2 * Copyright (c) 1998 Robert Nordier
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
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
16 /* Memory Locations */
17 .set MEM_REL,0x700 # Relocation address
18 .set MEM_ARG,0x900 # Arguments
19 .set MEM_ORG,0x7c00 # Origin
20 .set MEM_BUF,0x8c00 # Load area
21 .set MEM_BTX,0x9000 # BTX start
22 .set MEM_JMP,0x9010 # BTX entry point
23 .set MEM_USR,0xa000 # Client start
24 .set BDA_BOOT,0x472 # Boot howto flag
26 /* Partition Constants */
27 .set PRT_OFF,0x1be # Partition offset
28 .set PRT_NUM,0x4 # Partitions
29 .set PRT_BSD,0xa5 # Partition type
32 .set FL_PACKET,0x80 # Packet mode
35 .set SIZ_PAG,0x1000 # Page size
36 .set SIZ_SEC,0x200 # Sector size
43 start: jmp main # Start recognizably
46 * This is the start of a standard BIOS Parameter Block (BPB). Most bootable
47 * FAT disks have this at the start of their MBR. While normal BIOS's will
48 * work fine without this section, IBM's El Torito emulation "fixes" up the
49 * BPB by writing into the memory copy of the MBR. Rather than have data
50 * written into our xread routine, we'll define a BPB to work around it.
51 * The data marked with (T) indicates a field required for a ThinkPad to
52 * recognize the disk and (W) indicates fields written from IBM BIOS code.
53 * The use of the BPB is based on what OpenBSD and NetBSD implemented in
54 * their boot code but the required fields were determined by trial and error.
56 * Note: If additional space is needed in boot1, one solution would be to
57 * move the "prompt" message data (below) to replace the OEM ID.
60 oemid: .space 0x08, 0x00 # OEM ID
63 bpb: .word 512 # sector size (T)
64 .byte 0 # sectors/clustor
65 .word 0 # reserved sectors
66 .byte 0 # number of FATs
67 .word 0 # root entries
68 .word 0 # small sectors
69 .byte 0 # media type (W)
71 .word 18 # sectors per track (T)
72 .word 2 # number of heads (T)
73 .long 0 # hidden sectors (W)
74 .long 0 # large sectors
77 ebpb: .byte 0 # BIOS physical drive number (W)
81 * Trampoline used by boot2 to call read to read data from the disk via
82 * the BIOS. Call with:
84 * %cx:%ax - long - LBA to read in
85 * %es:(%bx) - caddr_t - buffer to read data into
86 * %dl - byte - drive to read from
87 * %dh - byte - num sectors to read
90 xread: push %ss # Address
93 * Setup an EDD disk packet and pass it to read
100 push %bx # transfer buffer
101 xor %ax,%ax # Number of
102 movb %dh,%al # blocks to
104 push $0x10 # Size of packet
105 mov %sp,%bp # Packet pointer
106 callw read # Read from disk
107 lea 0x10(%bp),%sp # Clear stack
110 * Load the rest of boot2 and BTX up, copy the parts to the right locations,
111 * and start it all up.
115 * Setup the segment registers to flat addressing (segment 0) and setup the
116 * stack to end just below the start of our code.
118 main: cld # String ops inc
120 mov %cx,%es # Address
123 mov $start,%sp # stack
125 * Relocate ourself to MEM_REL. Since %cx == 0, the inc %ch sets
126 * %cx == 0x100. Note that boot1 does not use this relocated copy
127 * of itself while loading boot2; however, BTX reclaims the memory
128 * used by boot1 during its initialization. As a result, boot2 uses
129 * xread from the relocated copy.
132 mov $MEM_REL,%di # Destination
133 incb %ch # Word count
137 * If we are on a hard drive, then load the MBR and look for the first
138 * FreeBSD slice. We use the fake partition entry below that points to
139 * the MBR when we call nread. The first pass looks for the first active
140 * FreeBSD slice. The second pass looks for the first non-active FreeBSD
141 * slice if the first one fails.
