mbr.s revision 62176 
1#
2# Copyright (c) 1999 Robert Nordier
3# All rights reserved.
4#
5# Redistribution and use in source and binary forms are freely
6# permitted provided that the above copyright notice and this
7# paragraph and the following disclaimer are duplicated in all
8# such forms.
9#
10# This software is provided "AS IS" and without any express or
11# implied warranties, including, without limitation, the implied
12# warranties of merchantability and fitness for a particular
13# purpose.
14#
15
16# $FreeBSD: head/sys/boot/i386/mbr/mbr.s 62176 2000-06-27 20:04:10Z jhb $
17
18# A 512 byte MBR boot manager that simply boots the active partition.
19
20		.set LOAD,0x7c00		# Load address
21		.set EXEC,0x600 		# Execution address
22		.set PT_OFF,0x1be		# Partition table
23		.set MAGIC,0xaa55		# Magic: bootable
24
25		.set NHRDRV,0x475		# Number of hard drives
26
27		.globl start			# Entry point
28		.code16
29
30#
31# Setup the segment registers for flat addressing and setup the stack.
32#
33start:		cld				# String ops inc
34		xorw %ax,%ax			# Zero
35		movw %ax,%es			# Address
36		movw %ax,%ds			#  data
37		movw %ax,%ss			# Set up
38		movw $LOAD,%sp			#  stack
39#
40# Relocate ourself to a lower address so that we are out of the way when
41# we load in the bootstrap from the partition to boot.
42#
43		movw $main-EXEC+LOAD,%si	# Source
44		movw $main,%di			# Destination
45		movw $0x200-(main-start),%cx	# Byte count
46		rep				# Relocate
47		movsb				#  code
48#
49# Jump to the relocated code.
50#
51		jmp main-LOAD+EXEC		# To relocated code
52#
53# Scan the partition table looking for an active entry.  Note that %ch is
54# zero from the repeated string instruction above.  We save the offset of
55# the active partition in %si and scan the entire table to ensure that only
56# one partition is marked active.
57#
58main:		xorw %si,%si			# No active partition
59		movw $partbl,%bx		# Partition table
60		movb $0x4,%cl			# Number of entries
61main.1: 	cmpb %ch,(%bx)			# Null entry?
62		je main.2			# Yes
63		jg err_pt			# If 0x1..0x7f
64		testw %si,%si	 		# Active already found?
65		jnz err_pt			# Yes
66		movw %bx,%si			# Point to active
67main.2: 	addb $0x10,%bl			# Till
68		loop main.1			#  done
69		testw %si,%si	 		# Active found?
70		jnz main.3			# Yes
71		int $0x18			# BIOS: Diskless boot
72#
73# Ok, we've found a possible active partition.  Check to see that the drive
74# is a valid hard drive number.
75#
76main.3: 	cmpb $0x80,%dl			# Drive valid?
77		jb main.4			# No
78		movb NHRDRV,%dh			# Calculate the highest
79		addb $0x80,%dh			#  drive number available
80		cmpb %dh,%dl			# Within range?
81		jb main.5			# Yes
82main.4: 	movb (%si),%dl			# Load drive
83#
84# Ok, now that we have a valid drive and partition entry, load the CHS from
85# the partition entry and read the sector from the disk.
86#
87main.5:		movw %sp,%di			# Save stack pointer
88		movb 0x1(%si),%dh		# Load head
89		movw 0x2(%si),%cx		# Load cylinder:sector
90		movw $LOAD,%bx			# Transfer buffer
91		cmpb $0xff,%dh			# Might we need to use LBA?
92		jnz main.7			# No.
93		cmpw $0xffff,%cx		# Do we need to use LBA?
94		jnz main.7			# No.
95		pushw %cx			# Save %cx
96		pushw %bx			# Save %bx
97		movw $0x55aa,%bx		# Magic
98		movb $0x41,%ah			# BIOS:	EDD extensions
99		int $0x13			#  present?
100		jc main.6			# No.
101		cmpw $0xaa55,%bx		# Magic ok?
102		jne main.6			# No.
103		testb $0x1,%cl			# Packet mode present?
104		jz main.6			# No.
105		popw %bx			# Restore %bx
106		pushl $0x0			# Set the LBA
107		pushl 0x8(%si)			#  address
108		pushw %es			# Set the address of
109		pushw %bx			#  the transfer buffer
110		pushw $0x1			# Read 1 sector
111		pushw $0x10			# Packet length
112		movw %sp,%si			# Packer pointer
113		movw $0x4200,%ax		# BIOS:	LBA Read from disk
114		jmp main.8			# Skip the CHS setup
115main.6:		popw %bx			# Restore %bx
116		popw %cx			# Restore %cx
117main.7:		movw $0x201,%ax			# BIOS: Read from disk
118main.8:		int $0x13			# Call the BIOS
119		movw %di,%sp			# Restore stack
120		jc err_rd			# If error
121#
122# Now that we've loaded the bootstrap, check for the 0xaa55 signature.  If it
123# is present, execute the bootstrap we just loaded.
124#
125		cmpw $MAGIC,0x1fe(%bx)		# Bootable?
126		jne err_os			# No
127		jmp *%bx			# Invoke bootstrap
128#
129# Various error message entry points.
130#
131err_pt: 	movw $msg_pt,%si		# "Invalid partition
132		jmp putstr			#  table"
133
134err_rd: 	movw $msg_rd,%si		# "Error loading
135		jmp putstr			#  operating system"
136
137err_os: 	movw $msg_os,%si		# "Missing operating
138		jmp putstr			#  system"
139#
140# Output an ASCIZ string to the console via the BIOS.
141#
142putstr.0:	movw $0x7,%bx	 		# Page:attribute
143		movb $0xe,%ah			# BIOS: Display
144		int $0x10			#  character
145putstr: 	lodsb				# Get character
146		testb %al,%al			# End of string?
147		jnz putstr.0			# No
148putstr.1:	jmp putstr.1			# Await reset
149
150msg_pt: 	.asciz "Invalid partition table"
151msg_rd: 	.asciz "Error loading operating system"
152msg_os: 	.asciz "Missing operating system"
153
154		.org PT_OFF
155
156partbl: 	.fill 0x10,0x4,0x0		# Partition table
157		.word MAGIC			# Magic number
158