altmbr: an alternative MBR which ignores the active flag

Impact: new feature

Create an alternative MBR which takes a partition number in byte 439
instead of looking at the active flag.  This is useful when
dual-booting legacy operating systems which assume that their
particular partition must be the active partition.

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

3 files changed, 326 insertions, 7 deletions

diff --git a/mbr/Makefile b/mbr/Makefile
index 0bdf7e3f..e9fc3875 100644
--- a/mbr/Makefile
+++ b/mbr/Makefile
@@ -17,7 +17,7 @@
 topdir = ..
 include $(topdir)/MCONFIG.embedded
 
-all:	mbr.bin gptmbr.bin isohdpfx.bin
+all:	mbr.bin altmbr.bin gptmbr.bin isohdpfx.bin
 
 .PRECIOUS: %.o
 %.o: %.S
@@ -31,6 +31,10 @@ mbr.bin: mbr.elf checksize.pl
 	$(OBJCOPY) -O binary $< $@
 	$(PERL) checksize.pl $@ 440
 
+altmbr.bin: altmbr.elf checksize.pl
+	$(OBJCOPY) -O binary $< $@
+	$(PERL) checksize.pl $@ 439 440
+
 isohdpfx.bin: isohdpfx.elf checksize.pl
 	$(OBJCOPY) -O binary $< $@
 	$(PERL) checksize.pl $@ 432
diff --git a/mbr/altmbr.S b/mbr/altmbr.S
new file mode 100644
index 00000000..4d9d7f3a
--- /dev/null
+++ b/mbr/altmbr.S
@@ -0,0 +1,307 @@
+/* -----------------------------------------------------------------------
+ *
+ *   Copyright 2007-2009 H. Peter Anvin - All Rights Reserved
+ *
+ *   Permission is hereby granted, free of charge, to any person
+ *   obtaining a copy of this software and associated documentation
+ *   files (the "Software"), to deal in the Software without
+ *   restriction, including without limitation the rights to use,
+ *   copy, modify, merge, publish, distribute, sublicense, and/or
+ *   sell copies of the Software, and to permit persons to whom
+ *   the Software is furnished to do so, subject to the following
+ *   conditions:
+ *
+ *   The above copyright notice and this permission notice shall
+ *   be included in all copies or substantial portions of the Software.
+ *
+ *   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ *   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+ *   OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ *   NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+ *   HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+ *   WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ *   FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+ *   OTHER DEALINGS IN THE SOFTWARE.
+ *
+ * ----------------------------------------------------------------------- */
+
+	.code16
+	.text
+
+	.globl	bootsec
+stack		= 0x7c00
+driveno		= (stack-6)
+sectors		= (stack-8)
+secpercyl	= (stack-12)
+
+BIOS_kbdflags	= 0x417
+BIOS_page	= 0x462
+
+	/* gas/ld has issues with doing this as absolute addresses... */
+	.section ".bootsec", "a", @nobits
+	.globl	bootsec
+bootsec:
+	.space	512
+
+	.text
+	.globl	_start
+_start:
+	cli
+	xorw	%ax, %ax
+	movw	%ax, %ds
+	movw	%ax, %ss
+	movw	$stack, %sp
+	movw	%sp, %si
+	pushw	%es		/* es:di -> $PnP header */
+	pushw	%di
+	movw	%ax, %es
+	sti
+	cld
+
+	/* Copy down to 0:0x600 */
+	movw	$_start, %di
+	movw	$(512/2), %cx
+	rep; movsw
+
+	ljmpw	$0, $next
+next:
+	
+	/* Escape for the user: if Ctrl is pressed, assume drive hd0 */
+	testb	$0x04, BIOS_kbdflags	/* Ctrl pressed */
+	jz	1f
+	movb	$0x80, %dl
+1:
