx = 4; if found we want part '0'
+	 *                 cx = 3;                  part '1'
+	 *                 cx = 2;                  part '2'
+	 *                 cx = 1;                  part '3'
+	 *
+	 * We'll come into this with no other values for cl.
+	 */
+	movb	$'0'+4, %al
+	subb	%cl, %al
+	movb	%al, part_num
+
+	/*
+	 * Tell operator what partition we're trying to boot.
+	 *
+	 * Using drive X, partition Y
+	 * - this used to be printed out after successfully loading the
+	 *   partition boot record; we now print it out before
+	 */
+	pushw	%si
+	movw	$info, %si
+	testb	$MBR_FLAGS_FORCE_CHS, flags
+	jnz	1f
+	incw	%si
+1:
+	call	Lmessage
+	popw	%si
+
+	/*
+	 * Partition table entry format:
+	 *
+	 * 0x00	BYTE boot indicator (0x80 = active, 0x00 = inactive)
+	 * 0x01	BYTE start head
+	 * 0x02	WORD start cylinder, sector
+	 * 0x04	BYTE system type (0xA6 = OpenBSD)
+	 * 0x05 BYTE end head
+	 * 0x06	WORD end cylinder, sector
+	 * 0x08	LONG start LBA sector
+	 * 0x0C	LONG number of sectors in partition
+	 *
+	 * In the case of a partition that extends beyond the 8GB boundary,
+	 * the LBA values will be correct, the CHS values will have their
+	 * maximums (typically (C,H,S) = (1023,255,63)).
+	 *
+	 * %ds:%si points to the active partition table entry.
+	 */
+
+	/* We will load the partition boot sector (biosboot) where we
+	 * were originally loaded.  We'll check to make sure something
+	 * valid comes in.  So that we don't find ourselves, zero out
+	 * the signature at the end.
+	 */
+	movw	$0, %es:signature(,1)
+
+	/*
+	 * Have we been instructed to ignore LBA?
+	 */
+	testb	$MBR_FLAGS_FORCE_CHS, flags
+	jnz	do_chs
+
+	/*
+	 * We will use the LBA sector number if we have LBA support,
+	 * so find out.
+	 */
+
+	/*
+	 * BIOS call "INT 0x13 Extensions Installation Check"
+	 *	Call with	%ah = 0x41
+	 *			%bx = 0x55AA
+	 *			%dl = drive (0x80 for 1st hd, 0x81 for 2nd, etc)
+	 *	Return:
+	 *			carry set: failure
+	 *				%ah = error code (0x01, invalid func)
+	 *			carry clear: success
+	 *				%bx = 0xAA55 (must verify)
+	 *				%ah = major version of extensions
+	 *				%al   (internal use)
+	 *				%cx = capabilities bitmap
+	 *					0x0001 - extnd disk access funcs
+	 *					0x0002 - rem. drive ctrl funcs
+	 *					0x0004 - EDD functions with EBP
+	 *				%dx   (extension version?)
+	 */
+
+	movb	%dl, (%si)		/* Store drive here temporarily */
+					/* (This call trashes %dl) */
+					/*
+					 * XXX This is actually the correct
+					 *     place to store this.  The 0x80
+					 *     value used to indicate the
+					 *     active partition is by intention
+					 *     the same as the BIOS drive value
+					 *     for the first hard disk (0x80).
+					 *     At one point, 0x81 would go here
+					 *     for the second hard disk; the
+					 *     0x80 value is often used as a
+					 *     bit flag for testing, rather
+					 *     than an exact byte value.
+					 */
+	movw	$0x55AA, %bx
+	movb	$0x41, %ah
 	int	$0x13
-	jnc	1f
-	puts(eread)
+
+	movb	(%si), %dl		/* Get back drive number */
+
+	jc	do_chs			/* Did the command work? Jump if not */
+	cmpw	$0xAA55, %bx		/* Check that bl, bh exchanged */
+	jne	do_chs			/* If not, don't have EDD extensions */
+	testb	$0x01, %cl		/* And do we have "read" available? */
+	jz	do_chs			/* Again, use CHS if not */
+
+do_lba:
+	/*
+	 * BIOS call "INT 0x13 Extensions Extended Read"
+	 *	Call with	%ah = 0x42
+	 *			%dl = drive (0x80 for 1st hd, 0x81 for 2nd, etc)
+	 *			%ds:%si = segment:offset of command packet
+	 *	Return:
+	 *			carry set: failure
+	 *				%ah = error code (0x01, invalid func)
+	 *				command packet's sector count field set
+	 *				to the number of sectors successfully
+	 *				transferred
+	 *			carry clear: success
+	 *				%ah = 0 (success)
+	 *	Command Packet:
+	 *			0x0000	BYTE	packet size (0x10 or 0x18)
+	 *			0x0001	BYTE	reserved (should be 0)
+	 *			0x0002	WORD	sectors to transfer (max 127)
+	 *			0x0004	DWORD	seg:offset of transfer buffer
+	 *			0x0008	QWORD	starting sector number
+	 */
+	movb	$CHAR_LBA_READ, %al
+	call	Lchr
+
+	/* Load LBA sector number from active partition table entry */
+	movl	8(%si), %ecx
+	movl	%ecx, lba_sector
+
+	pushw	%si			/* We'll need %si later */
+
+	movb	$0x42, %ah
+	movw	$lba_command, %si
+	int	$0x13
+
+	popw	%si			/* (get back %si) flags unchanged */
+
+	jnc	booting_os		/* If it worked, run the pbr we got */
+
+	/*
+	 * LBA read failed, fall through to try CHS read
+	 */
+
+do_chs:
+	/*
+	 * BIOS call "INT 0x13 Function 0x2" to read sectors from disk into
+	 * memory
+	 *       Call with       %ah = 0x2
+	 *                       %al = number of sectors
+	 *                       %ch = cylinder & 0xFF
+	 *                       %cl = sector (0-63) | rest of cylinder bits
+	 *                       %dh = head
+	 *                       %dl = drive (0x80 for hard disk)
+	 *                       %es:%bx = segment:offset of buffer
+	 *       Return:
+	 *                       carry set: failure
+	 *                           %ah = err code
+	 *                           %al = number of sectors transferred
+	 *                       carry clear: success
+	 *                           %al = 0x0 OR number of sectors transferred
+	 *                                 (depends on BIOS!)
+	 *                                 (according to Ralph Brown Int List)
+	 */
+	movb	$CHAR_CHS_READ, %al
+	call	Lchr
+
+	/* Load values from active partition table entry */
+	movb	1(%si), %dh		/* head */
+	movw	2(%si), %cx		/* sect, cyl */
+	movw	$0x201, %ax		/* function and number of blocks */
+	xorw	%bx, %bx		/* put it at %es:0 */
+	int	$0x13
+	jnc	booting_os
+
+read_error:
+	movw	$eread, %si
 	jmp	err_stop
 