143 mov $part4,%si # Partition
144 cmpb $0x80,%dl # Hard drive?
146 movb $0x1,%dh # Block count
147 callw nread # Read MBR
148 mov $0x1,%cx # Two passes
149 main.1: mov $MEM_BUF+PRT_OFF,%si # Partition table
150 movb $0x1,%dh # Partition
151 main.2: cmpb $PRT_BSD,0x4(%si) # Our partition type?
153 jcxz main.5 # If second pass
154 testb $0x80,(%si) # Active?
156 main.3: add $0x10,%si # Next entry
158 cmpb $0x1+PRT_NUM,%dh # In table?
163 * If we get here, we didn't find any FreeBSD slices at all, so print an
164 * error message and die.
166 mov $msg_part,%si # Message
169 * Floppies use partition 0 of drive 0.
171 main.4: xor %dx,%dx # Partition:drive
173 * Ok, we have a slice and drive in %dx now, so use that to locate and load
174 * boot2. %si references the start of the slice we are looking for, so go
175 * ahead and load up the first 16 sectors (boot1 + boot2) from that. When
176 * we read it in, we conveniently use 0x8c00 as our transfer buffer. Thus,
177 * boot1 ends up at 0x8c00, and boot2 starts at 0x8c00 + 0x200 = 0x8e00.
178 * The first part of boot2 is the disklabel, which is 0x200 bytes long.
179 * The second part is BTX, which is thus loaded into 0x9000, which is where
180 * it also runs from. The boot2.bin binary starts right after the end of
181 * BTX, so we have to figure out where the start of it is and then move the
182 * binary to 0xc000. Normally, BTX clients start at MEM_USR, or 0xa000, but
183 * when we use btxld to create boot2, we use an entry point of 0x2000. That
184 * entry point is relative to MEM_USR; thus boot2.bin starts at 0xc000.
186 main.5: mov %dx,MEM_ARG # Save args
187 movb $NSECT,%dh # Sector count
188 callw nread # Read disk
189 mov $MEM_BTX,%bx # BTX
190 mov 0xa(%bx),%si # Get BTX length and set
191 add %bx,%si # %si to start of boot2.bin
192 mov $MEM_USR+SIZ_PAG*2,%di # Client page 2
193 mov $MEM_BTX+(NSECT-1)*SIZ_SEC,%cx # Byte
199 * Enable A20 so we can access memory above 1 meg.
200 * Use the zero-valued %cx as a timeout for embedded hardware which do not
201 * have a keyboard controller.
203 seta20: cli # Disable interrupts
204 seta20.1: dec %cx # Timeout?
206 inb $0x64,%al # Get status
207 testb $0x2,%al # Busy?
209 movb $0xd1,%al # Command: Write
210 outb %al,$0x64 # output port
211 seta20.2: inb $0x64,%al # Get status
212 testb $0x2,%al # Busy?
214 movb $0xdf,%al # Enable
216 seta20.3: sti # Enable interrupts
218 jmp start+MEM_JMP-MEM_ORG # Start BTX
222 * Trampoline used to call read from within boot1.
224 nread: mov $MEM_BUF,%bx # Transfer buffer
225 mov 0x8(%si),%ax # Get
226 mov 0xa(%si),%cx # LBA
229 jnc return # If success, return
230 mov $msg_read,%si # Otherwise, set the error
231 # message and fall through to
234 * Print out the error message pointed to by %ds:(%si) followed
235 * by a prompt, wait for a keypress, and then reboot the machine.
237 error: callw putstr # Display message
238 mov $prompt,%si # Display
239 callw putstr # prompt
240 xorb %ah,%ah # BIOS: Get
242 movw $0x1234, BDA_BOOT # Do a warm boot
243 ljmp $0xf000,$0xfff0 # reboot the machine
245 * Display a null-terminated string using the BIOS output.
