boot1.S revision 215285 
1/*
2 * Copyright (c) 1998 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 * $FreeBSD: head/sys/boot/i386/boot2/boot1.S 215285 2010-11-14 08:31:02Z brucec $
16 */
17
18/* Memory Locations */
19		.set MEM_REL,0x700		# Relocation address
20		.set MEM_ARG,0x900		# Arguments
21		.set MEM_ORG,0x7c00		# Origin
22		.set MEM_BUF,0x8c00		# Load area
23		.set MEM_BTX,0x9000		# BTX start
24		.set MEM_JMP,0x9010		# BTX entry point
25		.set MEM_USR,0xa000		# Client start
26		.set BDA_BOOT,0x472		# Boot howto flag
27
28/* Partition Constants */
29		.set PRT_OFF,0x1be		# Partition offset
30		.set PRT_NUM,0x4		# Partitions
31		.set PRT_BSD,0xa5		# Partition type
32
33/* Flag Bits */
34		.set FL_PACKET,0x80		# Packet mode
35
36/* Misc. Constants */
37		.set SIZ_PAG,0x1000		# Page size
38		.set SIZ_SEC,0x200		# Sector size
39
40		.set NSECT,0x10
41		.globl start
42		.globl xread
43		.code16
44
45start:		jmp main			# Start recognizably
46
47/*
48 * This is the start of a standard BIOS Parameter Block (BPB). Most bootable
49 * FAT disks have this at the start of their MBR. While normal BIOS's will
50 * work fine without this section, IBM's El Torito emulation "fixes" up the
51 * BPB by writing into the memory copy of the MBR. Rather than have data
52 * written into our xread routine, we'll define a BPB to work around it.
53 * The data marked with (T) indicates a field required for a ThinkPad to
54 * recognize the disk and (W) indicates fields written from IBM BIOS code.
55 * The use of the BPB is based on what OpenBSD and NetBSD implemented in
56 * their boot code but the required fields were determined by trial and error.
57 *
58 * Note: If additional space is needed in boot1, one solution would be to
59 * move the "prompt" message data (below) to replace the OEM ID.
60 */
61		.org 0x03, 0x00
62oemid:		.space 0x08, 0x00	# OEM ID
63
64		.org 0x0b, 0x00
65bpb:		.word   512		# sector size (T)
66		.byte	0		# sectors/clustor
67		.word	0		# reserved sectors
68		.byte	0		# number of FATs
69		.word	0		# root entries
70		.word	0		# small sectors
71		.byte	0		# media type (W)
72		.word	0		# sectors/fat
73		.word	18		# sectors per track (T)
74		.word	2		# number of heads (T)
75		.long	0		# hidden sectors (W)
76		.long	0		# large sectors
77
78		.org 0x24, 0x00
79ebpb:		.byte	0		# BIOS physical drive number (W)
80
81		.org 0x25,0x90
82/*
83 * Trampoline used by boot2 to call read to read data from the disk via
84 * the BIOS.  Call with:
85 *
86 * %cx:%ax	- long    - LBA to read in
87 * %es:(%bx)	- caddr_t - buffer to read data into
88 * %dl		- byte    - drive to read from
89 * %dh		- byte    - num sectors to read
90 */
91
92xread:		push %ss			# Address
93		pop %ds				#  data
94/*
95 * Setup an EDD disk packet and pass it to read
96 */
97xread.1:					# Starting
98		pushl $0x0			#  absolute
99		push %cx			#  block
100		push %ax			#  number
101		push %es			# Address of
102		push %bx			#  transfer buffer
103		xor %ax,%ax			# Number of
104		movb %dh,%al			#  blocks to
105		push %ax			#  transfer
106		push $0x10			# Size of packet
107		mov %sp,%bp			# Packet pointer
108		callw read			# Read from disk
109		lea 0x10(%bp),%sp		# Clear stack
110		lret				# To far caller
111/*
112 * Load the rest of boot2 and BTX up, copy the parts to the right locations,
113 * and start it all up.
114 */
115
116/*
117 * Setup the segment registers to flat addressing (segment 0) and setup the
118 * stack to end just below the start of our code.
119 */
120main:		cld				# String ops inc
121		xor %cx,%cx			# Zero
122		mov %cx,%es			# Address
123		mov %cx,%ds			#  data
124		mov %cx,%ss			# Set up
125		mov $start,%sp			#  stack
126/*
127 * Relocate ourself to MEM_REL.  Since %cx == 0, the inc %ch sets
128 * %cx == 0x100.
129 */
130		mov %sp,%si			# Source
131		mov $MEM_REL,%di		# Destination
132		incb %ch			# Word count
133		rep				# Copy
134		movsw				#  code
135/*
136 * If we are on a hard drive, then load the MBR and look for the first
137 * FreeBSD slice.  We use the fake partition entry below that points to
138 * the MBR when we call nread.  The first pass looks for the first active
139 * FreeBSD slice.  The second pass looks for the first non-active FreeBSD
140 * slice if the first one fails.
