fatboot.S revision 1.1
1 /* $NetBSD: fatboot.S,v 1.1 2007/01/01 22:11:09 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 #define PBR_AFTERBPB 62 /* BPB size in floppy master BR */ 52 53 #ifdef TERSE_ERROR 54 /* 55 * Error codes. Done this way to save space. 56 */ 57 #define ERR_READ 'R' /* Read error */ 58 #define ERR_NO_BOOT 'B' /* No /boot */ 59 #define ERR_NOT_FAT16 'F' /* Not a FAT16 filesystem */ 60 #define ERR_NO_BOOT_MAGIC_2 'M' /* No magic in loaded /boot */ 61 62 #define set_err(err) movb $err, %al 63 64 #else 65 #define set_err(err) mov $err, %ax 66 #endif 67 68 .text 69 .code16 70 ENTRY(start) 71 jmp start0 72 nop 73 oem_name: .ascii "NetBSD40" /* 8 bytes */ 74 /* FAT16 BIOS/BOOT Parameter Block - see struct mbr_bpbFAT16 in bootblock.h */ 75 #define bpb_bytes_per_sec /* .word 0 */ 0x0b /* bytes per sector */ 76 #define bpb_sec_per_clust /* .byte 0 */ 0x0d /* sectors per cluster */ 77 #define bpb_res_sectors /* .word 0 */ 0x0e /* number of reserved sectors */ 78 #define bpb_FATs /* .byte 0 */ 0x10 /* number of FATs */ 79 #define bpb_root_dir_ents /* .word 0 */ 0x11 /* number of root dir entries */ 80 #define bpb_sectors /* .word 0 */ 0x13 /* total number of sectors */ 81 #define bpb_media /* .byte 0 */ 0x15 /* media descriptor */ 82 #define bpb_FAT_secs /* .word 0 */ 0x16 /* number of sectors per FAT */ 83 #define bpb_sec_per_track /* .word 0 */ 0x18 /* sectors per track */ 84 #define bpb_heads /* .word 0 */ 0x1a /* number of heads */ 85 #define bpb_hidden_secs /* .long 0 */ 0x1c /* # of hidden sectors */ 86 #define bpb_huge_sectors /* .long 0 */ 0x20 /* # of sectors if !bpbSectors*/ 87 /* Extended boot area */ 88 #define bs_drive_number /* .byte 0 */ 0x24 /* but we believe the BIOS ! */ 89 #define bs_reserved_1 /* .byte 0 */ 0x25 /* */ 90 #define bs_boot_sig /* .byte 0 */ 0x26 /* */ 91 #define bs_volume_id /* .long 0 */ 0x27 /* Volume ID number */ 92 #define bs_volume_label /* .space 11*/ 0x2b /* Volume label */ 93 #define bs_file_sys_type /* .space 8 */ 0x36 /* "FAT16 " */ 94 95 /* Some locals overlaying the end of the above */ 96 #define sec_p_cl_w 0x2c /* 16bit bpb_sec_per_clust */ 97 #define fat_sector 0x30 /* start of FAT in sectors */ 98 99 . = start + PBR_AFTERBPB /* skip BPB */ 100 start0: 101 xor %eax, %eax /* don't trust values of ds, es or ss */ 102 mov %ax, %ds 103 mov %ax, %es 104 mov %ax, %ss 105 mov $start, %sp 106 mov %sp, %bp /* to access the pbp */ 107 push %dx /* save drive at -2(%bp) */ 108 109 xchg %ax, %bx /* %bx = 0 */ 110 111 set_err(ERR_NOT_FAT16) 112 cmpl $'A'|'T'<<8|'1'<<16|'6'<<24, bs_file_sys_type+1(%bp) 113 jne error 114 115 /* Add 'reserved' (inside ptn) to 'hidden' (ptn offset) */ 116 mov bpb_res_sectors(%bp), %ax 117 addl bpb_hidden_secs(%bp), %eax 118 mov %eax, fat_sector(%bp) /* To get first sector of FAT */ 119 120 /* Determine base of root directory */ 121 movzbw bpb_FATs(%bp), %ax /* Count of FATs */ 122 mulw bpb_FAT_secs(%bp) /* FAT size in %dx:%ax */ 123 shl $16,%edx 124 xchg %ax,%dx /* FAT size now in %edx */ 125 addl fat_sector(%bp), %edx /* Directory is after FATs */ 126 push %bx /* %bx is zero */ 127 push %bx /* 64-bit for LBA read */ 128 pushl %edx /* Sector number of root dir */ 129 130 push $0x1000 /* Read to 0x10000:0 */ 131 pop %es /* Which we need in %es later */ 132 push %es 133 push %bx /* Offset zero */ 134 135 /* Convert the root directory size to sectors */ 136 push %dx 137 mov bpb_root_dir_ents(%bp), %ax 138 mov $0x20, %dx 139 mul %dx 140 divw bpb_bytes_per_sec(%bp) 141 add $0xffff, %dx /* Set carry if remainder non-zero */ 142 adc %bx, %ax /* %bx is still zero */ 143 pop %dx 144 145 /* Read in the entire root directory */ 146 push %ax /* Sectors in root directory */ 147 cwtl 148 addl %eax, %edx /* %edx now sector of first file */ 149 call read_lba /* Read entire directory */ 150 151 /* Scan directory for our file */ 152 xor %di, %di 153 scan_dir: 154 mov $boot_filename, %si 155 mov $11, %cx 156 repz cmpsb 157 je found_boot 158 or $31,%di 159 inc %di 160 cmp %ch, %es:(%di) /* %ch is zero - test end of dir */ 161 jz 1f 162 decw bpb_root_dir_ents(%bp) 163 jnz scan_dir 164 