247 putstr.0: mov $0x7,%bx # Page:attribute
248 movb $0xe,%ah # BIOS: Display
249 int $0x10 # character
250 putstr: lodsb # Get char
251 testb %al,%al # End of string?
255 * Overused return code. ereturn is used to return an error from the
256 * read function. Since we assume putstr succeeds, we (ab)use the
257 * same code when we return from putstr.
259 ereturn: movb $0x1,%ah # Invalid
261 return: retw # To caller
263 * Reads sectors from the disk. If EDD is enabled, then check if it is
264 * installed and use it if it is. If it is not installed or not enabled, then
265 * fall back to using CHS. Since we use a LBA, if we are using CHS, we have to
266 * fetch the drive parameters from the BIOS and divide it out ourselves.
269 * %dl - byte - drive number
270 * stack - 10 bytes - EDD Packet
272 read: testb $FL_PACKET,%cs:MEM_REL+flags-start # LBA support enabled?
273 jz read.1 # No, use CHS
274 cmpb $0x80,%dl # Hard drive?
275 jb read.1 # No, use CHS
276 mov $0x55aa,%bx # Magic
278 movb $0x41,%ah # BIOS: Check
279 int $0x13 # extensions present
281 jc read.1 # If error, use CHS
282 cmp $0xaa55,%bx # Magic?
283 jne read.1 # No, so use CHS
284 testb $0x1,%cl # Packet interface?
285 jz read.1 # No, so use CHS
286 mov %bp,%si # Disk packet
287 movb $0x42,%ah # BIOS: Extended
290 read.1: push %dx # Save
291 movb $0x8,%ah # BIOS: Get drive
292 int $0x13 # parameters
293 movb %dh,%ch # Max head number
296 andb $0x3f,%cl # Sectors per track
298 cli # Disable interrupts
299 mov 0x8(%bp),%eax # Get LBA
301 movzbl %cl,%ebx # Divide by
302 xor %edx,%edx # sectors
304 movb %ch,%bl # Max head number
305 movb %dl,%ch # Sector number
307 xorb %dl,%dl # number
309 movb %dl,%bh # Head number
311 cmpl $0x3ff,%eax # Cylinder number supportable?
312 sti # Enable interrupts
313 ja ereturn # No, return an error
314 xchgb %al,%ah # Set up cylinder
315 rorb $0x2,%al # number
318 xchg %ax,%cx # number
319 movb %bh,%dh # Head number
320 subb %ah,%al # Sectors this track
321 mov 0x2(%bp),%ah # Blocks to read
322 cmpb %ah,%al # To read
324 #ifdef TRACK_AT_A_TIME
327 movb $1,%al # one sector
329 read.2: mov $0x5,%di # Try count
330 read.3: les 0x4(%bp),%bx # Transfer buffer
332 movb $0x2,%ah # BIOS: Read
333 int $0x13 # from disk
335 jnc read.4 # If success
338 xorb %ah,%ah # BIOS: Reset
339 int $0x13 # disk system
340 xchg %bx,%ax # Block count
341 jmp read.3 # Continue
342 read.4: movzbw %bl,%ax # Sectors read
343 add %ax,0x8(%bp) # Adjust
345 incw 0xa(%bp) # transfer
346 read.5: shlb %bl # buffer
347 add %bl,0x5(%bp) # pointer,
348 sub %al,0x2(%bp) # block count
349 ja read.1 # If not done
350 read.6: retw # To caller
354 msg_read: .asciz "Read"
355 msg_part: .asciz "Boot"
357 prompt: .asciz " error\r\n"
359 flags: .byte FLAGS # Flags
363 /* Partition table */
366 part4: .byte 0x80, 0x00, 0x01, 0x00
367 .byte 0xa5, 0xfe, 0xff, 0xff
368 .byte 0x00, 0x00, 0x00, 0x00
369 .byte 0x50, 0xc3, 0x00, 0x00 # 50000 sectors long, bleh
371 .word 0xaa55 # Magic number