141 */
142		mov $part4,%si			# Partition
143		cmpb $0x80,%dl			# Hard drive?
144		jb main.4			# No
145		movb $0x1,%dh			# Block count
146		callw nread			# Read MBR
147		mov $0x1,%cx	 		# Two passes
148main.1: 	mov $MEM_BUF+PRT_OFF,%si	# Partition table
149		movb $0x1,%dh			# Partition
150main.2: 	cmpb $PRT_BSD,0x4(%si)		# Our partition type?
151		jne main.3			# No
152		jcxz main.5			# If second pass
153		testb $0x80,(%si)		# Active?
154		jnz main.5			# Yes
155main.3: 	add $0x10,%si	 		# Next entry
156		incb %dh			# Partition
157		cmpb $0x1+PRT_NUM,%dh		# In table?
158		jb main.2			# Yes
159		dec %cx				# Do two
160		jcxz main.1			#  passes
161/*
162 * If we get here, we didn't find any FreeBSD slices at all, so print an
163 * error message and die.
164 */
165		mov $msg_part,%si		# Message
166		jmp error			# Error
167/*
168 * Floppies use partition 0 of drive 0.
169 */
170main.4: 	xor %dx,%dx			# Partition:drive
171/*
172 * Ok, we have a slice and drive in %dx now, so use that to locate and load
173 * boot2.  %si references the start of the slice we are looking for, so go
174 * ahead and load up the first 16 sectors (boot1 + boot2) from that.  When
175 * we read it in, we conveniently use 0x8c00 as our transfer buffer.  Thus,
176 * boot1 ends up at 0x8c00, and boot2 starts at 0x8c00 + 0x200 = 0x8e00.
177 * The first part of boot2 is the disklabel, which is 0x200 bytes long.
178 * The second part is BTX, which is thus loaded into 0x9000, which is where
179 * it also runs from.  The boot2.bin binary starts right after the end of
180 * BTX, so we have to figure out where the start of it is and then move the
181 * binary to 0xc000.  Normally, BTX clients start at MEM_USR, or 0xa000, but
182 * when we use btxld to create boot2, we use an entry point of 0x2000.  That
183 * entry point is relative to MEM_USR; thus boot2.bin starts at 0xc000.
184 */
185main.5: 	mov %dx,MEM_ARG			# Save args
186		movb $NSECT,%dh			# Sector count
187		callw nread			# Read disk
188		mov $MEM_BTX,%bx		# BTX
189		mov 0xa(%bx),%si		# Get BTX length and set
190		add %bx,%si			#  %si to start of boot2.bin
191		mov $MEM_USR+SIZ_PAG*2,%di	# Client page 2
192		mov $MEM_BTX+(NSECT-1)*SIZ_SEC,%cx # Byte
193		sub %si,%cx			#  count
194		rep				# Relocate
195		movsb				#  client
196
197/*
198 * Enable A20 so we can access memory above 1 meg.
199 * Use the zero-valued %cx as a timeout for embedded hardware which do not
200 * have a keyboard controller.
201 */
202seta20: 	cli				# Disable interrupts
203seta20.1:	dec %cx				# Timeout?
204		jz seta20.3			# Yes
205		inb $0x64,%al			# Get status
206		testb $0x2,%al			# Busy?
207		jnz seta20.1			# Yes
208		movb $0xd1,%al			# Command: Write
209		outb %al,$0x64			#  output port
210seta20.2:	inb $0x64,%al			# Get status
211		testb $0x2,%al			# Busy?
212		jnz seta20.2			# Yes
213		movb $0xdf,%al			# Enable
214		outb %al,$0x60			#  A20
215seta20.3:	sti				# Enable interrupts
216
217		jmp start+MEM_JMP-MEM_ORG	# Start BTX
218
219
220/*
221 * Trampoline used to call read from within boot1.
222 */
223nread:		mov $MEM_BUF,%bx		# Transfer buffer
224		mov 0x8(%si),%ax		# Get
225		mov 0xa(%si),%cx		#  LBA
226		push %cs			# Read from
227		callw xread.1	 		#  disk
228		jnc return			# If success, return
229		mov $msg_read,%si		# Otherwise, set the error
230						#  message and fall through to
231						#  the error routine
232/*
233 * Print out the error message pointed to by %ds:(%si) followed
234 * by a prompt, wait for a keypress, and then reboot the machine.
235 */
236error:		callw putstr			# Display message
237		mov $prompt,%si			# Display
238		callw putstr			#  prompt
239		xorb %ah,%ah			# BIOS: Get
240		int $0x16			#  keypress
241		movw $0x1234, BDA_BOOT		# Do a warm boot
242		ljmp $0xf000,$0xfff0		# reboot the machine
243/*
244 * Display a null-terminated string using the BIOS output.
245 */
246putstr.0:	mov $0x7,%bx	 		# Page:attribute
247		movb $0xe,%ah			# BIOS: Display
248		int $0x10			#  character
249putstr: 	lodsb				# Get char
250		testb %al,%al			# End of string?
