cdbr.S revision 1.1
1/* $OpenBSD: cdbr.S,v 1.1 2004/08/21 18:48:37 tom Exp $ */ 2 3/* 4 * Copyright (c) 2004 Tom Cosgrove <tom.cosgrove@arches-consulting.com> 5 * Copyright (c) 2001 John Baldwin <jhb@FreeBSD.org> 6 * All rights reserved. 7 * 8 * Redistribution and use in source and binary forms, with or without 9 * modification, are permitted provided that the following conditions 10 * are met: 11 * 1. Redistributions of source code must retain the above copyright 12 * notice, this list of conditions and the following disclaimer. 13 * 2. Redistributions in binary form must reproduce the above copyright 14 * notice, this list of conditions and the following disclaimer in the 15 * documentation and/or other materials provided with the distribution. 16 * 3. Neither the name of the author nor the names of any co-contributors 17 * may be used to endorse or promote products derived from this software 18 * without specific prior written permission. 19 * 20 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 21 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 24 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30 * SUCH DAMAGE. 31 */ 32 33 .file "cdbr.S" 34 35/* #include <machine/asm.h> */ 36/* #include <assym.h> */ 37 38/* 39 * This program is a CD boot sector, similar to the partition boot record 40 * (pbr, also called biosboot) used by hard disks. It is implemented as a 41 * "no-emulation" boot sector, as described in the "El Torito" Bootable 42 * CD-ROM Format Specification. 43 * 44 * The function of this boot sector is to load and start the next-stage 45 * cdboot program, which will load the kernel. 46 * 47 * The El Torito standard allows us to specify where we want to be loaded, 48 * but for maximum compatibility we choose the default load address of 49 * 0x07C00. 50 * 51 * Memory layout: 52 * 53 * 0x00000 -> 0x003FF real mode interrupt vector table 54 * 0x00400 -> 0x00500 BIOS data segment 55 * 56 * 0x00000 -> 0x073FF our stack (grows down) (from 29k) 57 * 0x07400 -> 0x07BFF we relocate to here (at 29k) 58 * 0x07C00 -> 0x08400 BIOS loads us here (at 31k, for 2k) 59 * 0x07C00 -> ... /cdboot 60 * 61 * The BIOS loads us at physical address 0x07C00. We then relocate to 62 * 0x07400, seg:offset 0740:0000. We then load /cdboot at seg:offset 63 * 07C0:0000. 64 */ 65#define BOOTSEG 0x7c0 /* segment we're loaded to */ 66#define BOOTSECTSIZE 0x800 /* our size in bytes */ 67#define BOOTRELOCSEG 0x740 /* segment we relocate to */ 68#define BOOTSTACKOFF ((BOOTRELOCSEG << 4) - 4) /* starts here, grows down */ 69 70/* Constants for reading from the CD */ 71#define ERROR_TIMEOUT 0x80 /* BIOS timeout on read */ 72#define NUM_RETRIES 3 /* Num times to retry */ 73#define SECTOR_SIZE 0x800 /* size of a sector */ 74#define SECTOR_SHIFT 11 /* number of place to shift */ 75#define BUFFER_LEN 0x100 /* number of sectors in buffr */ 76#define MAX_READ 0x10000 /* max we can read at a time */ 77#define MAX_READ_PARAS MAX_READ >> 4 78#define MAX_READ_SEC MAX_READ >> SECTOR_SHIFT 79#define MEM_READ_BUFFER 0x9000 /* buffer to read from CD */ 80#define MEM_VOLDESC MEM_READ_BUFFER /* volume descriptor */ 81#define MEM_DIR MEM_VOLDESC+SECTOR_SIZE /* Lookup buffer */ 82#define VOLDESC_LBA 0x10 /* LBA of vol descriptor */ 83#define VD_PRIMARY 1 /* Primary VD */ 84#define VD_END 255 /* VD Terminator */ 85#define VD_ROOTDIR 156 /* Offset of Root Dir Record */ 86#define DIR_LEN 0 /* Offset of Dir Rec length */ 87#define DIR_EA_LEN 1 /* Offset of EA length */ 88#define DIR_EXTENT 2 /* Offset of 64-bit LBA */ 89#define DIR_SIZE 10 /* Offset of 64-bit length */ 90#define DIR_NAMELEN 32 /* Offset of 8-bit name len */ 91#define DIR_NAME 33 /* Offset of dir name */ 92 93 .text 94 .code16 95 96 .globl start 97start: 98 /* Set up stack */ 99 xorw %ax, %ax 100 movw %ax, %ss 101 movw $BOOTSTACKOFF, %sp 102 103 /* Relocate so we can load cdboot where we were */ 104 movw $BOOTSEG, %ax 105 movw %ax, %ds 106 movw $BOOTRELOCSEG, %ax 107 movw %ax, %es 108 xorw %si, %si 109 xorw %di, %di 110 movw $BOOTSECTSIZE, %cx /* Bytes in cdbr, relocate it all */ 111 cld 112 rep 113 movsb 114 115 /* Jump to relocated self */ 116 ljmp $BOOTRELOCSEG, $reloc 117reloc: 118 119 /* 120 * Set up %ds and %es: %ds is our data segment (= %cs), %es is 121 * used to specify the segment address of the destination buffer 122 * for cd reads. We initially have %es = %ds. 123 */ 124 movw %cs, %ax 125 movw %ax, %ds 126 movw %ax, %es 127 128 movb %dl, drive /* Store the boot drive number */ 129 130 movw $signon, %si /* Say "hi", and give boot drive */ 131 call display_string 132 movb drive, %al 133 call hex_byte 134 movw $crlf, %si 135 call display_string 136 137/* 138 * Load Volume Descriptor 139 */ 140 movl $VOLDESC_LBA, %eax /* Get the sector # for vol desc */ 141load_vd: 142 pushl %eax 143 movb $1, %dh /* One sector */ 144 movw $MEM_VOLDESC, %bx /* Destination */ 145 call read /* Read it in */ 146 popl %eax 147 cmpb $VD_PRIMARY, (%bx) /* Primary vol descriptor? */ 148 je have_vd /* Yes */ 149 inc %eax /* Try the next one */ 150 cmpb $VD_END, (%bx) /* Is it the last one? */ 151 jne load_vd /* No, so go try the next one */ 152 movw $msg_novd, %si /* No pri vol descriptor */ 153 jmp err_stop /* Panic */ 154have_vd: /* Have Primary VD */ 155 156/* 157 * Look for the next-stage loader binary at pre-defined paths (loader_paths) 158 */ 159 movw $loader_paths, %si /* Point to start of array */ 160lookup_path: 161 movw %si, loader /* remember the one we're looking for */ 162 pushw %si /* Save file name pointer */ 163 call lookup /* Try to find file */ 164 popw %di /* Restore file name pointer */ 165 jnc lookup_found /* Found this file */ 166 xorb %al, %al /* Look for next */ 167 movw $0xffff, %cx /* path name by */ 168 repnz /* scanning for */ 169 scasb /* nul char */ 170 movw %di, %si /* Point %si at next path */ 171 movb (%si), %al /* Get first char of next path */ 172 orb %al, %al /* Is it double nul? */ 173 jnz lookup_path /* No, try it */ 174 movw $msg_failed, %si /* Failed message */ 175 jmp err_stop /* Print it and halt */ 176 177lookup_found: /* Found a loader file */ 178 179/* 180 * Load the binary into the buffer. Due to real mode addressing limitations 181 * we have to read it in in 64k chunks. 