fatboot.S revision 1.2
1 /* $NetBSD: fatboot.S,v 1.2 2007/01/06 20:47:15 dsl Exp $ */ 2 3 /*- 4 * Copyright (c) 2007 The NetBSD Foundation, Inc. 5 * All rights reserved. 6 * 7 * This code is derived from software contributed to The NetBSD Foundation 8 * by David Laight. 9 * 10 * Redistribution and use in source and binary forms, with or without 11 * modification, are permitted provided that the following conditions 12 * are met: 13 * 1. Redistributions of source code must retain the above copyright 14 * notice, this list of conditions and the following disclaimer. 15 * 2. Redistributions in binary form must reproduce the above copyright 16 * notice, this list of conditions and the following disclaimer in the 17 * documentation and/or other materials provided with the distribution. 18 * 3. Neither the name of The NetBSD Foundation nor the names of its 19 * contributors may be used to endorse or promote products derived 20 * from this software without specific prior written permission. 21 * 22 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 23 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 24 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 25 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 26 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 27 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 28 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 29 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 30 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 31 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 32 * POSSIBILITY OF SUCH DAMAGE. 33 */ 34 35 /* 36 * i386 partition boot code 37 * This version reads boot directly from a FAT16 filesystem on LBA 38 * Addressable media - eg USB media. 39 * 40 * The code is read to address 0:7c00 by the mbr code (in sector zero). 41 * 42 * We assume that the partition contains a 'boot parameter block' that 43 * correctly identifies the filesystem. 44 * 45 * On entry and exit the BIOS drive number is in %dl. 46 */ 47 48 #include <machine/asm.h> 49 #include <sys/bootblock.h> 50 51 #ifndef FAT_ENTRY_SIZE 52 #error FAT_ENTRY_SIZE not defined 53 #endif 54 55 /* Support for FAT32 could be added - but hasn't been yet. */ 56 #if FAT_ENTRY_SIZE != 16 57 #error Unsupported FAT_ENTRY_SIZE value 58 #endif 59 60 #define PBR_AFTERBPB 62 /* BPB size in floppy master BR */ 61 62 #ifdef TERSE_ERROR 63 /* 64 * Error codes. Done this way to save space. 65 */ 66 #define ERR_READ 'R' /* Read error */ 67 #define ERR_NO_BOOT 'B' /* No /boot */ 68 #define ERR_NOT_FAT16 'F' /* Not a FAT16 filesystem */ 69 #define ERR_NO_BOOT_MAGIC_2 'M' /* No magic in loaded /boot */ 70 71 #define set_err(err) movb $err, %al 72 73 #else 74 #define set_err(err) mov $err, %ax 75 #endif 76 77 .text 78 .code16 79 ENTRY(start) 80 jmp start0 81 nop 82 oem_name: .ascii "NetBSD40" /* 8 bytes */ 83 /* FAT16 BIOS/BOOT Parameter Block - see struct mbr_bpbFAT16 in bootblock.h */ 84 #define bpb_bytes_per_sec /* .word 0 */ 0x0b /* bytes per sector */ 85 #define bpb_sec_per_clust /* .byte 0 */ 0x0d /* sectors per cluster */ 86 #define bpb_res_sectors /* .word 0 */ 0x0e /* number of reserved sectors */ 87 #define bpb_FATs /* .byte 0 */ 0x10 /* number of FATs */ 88 #define bpb_root_dir_ents /* .word 0 */ 0x11 /* number of root dir entries */ 89 #define bpb_sectors /* .word 0 */ 0x13 /* total number of sectors */ 90 #define bpb_media /* .byte 0 */ 0x15 /* media descriptor */ 91 #define bpb_FAT_secs /* .word 0 */ 0x16 /* number of sectors per FAT */ 92 #define bpb_sec_per_track /* .word 0 */ 0x18 /* sectors per track */ 93 #define bpb_heads /* .word 0 */ 0x1a /* number of heads */ 94 #define bpb_hidden_secs /* .long 0 */ 0x1c /* # of hidden sectors */ 95 #define bpb_huge_sectors /* .long 0 */ 0x20 /* # of sectors if !bpbSectors*/ 96 /* Extended boot area */ 97 #define bs_drive_number /* .byte 0 */ 0x24 /* but we believe the BIOS ! */ 98 #define bs_reserved_1 /* .byte 0 */ 0x25 /* */ 99 #define bs_boot_sig /* .byte 0 */ 0x26 /* */ 100 #define bs_volume_id /* .long 0 */ 0x27 /* Volume ID number */ 101 #define bs_volume_label /* .space 11*/ 0x2b /* Volume label */ 102 #define bs_file_sys_type /* .space 8 */ 0x36 /* "FAT16 " */ 103 104 /* Some locals overlaying the end of the above */ 105 #define sec_p_cl_w 0x2c /* 16bit bpb_sec_per_clust */ 106 #define fat_sector 0x30 /* start of FAT in sectors */ 107 108 . = start + PBR_AFTERBPB /* skip BPB */ 109 start0: 110 xor %eax, %eax /* don't trust values of ds, es or ss */ 111 mov %ax, %ds 112 mov %ax, %es 113 mov %ax, %ss 114 mov $start, %sp 115 mov %sp, %bp /* to access the pbp */ 116 push %dx /* save drive at -2(%bp) */ 117 118 set_err(ERR_NOT_FAT16) 119 cmpl $'A'|'T'<<8|'1'<<16|'6'<<24, bs_file_sys_type+1(%bp) 120 jne error 121 122 /* Add 'reserved' (inside ptn) to 'hidden' (ptn offset) */ 123 mov bpb_res_sectors(%bp), %ax 124 addl bpb_hidden_secs(%bp), %eax 125 mov %eax, fat_sector(%bp) /* To get first sector of FAT */ 126 127 /* Determine base of root directory */ 128 movzbw bpb_FATs(%bp), %ax /* Count of FATs */ 129 mulw bpb_FAT_secs(%bp) /* FAT size in %dx:%ax */ 130 shl $16,%edx 131 xchg %ax,%dx /* FAT size now in %edx */ 132 addl fat_sector(%bp), %edx /* Directory is after FATs */ 133 push %cs /* %cs is zero */ 134 push %cs /* 64-bit for LBA read */ 135 pushl %edx /* Sector number of root dir */ 136 137 push $0x1000 /* Read to 0x10000:0 */ 138 pop %es /* Which we need in %es later */ 139 push %es 140 push %cs /* Offset zero */ 141 142 /* Convert the root directory size to sectors */ 143 push %dx 144 mov bpb_root_dir_ents(%bp), %ax 145 mov $0x20, %dx 146 mul %dx 147 divw bpb_bytes_per_sec(%bp) 148 add $0xffff, %dx /* Set carry if remainder non-zero */ 149 adc $0, %ax /* and round up the division */ 150 pop %dx 151 152 /* Read in the entire root directory */ 153 push %ax /* Sectors in root directory */ 154 cwtl 155 addl %eax, %edx /* %edx now sector of first file */ 156 call read_lba /* Read entire directory */ 157 158 /* Scan directory for our file */ 159 xor %di, %di 160 scan_dir: 161 mov $boot_filename, %si 162 mov $11, %cx 163 repz cmpsb 164 je found_boot 165 or $31,%di 166 inc %di 167 cmp %ch, %es:(%di) /* %ch is zero - test end of dir */ 168 jz 1f 169 decw bpb_root_dir_ents(%bp) 170 jnz scan_dir 171 1: set_err(ERR_NO_BOOT) 172 173 error: 174 #ifdef TERSE_ERROR 175 movb %al, errcod 176 movw $errtxt, %si 177 call message 178 #else 179 push %ax 180 movw $errtxt, %si 181 call message 182 pop %si 183 call message 184 movw $newline, %si 185 call message 186 #endif 187 1: sti 188 hlt 189 jmp 1b 190 191 found_boot: 192 movzbl bpb_sec_per_clust(%bp), %eax 193 movw %ax, sec_p_cl_w(%bp) 194 add %ax, %ax 195 subl %eax, %edx /* 1st file sector is cluster 2 */ 196 1: inc %cl /* Convert power of 2 ... */ 197 shr $1, %ax /* ... to shift */ 198 jnz 1b 199 dec %cx 200 dec %cx 201 movw %es:15(%di), %ax /* Cluster number for file start */ 202 push %es /* We increment the 'segment' ... */ 