/* $NetBSD: fatboot.S,v 1.5 2025/02/28 09:07:12 andvar Exp $ */ /*- * Copyright (c) 2007 The NetBSD Foundation, Inc. * All rights reserved. * * This code is derived from software contributed to The NetBSD Foundation * by David Laight. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * 1. Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * 2. Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. * * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE * POSSIBILITY OF SUCH DAMAGE. */ /* * i386 partition boot code * This version reads boot directly from a FAT16 filesystem on LBA * Addressable media - eg USB media. * * The code is read to address 0:7c00 by the mbr code (in sector zero). * * We assume that the partition contains a 'boot parameter block' that * correctly identifies the filesystem. * * On entry and exit the BIOS drive number is in %dl. */ #include <machine/asm.h> #include <sys/bootblock.h> #ifndef FAT_ENTRY_SIZE #error FAT_ENTRY_SIZE not defined #endif /* Support for FAT32 could be added - but hasn't been yet. */ #if FAT_ENTRY_SIZE != 16 && FAT_ENTRY_SIZE != 12 #error Unsupported FAT_ENTRY_SIZE value #endif #define FAT_SIZE_STR (('0'+ FAT_ENTRY_SIZE / 10) | ('0' + FAT_ENTRY_SIZE % 10) << 8) #define PBR_AFTERBPB 62 /* BPB size in floppy master BR */ #ifdef TERSE_ERROR /* * Error codes. Done this way to save space. */ #define ERR_READ 'R' /* Read error */ #define ERR_NO_BOOT 'B' /* No /boot */ #define ERR_NOT_FAT16 'F' /* Not a FAT16 filesystem */ #define ERR_NO_BOOT_MAGIC_2 'M' /* No magic in loaded /boot */ #define set_err(err) movb $err, %al #else #define set_err(err) mov $err, %ax #endif .text .code16 ENTRY(start) jmp start0 nop oem_name: .ascii "NetBSD40" /* 8 bytes */ /* FAT16 BIOS/BOOT Parameter Block - see struct mbr_bpbFAT16 in bootblock.h */ #define bpb_bytes_per_sec /* .word 0 */ 0x0b /* bytes per sector */ #define bpb_sec_per_clust /* .byte 0 */ 0x0d /* sectors per cluster */ #define bpb_res_sectors /* .word 0 */ 0x0e /* number of reserved sectors */ #define bpb_FATs /* .byte 0 */ 0x10 /* number of FATs */ #define bpb_root_dir_ents /* .word 0 */ 0x11 /* number of root dir entries */ #define bpb_sectors /* .word 0 */ 0x13 /* total number of sectors */ #define bpb_media /* .byte 0 */ 0x15 /* media descriptor */ #define bpb_FAT_secs /* .word 0 */ 0x16 /* number of sectors per FAT */ #define bpb_sec_per_track /* .word 0 */ 0x18 /* sectors per track */ #define bpb_heads /* .word 0 */ 0x1a /* number of heads */ #define bpb_hidden_secs /* .long 0 */ 0x1c /* # of hidden sectors */ #define bpb_huge_sectors /* .long 0 */ 0x20 /* # of sectors if !bpbSectors*/ /* Extended boot area */ #define bs_drive_number /* .byte 0 */ 0x24 /* but we believe the BIOS ! */ #define bs_reserved_1 /* .byte 0 */ 0x25 /* */ #define bs_boot_sig /* .byte 0 */ 0x26 /* */ #define bs_volume_id /* .long 0 */ 0x27 /* Volume ID number */ #define bs_volume_label /* .space 11*/ 0x2b /* Volume label */ #define bs_file_sys_type /* .space 8 */ 0x36 /* "FAT16 " */ /* Some locals overlaying the end of the above */ #define sec_p_cl_w 0x2c /* 16bit bpb_sec_per_clust */ #define fat_sector 0x30 /* start of FAT in sectors */ . = start + PBR_AFTERBPB /* skip BPB */ start0: xor %eax, %eax /* don't trust values of ds, es or ss */ mov %ax, %ds mov %ax, %es mov %ax, %ss mov $start, %sp mov %sp, %bp /* to access the pbp */ push %dx /* save drive at -2(%bp) */ /* We put the LBA bios command block on stack. * Since we only want a 32bit sector number, stack a zero */ push %cs /* %cs is zero */ push %cs /* 64-bit for LBA read */ set_err(ERR_NOT_FAT16) cmpl $'A'|'T'<<8|FAT_SIZE_STR<<16, bs_file_sys_type+1(%bp) jne error /* Add 'reserved' (inside ptn) to 'hidden' (ptn offset) */ mov bpb_res_sectors(%bp), %ax addl bpb_hidden_secs(%bp), %eax mov %eax, fat_sector(%bp) /* To get first sector of FAT */ #if FAT_ENTRY_SIZE == 12 /* Read the entire FAT */ push %eax push %ds push $fat_buffer push $12 /* 12 sectors is assumed 6k */ call read_lba #endif /* Determine base of root directory */ movzbw bpb_FATs(%bp), %ax /* Count of FATs */ mulw bpb_FAT_secs(%bp) /* FAT size in %dx:%ax */ shl $16,%edx xchg %ax,%dx /* FAT size now in %edx */ addl fat_sector(%bp), %edx /* Directory is after FATs */ pushl %edx /* Sector number of root dir */ push $0x1000 /* Read to 0x10000:0 */ pop %es /* Which we need in %es later */ push %es push %cs /* Offset zero */ /* Convert the