182 */ 183 movl DIR_SIZE(%bx), %eax /* Read file length */ 184 add $SECTOR_SIZE-1, %eax /* Convert length to sectors */ 185 shr $SECTOR_SHIFT, %eax 186 cmp $BUFFER_LEN, %eax 187 jbe load_sizeok 188 movw $msg_load2big, %si /* Error message */ 189 jmp err_stop 190load_sizeok: 191 movzbw %al, %cx /* Num sectors to read */ 192 movl DIR_EXTENT(%bx), %eax /* Load extent */ 193 xorl %edx, %edx 194 movb DIR_EA_LEN(%bx), %dl 195 addl %edx, %eax /* Skip extended */ 196 197 /* Use %bx to hold the segment (para) number */ 198 movw $BOOTSEG, %bx /* We put cdboot here too */ 199load_loop: 200 movb %cl, %dh 201 cmpb $MAX_READ_SEC, %cl /* Truncate to max read size */ 202 jbe load_notrunc 203 movb $MAX_READ_SEC, %dh 204load_notrunc: 205 subb %dh, %cl /* Update count */ 206 pushl %eax /* Save */ 207 movw %bx, %es /* %bx had the segment (para) number */ 208 xorw %bx, %bx /* %es:0000 for destination */ 209 call read /* Read it in */ 210 popl %eax /* Restore */ 211 addl $MAX_READ_SEC, %eax /* Update LBA */ 212 addw $MAX_READ_PARAS, %bx /* Update dest addr */ 213 jcxz load_done /* Done? */ 214 jmp load_loop /* Keep going */ 215load_done: 216 217 /* Now we can start the loaded program */ 218 219 movw loader, %cx /* Tell cdboot where it is */ 220 /* (Older versions of cdbr have */ 221 /* %cx == 0 from the jcxz load_done) */ 222 movb drive, %dl /* Get the boot drive number */ 223 ljmp $BOOTSEG, $0 /* Go run cdboot */ 224 225/* 226 * Lookup the file in the path at [SI] from the root directory. 227 * 228 * Trashes: All but BX 229 * Returns: CF = 0 (success), BX = pointer to record 230 * CF = 1 (not found) 231 */ 232lookup: 233 movw $VD_ROOTDIR + MEM_VOLDESC, %bx /* Root directory record */ 234 235lookup_dir: 236 lodsb /* Get first char of path */ 237 cmpb $0, %al /* Are we done? */ 238 je lookup_done /* Yes */ 239 cmpb $'/', %al /* Skip path separator */ 240 je lookup_dir 241 decw %si /* Undo lodsb side effect */ 242 call find_file /* Lookup first path item */ 243 jnc lookup_dir /* Try next component */ 244 ret 245lookup_done: 246 movw $msg_loading, %si /* Success message - say which file */ 247 call display_string 248 mov loader, %si 249 call display_string 250 mov $crlf, %si 251 call display_string 252 clc /* Clear carry */ 253 ret 254 255/* 256 * Lookup file at [SI] in directory whose record is at [BX]. 257 * 258 * Trashes: All but returns 259 * Returns: CF = 0 (success), BX = pointer to record, SI = next path item 260 * CF = 1 (not found), SI = preserved 261 */ 262find_file: 263 mov DIR_EXTENT(%bx), %eax /* Load extent */ 264 xor %edx, %edx 265 mov DIR_EA_LEN(%bx), %dl 266 add %edx, %eax /* Skip extended attributes */ 267 mov %eax, rec_lba /* Save LBA */ 268 mov DIR_SIZE(%bx), %eax /* Save size */ 269 mov %eax, rec_size 270 xor %cl, %cl /* Zero length */ 271 push %si /* Save */ 272ff_namelen: 273 inc %cl /* Update length */ 274 lodsb /* Read char */ 275 cmp $0, %al /* Nul? */ 276 je ff_namedone /* Yes */ 277 cmp $'/', %al /* Path separator? */ 278 jnz ff_namelen /* No, keep going */ 279ff_namedone: 280 dec %cl /* Adjust length and save */ 281 mov %cl, name_len 282 pop %si /* Restore */ 283ff_load: 284 mov rec_lba, %eax /* Load LBA */ 