-1:
+booting_os:
+	puts(crlf)
 	DBGMSG(CHAR_G)
-	puts(info)
 
-	# jump to the new code (%ds:%si is at the right point)
-	data32
-	ljmp	$0, $BOOTBIOS << 4
+	/*
+	 * Make sure the pbr we loaded has a valid signature at the end.
+	 * This also ensures that something did load where we were expecting
+	 * it, as there's still a copy of our code there...
+	 */
+	cmpw	$DOSMBR_SIGNATURE, %es:signature(,1)
+	jne	missing_os
+
+	/* jump to the new code (%ds:%si is at the right point) */
+	ljmp	$0, $BOOTSEG << 4
 	/* not reached */
 
+missing_os:
+	movw	$enoos, %si
+	jmp	err_stop
+
 /*
  * Display string
  */
 Lmessage:
-	pushl	%eax
+	pushw	%ax
 	cld
 1:
-	lodsb			# load a byte into %al
+	lodsb			/* %al = *%si++ */
 	testb	%al, %al
 	jz	1f
-	/* call	Lchr */
-	.byte	0xe8
-	.word	Lchr - . - 2
+	call    Lchr
 	jmp	1b
 
-#
-#	Lchr: write the error message in %ds:%si to console
-#
+/*
+ *	Lchr: write the error message in %ds:%si to console
+ */
 Lchr:
-	pushl	%eax
+	pushw	%ax
 
-#ifndef SERIAL
-	pushl	%ebx
-	movb	$0x0e, %ah
-	xor	%bx, %bx
-	inc	%bx		/* movw $0x01, %bx */
-	int	$0x10
-	popl	%ebx
-#else
-	pushl	%edx
+#ifdef SERIAL
+	pushw	%dx
 	movb	$0x01, %ah
-	data32
-	movl	SERIAL, %dx
+	movw	SERIAL, %dx
 	int	$0x14
-	popl	%edx
+	popw	%dx
+#else
+	pushw	%bx
+	movb	$0x0e, %ah
+	movw	$1, %bx
+	int	$0x10
+	popw	%bx
 #endif
-1:	popl	%eax
+1:	popw	%ax
 	ret
 
+/* command packet for LBA read of boot sector */
+lba_command:
+	.byte	0x10			/* size of command packet */
+	.byte	0x00			/* reserved */
+	.word	0x0001			/* sectors to transfer, just 1 */
+	.word	0			/* target buffer, offset */
+	.word	BOOTSEG			/* target buffer, segment */
+lba_sector:
+	.long	0, 0			/* sector number */
+
 /* Info messages */
-info:	.ascii		"Using Drive: "
-adrive:	.byte		'X'
-	.ascii		" Partition: "
-aprtn:	.byte		'Y'
-	.asciz		"\r\n"
+info:	.ascii		"!Using drive "
+drive_num:
+	.byte		'X'
+	.ascii		", partition "
+part_num:
+	.asciz		"Y"
 
 /* Error messages */
-fdmbr:	.asciz		"MBR on floppy or old BIOS\r\n"
-eread:	.asciz		"Read error\r\n"
-noboot: .asciz		"No active partition\r\n"
-esig:	.asciz		"Invalid Signature\r\n"
+efdmbr:	.asciz		"MBR on floppy or old BIOS\r\n"
+eread:	.asciz		"\r\nRead error\r\n"
+enoos:	.asciz		"No O/S\r\n"
+enoboot: .ascii		"No active partition"	/* runs into crlf... */
+crlf:	.asciz		"\r\n"
 
 endofcode:
 	nop
 
-/* (MBR) NT registry offset */
+/* We're going to store a flags word here */
+
+	. = 0x1b4
+flags:
+	.word	0x0000
+	.ascii	"Ox"			/* Indicate that the two bytes */
+					/* before us are the flags word */
+
+/* (MBR) NT disk signature offset */
 	. = 0x1b8
 	.space  4, 0
 
 /* partition table */
 /* flag, head, sec, cyl, type, ehead, esect, ecyl, start, len */
-	. = DOSPARTOFF	# starting address of partition table
+	. = DOSPARTOFF	/* starting address of partition table */
 pt:
 	.byte	0x0,0,0,0,0,0,0,0
 	.long	0,0
@@ -355,4 +568,3 @@ pt:
 signature:
 	.short	DOSMBR_SIGNATURE
 	. = 0x200
-