1#-
2# Copyright (c) 2007 Yahoo!, Inc.
3# All rights reserved.
4# Written by: John Baldwin <jhb@FreeBSD.org>
5#
6# Redistribution and use in source and binary forms, with or without
7# modification, are permitted provided that the following conditions
8# are met:
9# 1. Redistributions of source code must retain the above copyright
10#    notice, this list of conditions and the following disclaimer.
11# 2. Redistributions in binary form must reproduce the above copyright
12#    notice, this list of conditions and the following disclaimer in the
13#    documentation and/or other materials provided with the distribution.
14# 3. Neither the name of the author nor the names of any co-contributors
15#    may be used to endorse or promote products derived from this software
16#    without specific prior written permission.
17#
18# THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
19# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
20# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
21# ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
22# FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
23# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
24# OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
25# HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
26# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
27# OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
28# SUCH DAMAGE.
29#
30# $FreeBSD$
31#
32# Partly from: src/sys/boot/i386/mbr/mbr.s 1.7
33
34# A 512 byte PMBR boot manager to read a boot program and run it.
35# The embedded MBR is set up for PMBR and default bootblock sector
36# is hardcoded to 256 and size 1. The actual values are supposed to be
37# updated by installboot.
38
39		.set LOAD,0x7c00		# Load address
40		.set EXEC,0x600 		# Execution address
41		.set MAGIC,0xaa55		# Magic: bootable
42		.set SECSIZE,0x200		# Size of a single disk sector
43		.set DISKSIG,440		# Disk signature offset
44		.set STACK,EXEC+SECSIZE*4	# Stack address
45		.set DPBUF,STACK
46
47		.set NHRDRV,0x475		# Number of hard drives
48
49		.globl start			# Entry point
50		.code16
51		.text
52
53start:		jmp real_code
54		.fill 0x3c,0x1,0x90		# fill with nop to ease disasm
55#
56# BIOS Parameter Block. Reserved space from 0xb to 0x3e, the FAT32 BPB
57# is 60 (3Ch) bytes.
58#
59		. = start + 0x3e
60
61#
62# Setup the segment registers for flat addressing and setup the stack.
63#
64real_code:	cld				# String ops inc
65		xorw %ax,%ax			# Zero
66		movw %ax,%es			# Address
67		movw %ax,%ds			#  data
68		movw %ax,%ss			# Set up
69		movw $STACK,%sp			#  stack
70#
71# Relocate ourself to a lower address so that we have more room to load
72# other sectors.
73#
74		movw $main-EXEC+LOAD,%si	# Source
75		movw $main,%di			# Destination
76		movw $SECSIZE-(main-start),%cx	# Byte count
77		rep				# Relocate
78		movsb				#  code
79#
80# Jump to the relocated code.
81#
82		jmp main-LOAD+EXEC		# To relocated code
83#
84# Validate drive number in %dl.
85#
86main:	 	cmpb $0x80,%dl			# Drive valid?
87		jb main.1			# No
88		movb NHRDRV,%dh			# Calculate the highest
89		addb $0x80,%dh			#  drive number available
90		cmpb %dh,%dl			# Within range?
91		jb main.2			# Yes
92main.1: 	movb $0x80,%dl			# Assume drive 0x80
93#
94# Load stage2 and start it. location and size is written by installboot
95# and if size is 0, we can not do anything...
96#
97main.2:		movw stage2_size, %ax
98		cmpw $0, %ax
99		je err_noboot			# the stage2 size is not set
100		pushw %dx			# save drive
101		movb $0x41, %ah			# check extensions
102		movw $0x55aa, %bx
103		int $0x13
104		popw %dx			# restore drive
105		jc err_rd			# need lba mode for now
106		cmpw $0xaa55, %bx		# chs support is not
107		jne err_rd			# implemented.
108		movw $stage2_sector, %si	# pointer to lba
109		movw $LOAD/16,%bx		# set buffer segment
110		movw %bx,%es
111		xorw %bx,%bx			# and offset
112load_boot:	push %si			# Save %si
113		call read
114		pop %si				# Restore
115		decw stage2_size		# stage2_size--
116		jnz next_boot
117boot:		mov %bx,%es			# Reset %es to zero
118		jmp LOAD			# Jump to boot code
119next_boot:	incl (%si)			# Next LBA
120		adcl $0,4(%si)
121		mov %es,%ax			# Adjust segment for next
122		addw $SECSIZE/16,%ax		#  sector
123		mov %ax,%es			#
124		jmp load_boot
125#
126# Load a sector (64-bit LBA at %si) from disk %dl into %es:%bx by creating
127# a EDD packet on the stack and passing it to the BIOS.  Trashes %ax and %si.
128#
129read:		pushl 0x4(%si)			# Set the LBA
130		pushl 0x0(%si)			#  address
131		pushw %es			# Set the address of
132		pushw %bx			#  the transfer buffer
133		pushw $0x1			# Read 1 sector
134		pushw $0x10			# Packet length
135		movw %sp,%si			# Packer pointer
136		movw $0x4200,%ax		# BIOS:	LBA Read from disk
137		int $0x13			# Call the BIOS
138		add $0x10,%sp			# Restore stack
139		jc err_rd			# If error
140		ret
141#
142# Various error message entry points.
143#
144err_rd: 	movw $msg_rd,%si		# "I/O error loading
145		jmp putstr			#  boot loader"
146
147err_noboot: 	movw $msg_noboot,%si		# "Missing boot
148		jmp putstr			#  loader"
149#
150# Output an ASCIZ string to the console via the BIOS.
151#
152putstr.0:	movw $0x7,%bx	 		# Page:attribute
153		movb $0xe,%ah			# BIOS: Display
154		int $0x10			#  character
155putstr: 	lodsb				# Get character
156		testb %al,%al			# End of string?
157		jnz putstr.0			# No
158putstr.1:	jmp putstr.1			# Await reset
159
160msg_rd: 	.asciz "I/O error"
161msg_noboot: 	.asciz "No boot loader"
162
163		nop
164mbr_version:	.byte 1, 1			# 1.1
165		.align 4
166stage2_size:	.word 1				# bootblock size in sectors
167stage2_sector:	.quad 256			# lba of bootblock
168disk_uuid:	.quad 0				# uuid
169		.quad 0
170
171# this is the end of the code block we can use, next is space for
172# signature, partition table 4 entries and signature.
173		.org DISKSIG,0x1b8		#
174sig:		.long 0				# OS Disk Signature
175		.word 0				# "Unknown" in PMBR
176
177partbl:		.byte 0x00			# non-bootable
178		.byte 0x00			# head 0
179		.byte 0x02			# sector
180		.byte 0x00			# cylinder
181		.byte 0xEE			# ID
182		.byte 0xFF			# ending head
183		.byte 0xFF			# ending sector
184		.byte 0xFF			# ending cylinder
185		.long 0x00000001		# starting LBA
186		.long 0xFFFFFFFF		# size
187		.fill 0x10,0x3,0x0		# other 3 entries
188		.word MAGIC			# Magic number
189