285 mov $MEM_DIR, %ebx /* Address buffer */ 286 mov $1, %dh /* One sector */ 287 call read /* Read directory block */ 288 incl rec_lba /* Update LBA to next block */ 289ff_scan: 290 mov %ebx, %edx /* Check for EOF */ 291 sub $MEM_DIR, %edx 292 cmp %edx, rec_size 293 ja ff_scan_1 294 stc /* EOF reached */ 295 ret 296ff_scan_1: 297 cmpb $0, DIR_LEN(%bx) /* Last record in block? */ 298 je ff_nextblock 299 push %si /* Save */ 300 movzbw DIR_NAMELEN(%bx), %si /* Find end of string */ 301ff_checkver: 302 cmpb $'0', DIR_NAME-1(%bx,%si) /* Less than '0'? */ 303 jb ff_checkver_1 304 cmpb $'9', DIR_NAME-1(%bx,%si) /* Greater than '9'? */ 305 ja ff_checkver_1 306 dec %si /* Next char */ 307 jnz ff_checkver 308 jmp ff_checklen /* All numbers in name, so */ 309 /* no version */ 310ff_checkver_1: 311 movzbw DIR_NAMELEN(%bx), %cx 312 cmp %cx, %si /* Did we find any digits? */ 313 je ff_checkdot /* No */ 314 cmpb $';', DIR_NAME-1(%bx,%si) /* Check for semicolon */ 315 jne ff_checkver_2 316 dec %si /* Skip semicolon */ 317 mov %si, %cx 318 mov %cl, DIR_NAMELEN(%bx) /* Adjust length */ 319 jmp ff_checkdot 320ff_checkver_2: 321 mov %cx, %si /* Restore %si to end of string */ 322ff_checkdot: 323 cmpb $'.', DIR_NAME-1(%bx,%si) /* Trailing dot? */ 324 jne ff_checklen /* No */ 325 decb DIR_NAMELEN(%bx) /* Adjust length */ 326ff_checklen: 327 pop %si /* Restore */ 328 movzbw name_len, %cx /* Load length of name */ 329 cmp %cl, DIR_NAMELEN(%bx) /* Does length match? */ 330 je ff_checkname /* Yes, check name */ 331ff_nextrec: 332 add DIR_LEN(%bx), %bl /* Next record */ 333 adc $0, %bh 334 jmp ff_scan 335ff_nextblock: 336 subl $SECTOR_SIZE, rec_size /* Adjust size */ 337 jnc ff_load /* If subtract ok, keep going */ 338 ret /* End of file, so not found */ 339ff_checkname: 340 lea DIR_NAME(%bx), %di /* Address name in record */ 341 push %si /* Save */ 342 repe cmpsb /* Compare name */ 343 jcxz ff_match /* We have a winner! */ 344 pop %si /* Restore */ 345 jmp ff_nextrec /* Keep looking */ 346ff_match: 347 add $2, %sp /* Discard saved %si */ 348 clc /* Clear carry */ 349 ret 350 351/* 352 * Load DH sectors starting at LBA %eax into address %es:%bx. 353 * 354 * Preserves %bx, %cx, %dx, %si, %es 355 * Trashes %eax 356 */ 357read: 358 pushw %si /* Save */ 359 pushw %cx /* Save since some BIOSs trash */ 360 movl %eax, edd_lba /* LBA to read from */ 361 movw %es, %ax /* Get the segment */ 362 movw %ax, edd_addr + 2 /* and store */ 363 movw %bx, edd_addr /* Store offset too */ 364read_retry: 365 call twiddle /* Entertain the user */ 366 pushw %dx /* Save */ 367 movw $edd_packet, %si /* Address Packet */ 368 movb %dh, edd_len /* Set length */ 369 movb drive, %dl /* BIOS Device */ 370 movb $0x42, %ah /* BIOS: Extended Read */ 371 int $0x13 /* Call BIOS */ 372 popw %dx /* Restore */ 373 jc read_fail /* Worked? */ 374 popw %cx /* Restore */ 375 popw %si 376 ret /* Return */ 377read_fail: 378 cmpb $ERROR_TIMEOUT, %ah /* Timeout? */ 379 je read_retry /* Yes, Retry */ 380read_error: 381 pushw %ax /* Save error */ 382 movw $msg_badread, %si /* "Read error: 0x" */ 383 call display_string 384 popw %ax /* Retrieve error code */ 