251		jne putstr.0			# No
252
253/*
254 * Overused return code.  ereturn is used to return an error from the
255 * read function.  Since we assume putstr succeeds, we (ab)use the
256 * same code when we return from putstr.
257 */
258ereturn:	movb $0x1,%ah			# Invalid
259		stc				#  argument
260return: 	retw				# To caller
261/*
262 * Reads sectors from the disk.  If EDD is enabled, then check if it is
263 * installed and use it if it is.  If it is not installed or not enabled, then
264 * fall back to using CHS.  Since we use a LBA, if we are using CHS, we have to
265 * fetch the drive parameters from the BIOS and divide it out ourselves.
266 * Call with:
267 *
268 * %dl	- byte     - drive number
269 * stack - 10 bytes - EDD Packet
270 */
271read:		testb $FL_PACKET,%cs:MEM_REL+flags-start # LBA support enabled?
272		jz read.1			# No, use CHS
273		cmpb $0x80,%dl			# Hard drive?
274		jb read.1			# No, use CHS
275		mov $0x55aa,%bx			# Magic
276		push %dx			# Save
277		movb $0x41,%ah			# BIOS: Check
278		int $0x13			#  extensions present
279		pop %dx				# Restore
280		jc read.1			# If error, use CHS
281		cmp $0xaa55,%bx			# Magic?
282		jne read.1			# No, so use CHS
283		testb $0x1,%cl			# Packet interface?
284		jz read.1			# No, so use CHS
285		mov %bp,%si			# Disk packet
286		movb $0x42,%ah			# BIOS: Extended
287		int $0x13			#  read
288		retw				# To caller
289read.1:	 	push %dx			# Save
290		movb $0x8,%ah			# BIOS: Get drive
291		int $0x13			#  parameters
292		movb %dh,%ch			# Max head number
293		pop %dx				# Restore
294		jc return			# If error
295		andb $0x3f,%cl			# Sectors per track
296		jz ereturn			# If zero
297		cli				# Disable interrupts
298		mov 0x8(%bp),%eax		# Get LBA
299		push %dx			# Save
300		movzbl %cl,%ebx			# Divide by
301		xor %edx,%edx			#  sectors
302		div %ebx			#  per track
303		movb %ch,%bl			# Max head number
304		movb %dl,%ch			# Sector number
305		inc %bx				# Divide by
306		xorb %dl,%dl			#  number
307		div %ebx			#  of heads
308		movb %dl,%bh			# Head number
309		pop %dx				# Restore
310		cmpl $0x3ff,%eax		# Cylinder number supportable?
311		sti				# Enable interrupts
312		ja ereturn			# No, return an error
313		xchgb %al,%ah			# Set up cylinder
314		rorb $0x2,%al			#  number
315		orb %ch,%al			# Merge
316		inc %ax				#  sector
317		xchg %ax,%cx	 		#  number
318		movb %bh,%dh			# Head number
319		subb %ah,%al			# Sectors this track
320		mov 0x2(%bp),%ah		# Blocks to read
321		cmpb %ah,%al			# To read
322		jb read.2			#  this
323#ifdef	TRACK_AT_A_TIME
324		movb %ah,%al			#  track
325#else
326		movb $1,%al			#  one sector
327#endif
328read.2: 	mov $0x5,%di	 		# Try count
329read.3: 	les 0x4(%bp),%bx		# Transfer buffer
330		push %ax			# Save
331		movb $0x2,%ah			# BIOS: Read
332		int $0x13			#  from disk
333		pop %bx				# Restore
334		jnc read.4			# If success
335		dec %di				# Retry?
336		jz read.6			# No
337		xorb %ah,%ah			# BIOS: Reset
338		int $0x13			#  disk system
339		xchg %bx,%ax	 		# Block count
340		jmp read.3			# Continue
341read.4: 	movzbw %bl,%ax	 		# Sectors read
342		add %ax,0x8(%bp)		# Adjust
343		jnc read.5			#  LBA,
344		incw 0xa(%bp)	 		#  transfer
345read.5: 	shlb %bl			#  buffer
346		add %bl,0x5(%bp)		#  pointer,
347		sub %al,0x2(%bp)		#  block count
348		ja read.1			# If not done
349read.6: 	retw				# To caller
350
351/* Messages */
352
353msg_read:	.asciz "Read"
354msg_part:	.asciz "Boot"
355
356prompt: 	.asciz " error\r\n"
357
358flags:		.byte FLAGS			# Flags
359
360		.org PRT_OFF,0x90
361
362/* Partition table */
363
364		.fill 0x30,0x1,0x0
365part4:		.byte 0x80, 0x00, 0x01, 0x00
366		.byte 0xa5, 0xfe, 0xff, 0xff
367		.byte 0x00, 0x00, 0x00, 0x00
368		.byte 0x50, 0xc3, 0x00, 0x00	# 50000 sectors long, bleh
369
370		.word 0xaa55			# Magic number
371