203 pop %di /* ... after each read, offset is 0 */ 204 205 read_data_block: 206 mov %ax, %bx /* Save cluster number */ 207 shl %cl, %eax /* Convert to sector number */ 208 add %eax, %edx 209 pushl %edx /* Sector to read */ 210 sub %eax, %edx /* Recover base segment */ 211 push %di /* Target address segment! */ 212 push $0 213 push sec_p_cl_w(%bp) 214 call read_lba /* Read a cluster */ 215 216 /* Update read ptr for next cluster */ 217 mov bpb_bytes_per_sec(%bp), %ax 218 shr $4, %ax /* x86 segment count */ 219 shl %cl, %ax /* for a cluster */ 220 add %ax, %di 221 222 /* Lookup FAT slot number in FAT table */ 223 mov %bx, %ax /* Recover cluster number */ 224 push %dx 225 xor %dx, %dx 226 divw bpb_bytes_per_sec(%bp) 227 mov %dx, %bx /* Entry in FAT block */ 228 pop %dx 229 cmp %ax, fat_cache 230 je lookup_fat 231 232 /* We must read a different chuck of the FAT */ 233 mov %ax, fat_cache 234 cwtl 235 shl $1, %ax 236 addl fat_sector(%bp), %eax 237 push %eax 238 push %ds 239 push $fat_buffer 240 push $2 /* Always read 2 sectors of FAT */ 241 call read_lba 242 243 /* Now use low part of cluster number to index FAT sector */ 244 lookup_fat: 245 add %bx, %bx /* 2 bytes per entry... */ 246 movzwl fat_buffer(%bx), %eax /* Next FAT slot */ 247 cmp $0xfff0, %ax 248 jb read_data_block 249 250 /* Found end of FAT chain - must be EOF - leap into loaded code */ 251 mov $0x1000, %ax 252 mov %ax, %es 253 cmpl $X86_BOOT_MAGIC_2, %es:4 254 je magic_ok 255 set_err(ERR_NO_BOOT_MAGIC_2) 256 err1: jmp error 257 258 /* Set parameters expected by /boot */ 259 magic_ok: 260 mov bpb_hidden_secs(%bp), %ebx /* ptn base sector */ 261 movb -2(%bp), %dl /* disk number (could pop %dx) */ 262 mov $boot_params + 4, %si 263 push %es 264 push $0 265 lret 266 267 /* Read disk using on-stack int13-extension parameter block */ 268 read_lba: 269 pop %ax /* Save rtn addr */ 270 pushw $16 /* Stack ctl block length */ 271 mov %sp, %si /* Address ctl block */ 272 push %ax /* restack rtn addr */ 273 pushal /* Save everything except %si and %ax*/ 274 mov -2(%bp), %dl /* Disk # saved on entry */ 275 movb $0x42, %ah 276 int $0x13 277 popal 278 279 set_err(ERR_READ) 280 jc err1 281 ret $12 /* Discard all except high LBA zeros */ 282 283 /* 284 * I hate #including source files, but pbr_magic below has to be at 285 * the correct absolute address. 286 * Clearly this could be done with a linker script. 287 */ 288 289 #include <message.S> 290 #if 0 291 #include <dump_eax.S> 292 #endif 293 294 errtxt: .ascii "Error " /* runs into newline... */ 295 errcod: .byte 0 /* ... if errcod set */ 296 newline: 297 .asciz "\r\n" 298 299 #ifndef TERSE_ERROR 300 ERR_READ: .asciz "Disk read" 301 ERR_NO_BOOT: .asciz "No /boot" 302 ERR_NOT_FAT16: .asciz "Not FAT16 ptn" 303 ERR_NO_BOOT_MAGIC_2: .asciz "No magic in /boot" 304 #endif 305 306 boot_filename: .ascii "BOOT " 307 308 space: 309 pbr_space = boot_params - . 310 311 /* 312 * Add magic number, with a zero sized patchable area - just in case something 313 * finds it and tries to update the area. 314 * Boot options can be set using 'installboot -e boot' so we don't need to 315 * use any of our valuable bytes. 316 */ 317 318 . = _C_LABEL(start) + 0x1fe - 2 - 4 - 4 319 boot_params: 320 .long X86_BOOT_MAGIC_FAT 321 .long 1f - . 322 1: 323 fat_cache: .word 0xffff /* Sector number in buffer */ 324 . = _C_LABEL(start) + 0x1fe 325 .word 0xaa55 326 fat_buffer: /* 2 sectors worth of FAT table */ 327
Indexes created Thu Oct 02 07:10:07 GMT 2025