root directory size to sectors */ push %dx mov bpb_root_dir_ents(%bp), %ax mov $0x20, %dx mul %dx divw bpb_bytes_per_sec(%bp) add $0xffff, %dx /* Set carry if remainder non-zero */ adc $0, %ax /* and round up the division */ pop %dx /* Read in the entire root directory */ push %ax /* Sectors in root directory */ cwtl addl %eax, %edx /* %edx now sector of first cluster */ call read_lba /* Read entire directory */ /* Scan directory for our file */ xor %di, %di scan_dir: mov $boot_filename, %si mov $11, %cx repz cmpsb je found_boot or $31,%di inc %di cmp %ch, %es:(%di) /* %ch is zero - test end of dir */ jz 1f decw bpb_root_dir_ents(%bp) jnz scan_dir 1: set_err(ERR_NO_BOOT) error: #ifdef TERSE_ERROR movb %al, errcod movw $errtxt, %si call message #else push %ax movw $errtxt, %si call message pop %si call message movw $newline, %si call message #endif 1: sti hlt jmp 1b found_boot: movzbl bpb_sec_per_clust(%bp), %eax movw %ax, sec_p_cl_w(%bp) add %ax, %ax subl %eax, %edx /* 1st file sector is cluster 2 */ 1: inc %cl /* Convert power of 2 ... */ shr $1, %ax /* ... to shift */ jnz 1b dec %cx dec %cx movw %es:(26-11)(%di), %ax /* Cluster number for file start */ push %es /* We increment the 'segment' ... */ pop %di /* ... after each read, offset is 0 */ read_data_block: mov %ax, %bx /* Save cluster number */ shl %cl, %eax /* Convert to sector number */ jz error /* Sanity bail-out */ add %edx, %eax pushl %eax /* Sector to read */ push %di /* Target address segment! */ push $0 push sec_p_cl_w(%bp) call read_lba /* Read a cluster */ /* Update read ptr for next cluster */ mov bpb_bytes_per_sec(%bp), %ax shr $4, %ax /* x86 segment count */ shl %cl, %ax /* for a cluster */ add %ax, %di /* Lookup FAT slot number in FAT table */ mov %bx, %ax /* Recover cluster number */ #if FAT_ENTRY_SIZE == 12 shr $1, %ax jc 1f add %ax, %bx mov fat_buffer(%bx), %ax and $0xf,%ah jmp 2f 1: add %ax, %bx mov fat_buffer(%bx), %ax shr $4, %ax 2: cmp $0x0fff, %ax jb read_data_block #else push %dx xor %dx, %dx divw bpb_bytes_per_sec(%bp) mov %dx, %bx /* Entry in FAT block */ pop %dx cmp %ax, fat_cache je lookup_fat /* We must read a different chunk of the FAT */ mov %ax, fat_cache cwtl shl $1, %ax addl fat_sector(%bp), %eax push %eax push %ds push $fat_buffer push $2 /* Always read 2 sectors of FAT */ call read_lba /* Now use low part of cluster number to index FAT sector */ lookup_fat: add %bx, %bx /* 2 bytes per entry... */ movzwl fat_buffer(%bx), %eax /* Next FAT slot */ cmp $0xfff0, %ax jb read_data_block #endif /* Found end of FAT chain - must be EOF - leap into loaded code */ mov $0x1000, %ax mov %ax, %es cmpl $X86_BOOT_MAGIC_2, %es:4 je magic_ok set_err(ERR_NO_BOOT_MAGIC_2) err1: jmp error /* Set parameters expected by /boot */ magic_ok: mov bpb_hidden_secs(%bp), %ebx /* ptn base sector */ movb -2(%bp), %dl /* disk number */ mov $boot_params + 4, %si push %es push $0 lret /* Read disk using on-stack int13-extension parameter block */ read_lba: pop %ax /* Save rtn addr */ pushw $16 /* Stack ctl block length */ mov %sp, %si /* Address ctl block */ push %ax /* restack rtn addr */ pushal /* Save everything except %si and %ax*/ mov -2(%bp), %dl /* Disk # saved on entry */ movb $0x42, %ah int $0x13 popal set_err(ERR_READ) jc err1 ret $12 /* Discard all except high LBA zeros */ /* * I hate #including source files, but pbr_magic below has to be at * the correct absolute address. * Clearly this could be done with a linker script. */ #include <message.S> #if 0 #include <dump_eax.S> dump_eax_buff = start #endif errtxt: .ascii "Error " /* runs into newline... */ errcod: .byte 0 /* ... if errcod set */ newline: .asciz "\r\n" #ifndef TERSE_ERROR ERR_READ: .asciz "Disk read" ERR_NO_BOOT: .asciz "No /boot" ERR_NOT_FAT16: .ascii "Not FAT" .word FAT_SIZE_STR .asciz " ptn" ERR_NO_BOOT_MAGIC_2: .asciz "No magic in /boot" #endif boot_filename: .ascii "BOOT " space: pbr_space = boot_params - . /* * Add magic number, with a zero sized patchable area - just in case something * finds it and tries to update the area. * Boot options can be set using 'installboot -e boot' so we don't need to * use any of our valuable bytes. */ . = _C_LABEL(start) + 0x1fe - 2 - 4 - 4 boot_params: .long X86_BOOT_MAGIC_FAT .long 1f - . 1: fat_cache: .word 0xffff /* Sector number in buffer */ . = _C_LABEL(start) + 0x1fe .word 0xaa55 fat_buffer: /* 2 sectors worth of FAT table */ /* 6k for FAT12 */