+	pushw	%dx		/* dl -> drive number */
+
+	/* Check to see if we have EBIOS */
+	pushw	%dx		/* drive number */
+	movb	$0x41, %ah	/* %al == 0 already */
+	movw	$0x55aa, %bx
+	xorw	%cx, %cx
+	xorb	%dh, %dh
+	stc
+	int	$0x13
+	jc	1f
+	cmpw	$0xaa55, %bx
+	jne	1f
+	shrw	%cx		/* Bit 0 = fixed disk subset */
+	jnc	1f
+
+	/* We have EBIOS; patch in the following code at
+	   read_sector_cbios: movb $0x42, %ah ;  jmp read_common */
+	movl	$0xeb42b4+((read_common-read_sector_cbios-4) << 24), \
+		(read_sector_cbios)
+
+1:
+	popw	%dx
+
+	/* Get (C)HS geometry */
+	movb	$0x08, %ah
+	int	$0x13
+	andw	$0x3f, %cx	/* Sector count */
+	pushw	%cx		/* Save sectors on the stack */
+	movzbw	%dh, %ax	/* dh = max head */
+	incw	%ax		/* From 0-based max to count */
+	mulw	%cx		/* Heads*sectors -> sectors per cylinder */
+
+	/* Save sectors/cylinder on the stack */
+	pushw	%dx		/* High word */
+	pushw	%ax		/* Low word */
+
+	xorl	%eax, %eax	/* Base */
+	cdq			/* Root (%edx <- 0) */
+	call	scan_partition_table
+
+	/* If we get here, we have no OS */
+missing_os:
+	call	error
+	.ascii	"Missing operating system.\r\n"
+
+/*
+ * read_sector: read a single sector pointed to by %eax to 0x7c00.
+ * CF is set on error.  All registers saved.
+ */
+read_sector:
+	pushal
+	xorl	%edx, %edx
+	movw	$bootsec, %bx
+	pushl	%edx	/* MSW of LBA */
+	pushl	%eax	/* LSW of LBA */
+	pushw	%es	/* Buffer segment */
+	pushw	%bx	/* Buffer offset */
+	pushw	$1	/* Sector count */
+	pushw	$16	/* Size of packet */
+	movw	%sp, %si
+
+	/* This chunk is skipped if we have ebios */
+	/* Do not clobber %eax before this chunk! */
+	/* This also relies on %bx and %edx as set up above. */
+read_sector_cbios:
+	divl	(secpercyl)
+	shlb	$6, %ah
+	movb	%ah, %cl
+	movb	%al, %ch
+	xchgw	%dx, %ax
+	divb	(sectors)
+	movb	%al, %dh
+	orb	%ah, %cl
+	incw	%cx	/* Sectors are 1-based */
+	movw	$0x0201, %ax
+
+read_common:
+	movb	(driveno), %dl
+	int	$0x13
+	leaw	16(%si), %sp	/* Drop DAPA */
+	popal
+	ret
+
+/*
+ * read_partition_table:
+ *	Read a partition table (pointed to by %eax), and copy
+ *	the partition table into the ptab buffer.
+ *
+ *	Clobbers %si, %di, and %cx, other registers preserved.
+ *	%cx = 0 on exit.
+ *
+ *	On error, CF is set and ptab is overwritten with junk.
+ */
+ptab	= _start+446
+
+read_partition_table:
+	call	read_sector
+	movw	$bootsec+446, %si
+	movw	$ptab, %di
+	movw	$(16*4/2), %cx
+	rep ; movsw
+	ret
+
+/*
+ * scan_partition_table:
+ *	Scan a partition table currently loaded in the partition table
+ *	area.  Preserve all registers.
+ *
+ *      On entry:
+ *	  %eax - base (location of this partition table)
+ *	  %edx - root (offset from MBR, or 0 for MBR)
+ *
+ *      These get pushed into stack slots:
+ *        28(%bp) - %eax - base
+ *	  20(%bp) - %edx - root
+ */
+
+scan_partition_table:
+	pushal
+	movw	%sp, %bp
+
+	/* Scan the primary partition table */
+	movw	$ptab, %si
+	movw	$4, %cx
+	push	%si
+	push	%cx
+
+	/* Is it a primary partition? */
+	andl	%edx, %edx
+	jnz	6f
+
+5:
+	decb	(partition)
+	jz	boot
+	addw	$16, %bx
+	loopw	5b
+
+6:
+	/* No active partitions found, look for extended partitions */
+	popw	%cx		/* %cx <- 4    */
+	popw	%si		/* %si <- ptab */
+7:
+	movl	20(%bp), %edx		/* "Root" */
+
+	movb	4(%si), %al
+	cmpb	$0x0f, %al	/* 0x0f = Win9x extended */