385 movb %ah, %al /* Into %al */ 386 call hex_byte /* Display error code */ 387 jmp stay_stopped /* ... then hang */ 388 389/* 390 * Display the ASCIZ error message in %esi then halt 391 */ 392err_stop: 393 call display_string 394 395stay_stopped: 396 sti /* Ensure Ctl-Alt-Del will work */ 397 hlt /* (don't require power cycle) */ 398 jmp stay_stopped /* (Just to make sure) */ 399 400/* 401 * Output the "twiddle" 402 */ 403twiddle: 404 push %ax /* Save */ 405 push %bx /* Save */ 406 mov twiddle_index, %al /* Load index */ 407 mov twiddle_chars, %bx /* Address table */ 408 inc %al /* Next */ 409 and $3, %al /* char */ 410 mov %al, twiddle_index /* Save index for next call */ 411 xlat /* Get char */ 412 call display_char /* Output it */ 413 mov $8, %al /* Backspace */ 414 call display_char /* Output it */ 415 pop %bx /* Restore */ 416 pop %ax /* Restore */ 417 ret 418 419/* 420 * Display the ASCIZ string pointed to by %si. 421 * 422 * Destroys %si, possibly others. 423 */ 424display_string: 425 pushw %ax 426 cld 4271: 428 lodsb /* %al = *%si++ */ 429 testb %al, %al 430 jz 1f 431 call display_char 432 jmp 1b 433 434/* 435 * Write out value in %eax in hex 436 */ 437hex_long: 438 pushl %eax 439 shrl $16, %eax 440 call hex_word 441 popl %eax 442 /* fall thru */ 443 444/* 445 * Write out value in %ax in hex 446 */ 447hex_word: 448 pushw %ax 449 mov %ah, %al 450 call hex_byte 451 popw %ax 452 /* fall thru */ 453/* 454 * Write out value in %al in hex 455 */ 456hex_byte: 457 pushw %ax 458 shrb $4, %al 459 call hex_nibble 460 popw %ax 461 /* fall thru */ 462 463/* Write out nibble in %al */ 464hex_nibble: 465 and $0x0F, %al 466 add $'0', %al 467 cmpb $'9', %al 468 jbe display_char 469 addb $'A'-'9'-1, %al 470 /* fall thru to display_char */ 471 472/* 473 * Display the character in %al 474 */ 475display_char: 476 pushw %ax 477 478 pushw %bx 479 movb $0x0e, %ah 480 movw $1, %bx 481 int $0x10 482 popw %bx 4831: popw %ax 484 ret 485 486/* 487 * Data 488 */ 489drive: .byte 0 /* Given to us by the BIOS */ 490signon: .asciz "CD-ROM: " 491crlf: .asciz "\r\n" 492msg_load2big: .asciz "File too big" 493msg_badread: .asciz "Read error: 0x" 494msg_novd: .asciz "Can't find Primary Volume Descriptor" 495msg_loading: .asciz "Loading " 496 497/* State for searching dir */ 498rec_lba: .long 0x0 /* LBA (adjusted for EA) */ 499rec_size: .long 0x0 /* File size */ 500name_len: .byte 0x0 /* Length of current name */ 501 502twiddle_index: .byte 0x0 503twiddle_chars: .ascii "|/-\\" 504 505/* Packet for LBA (CD) read */ 506edd_packet: .byte 0x10 /* Length */ 507 .byte 0 /* Reserved */ 508edd_len: .byte 0x0 /* Num to read */ 509 .byte 0 /* Reserved */ 510edd_addr: .word 0x0, 0x0 /* Seg:Off */ 511edd_lba: .quad 0x0 /* LBA */ 512 513/* The data from here must be last in the file, only followed by zero bytes */ 514 515loader: .word 0 /* The path we end up using */ 516 517msg_failed: .ascii "Can't find " /* This string runs into... */ 518 519/* loader_paths is a list of ASCIZ strings followed by a term NUL byte */ 520loader_paths: .asciz "/cdboot" 521 .asciz "/CDBOOT" 522 .byte 0 523 524 . = BOOTSECTSIZE 525 526 .end 527