1: set_err(ERR_NO_BOOT) 165 166 error: 167 #ifdef TERSE_ERROR 168 movb %al, errcod 169 movw $errtxt, %si 170 call message 171 #else 172 push %ax 173 movw $errtxt, %si 174 call message 175 pop %si 176 call message 177 movw $newline, %si 178 call message 179 #endif 180 1: sti 181 hlt 182 jmp 1b 183 184 found_boot: 185 movzbl bpb_sec_per_clust(%bp), %eax 186 movw %ax, sec_p_cl_w(%bp) 187 add %ax, %ax 188 subl %eax, %edx /* 1st file sector is cluster 2 */ 189 1: inc %cl /* Convert power of 2 ... */ 190 shr $1, %ax /* ... to shift */ 191 jnz 1b 192 dec %cx 193 dec %cx 194 movw %es:15(%di), %ax /* Cluster number for file start */ 195 push %es /* We increment the 'segment' ... */ 196 pop %di /* ... after each read, offset is 0 */ 197 198 read_data_block: 199 mov %ax, %bx /* Save cluster number */ 200 shl %cl, %eax /* Convert to sector number */ 201 add %eax, %edx 202 pushl %edx /* Sector to read */ 203 sub %eax, %edx /* Recover base segment */ 204 push %di /* Target address segment! */ 205 push $0 206 push sec_p_cl_w(%bp) 207 call read_lba /* Read a cluster */ 208 209 /* Update read ptr for next cluster */ 210 mov bpb_bytes_per_sec(%bp), %ax 211 shr $4, %ax /* x86 segment count */ 212 shl %cl, %ax /* for a cluster */ 213 add %ax, %di 214 215 /* Lookup FAT slot number in FAT table */ 216 mov %bx, %ax /* Recover cluster number */ 217 push %dx 218 xor %dx, %dx 219 divw bpb_bytes_per_sec(%bp) 220 mov %dx, %bx /* Entry in FAT block */ 221 pop %dx 222 cmp %ax, fat_cache 223 je lookup_fat 224 225 /* We must read a different chuck of the FAT */ 226 mov %ax, fat_cache 227 cwtl 228 shl $1, %ax 229 addl fat_sector(%bp), %eax 230 push %eax 231 push %ds 232 push $fat_buffer 233 push $2 /* Always read 2 sectors of FAT */ 234 call read_lba 235 236 /* Now use low part of cluster number to index FAT sector */ 237 lookup_fat: 238 add %bx, %bx /* 2 bytes per entry... */ 239 movzwl fat_buffer(%bx), %eax /* Next FAT slot */ 240 cmp $0xfff0, %ax 241 jb read_data_block 242 243 /* Found end of FAT chain - must be EOF - leap into loaded code */ 244 mov $0x1000, %ax 245 mov %ax, %es 246 cmpl $X86_BOOT_MAGIC_2, %es:4 247 je magic_ok 248 set_err(ERR_NO_BOOT_MAGIC_2) 249 err1: jmp error 250 251 /* Set parameters expected by /boot */ 252 magic_ok: 253 mov bpb_hidden_secs(%bp), %ebx /* ptn base sector */ 254 movb -2(%bp), %dl /* disk number (could pop %dx) */ 255 mov $boot_params + 4, %si 256 push %es 257 push $0 258 lret 259 260 /* Read disk using on-stack int13-extension parameter block */ 261 read_lba: 262 pop %ax /* Save rtn addr */ 263 pushw $16 /* Stack ctl block length */ 264 mov %sp, %si /* Address ctl block */ 265 push %ax /* restack rtn addr */ 266 pushal 267 mov -2(%bp), %dl /* Disk # saved on entry */ 268 movb $0x42, %ah 269 int $0x13 270 popal 271 272 set_err(ERR_READ) 273 jc err1 274 ret $12 /* Discard all except high LBA zeros */ 275 276 /* 277 * I hate #including source files, but pbr_magic below has to be at 278 * the correct absolute address. 279 * Clearly this could be done with a linker script. 280 */ 281 282 #include <message.S> 283 #if 0 284 #include <dump_eax.S> 285 #endif 286 287 errtxt: .ascii "Error " /* runs into newline... */ 288 errcod: .byte 0 /* ... if errcod set */ 289 newline: 290 .asciz "\r\n" 291 292 #ifndef TERSE_ERROR 293 ERR_READ: .asciz "Disk read" 294 ERR_NO_BOOT: .asciz "No /boot" 295 ERR_NOT_FAT16: .asciz "Not FAT16 ptn" 296 ERR_NO_BOOT_MAGIC_2: .asciz "No magic in /boot" 297 #endif 298 299 boot_filename: .ascii "BOOT " 300 301 space: 302 pbr_space = boot_params - . 303 304 /* 305 * Add magic number, with a zero sized patchable area - just in case something 306 * finds it and tries to update the area. 307 * Boot options can be set using 'installboot -e boot' so we don't need to 308 * use any of our valuable bytes. 309 */ 310 311 . = _C_LABEL(start) + 0x1fe - 2 - 4 - 4 312 boot_params: 313 .long X86_BOOT_MAGIC_FAT 314 .long 1f - . 315 1: 316 fat_cache: .word 0xffff /* Sector number in buffer */ 317 . = _C_LABEL(start) + 0x1fe 318 .word 0xaa55 319 fat_buffer: /* 2 sectors worth of FAT table */ 320
Indexes created Thu Oct 02 14:10:14 GMT 2025