+	je	8f
+	andb	$~0x80, %al	/* 0x85 = Linux extended */
+	cmpb	$0x05, %al	/* 0x05 = MS-DOS extended */
+	jne	9f
+
+	/* It is an extended partition.  Read the extended partition and
+	   try to scan it.  If the scan returns, re-load the current
+	   partition table and resume scan. */
+8:
+	movl	8(%si), %eax		/* Partition table offset */
+	addl	%edx, %eax		/* Compute location of new ptab */
+	andl	%edx, %edx		/* Is this the MBR? */
+	jnz	10f
+	movl	%eax, %edx		/* Offset -> root if this was MBR */
+10:
+	call	read_partition_table
+	jc	11f
+	call	scan_partition_table
+11:
+	/* This returned, so we need to reload the current partition table */
+	movl	28(%bp), %eax		/* "Base" */
+	call	read_partition_table
+
+	/* fall through */
+9:
+	/* Not an extended partition */
+	andb	%al, %al
+	jz	12f			/* Not a valid partition */
+	andl	%edx, %edx		/* Are we inside an extended part? */
+	/* If so, this is a logical partition */
+	decb	(partition)
+	je	boot
+12:
+	addw	$16, %si
+	loopw	7b
+
+	/* Nothing found, return */
+	popal
+	ret
+
+/*
+ * boot: invoke the actual bootstrap. (%si) points to the partition
+ *	 table entry, and 28(%bp) has the partition table base.
+ */
+boot:
+	cmpb	$0, 4(%si)
+	je	missing_os
+	movl	8(%si), %eax
+	addl	28(%bp), %eax
+	movl	%eax, 8(%si)	/* Adjust in-memory partition table entry */
+	call	read_sector
+	jc	disk_error
+	cmpw	$0xaa55, (bootsec+510)
+	jne	missing_os		/* Not a valid boot sector */
+	movw	$driveno, %sp	/* driveno == bootsec-6 */
+	popw	%dx		/* dl -> drive number */
+	popw	%di		/* es:di -> $PnP vector */
+	popw	%es
+	cli
+	jmpw	*%sp		/* %sp == bootsec */
+
+disk_error:
+	call	error
+	.ascii	"Operating system load error.\r\n"
+
+/*
+ * Print error messages.  This is invoked with "call", with the
+ * error message at the return address.
+ */
+error:
+	popw	%si
+2:
+	lodsb
+	movb	$0x0e, %ah
+	movb	(BIOS_page), %bh
+	movb	$0x07, %bl
+	int	$0x10		/* May destroy %bp */
+	cmpb	$10, %al	/* Newline? */
+	jne	2b
+
+	int	$0x18		/* Boot failure */
+die:
+	hlt
+	jmp	die
+
+/* Location of the partition configuration byte */
+partition = _start + 439
diff --git a/mbr/checksize.pl b/mbr/checksize.pl
index 68a035b7..a3643101 100755
--- a/mbr/checksize.pl
+++ b/mbr/checksize.pl
@@ -1,6 +1,6 @@
 ## -----------------------------------------------------------------------
 ##
-##   Copyright 2007-2008 H. Peter Anvin - All Rights Reserved
+##   Copyright 2007-2009 H. Peter Anvin - All Rights Reserved
 ##
 ##   This program is free software; you can redistribute it and/or modify
 ##   it under the terms of the GNU General Public License as published by
@@ -13,18 +13,26 @@
 ##
 ## checksize.pl
 ##
-## Check the size of a binary file
+## Check the size of a binary file and pad it with zeroes to that size
 ##
 
-($file, $maxsize) = @ARGV;
+use bytes;
 
-@st = stat($file);
+($file, $maxsize, $padsize) = @ARGV;
+
+$padsize = $maxsize unless(defined($padsize));
+
+open(FILE, '+<', $file) or die;
+@st = stat(FILE);
 if (!defined($size = $st[7])) {
     die "$0: $file: $!\n";
 }
 if ($size > $maxsize) {
     print STDERR "$file: too big ($size > $maxsize)\n";
     exit 1;
-} else {
-    exit 0;
+} elsif ($size < $padsize) {
+    seek(FILE, $size, 0);
+    print FILE "\0" x ($padsize-$size);
 }
+
+exit 0;