mbr.s revision 62176
1# 2# Copyright (c) 1999 Robert Nordier 3# All rights reserved. 4# 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 8# such forms. 9# 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 13# purpose. 14# 15 16# $FreeBSD: head/sys/boot/i386/mbr/mbr.s 62176 2000-06-27 20:04:10Z jhb $ 17 18# A 512 byte MBR boot manager that simply boots the active partition. 19 20 .set LOAD,0x7c00 # Load address 21 .set EXEC,0x600 # Execution address 22 .set PT_OFF,0x1be # Partition table 23 .set MAGIC,0xaa55 # Magic: bootable 24 25 .set NHRDRV,0x475 # Number of hard drives 26 27 .globl start # Entry point 28 .code16 29 30# 31# Setup the segment registers for flat addressing and setup the stack. 32# 33start: cld # String ops inc 34 xorw %ax,%ax # Zero 35 movw %ax,%es # Address 36 movw %ax,%ds # data 37 movw %ax,%ss # Set up 38 movw $LOAD,%sp # stack 39# 40# Relocate ourself to a lower address so that we are out of the way when 41# we load in the bootstrap from the partition to boot. 42# 43 movw $main-EXEC+LOAD,%si # Source 44 movw $main,%di # Destination 45 movw $0x200-(main-start),%cx # Byte count 46 rep # Relocate 47 movsb # code 48# 49# Jump to the relocated code. 50# 51 jmp main-LOAD+EXEC # To relocated code 52# 53# Scan the partition table looking for an active entry. Note that %ch is 54# zero from the repeated string instruction above. We save the offset of 55# the active partition in %si and scan the entire table to ensure that only 56# one partition is marked active. 57# 58main: xorw %si,%si # No active partition 59 movw $partbl,%bx # Partition table 60 movb $0x4,%cl # Number of entries 61main.1: cmpb %ch,(%bx) # Null entry? 62 je main.2 # Yes 63 jg err_pt # If 0x1..0x7f 64 testw %si,%si # Active already found? 65 jnz err_pt # Yes 66 movw %bx,%si # Point to active 67main.2: addb $0x10,%bl # Till 68 loop main.1 # done 69 testw %si,%si # Active found? 70 jnz main.3 # Yes 71 int $0x18 # BIOS: Diskless boot 72# 73# Ok, we've found a possible active partition. Check to see that the drive 74# is a valid hard drive number. 75# 76main.3: cmpb $0x80,%dl # Drive valid? 77 jb main.4 # No 78 movb NHRDRV,%dh # Calculate the highest 79 addb $0x80,%dh # drive number available 80 cmpb %dh,%dl # Within range? 81 jb main.5 # Yes 82main.4: movb (%si),%dl # Load drive 83# 84# Ok, now that we have a valid drive and partition entry, load the CHS from 85# the partition entry and read the sector from the disk. 86# 87main.5: movw %sp,%di # Save stack pointer 88 movb 0x1(%si),%dh # Load head 89 movw 0x2(%si),%cx # Load cylinder:sector 90 movw $LOAD,%bx # Transfer buffer 91 cmpb $0xff,%dh # Might we need to use LBA? 92 jnz main.7 # No. 93 cmpw $0xffff,%cx # Do we need to use LBA? 94 jnz main.7 # No. 95 pushw %cx # Save %cx 96 pushw %bx # Save %bx 97 movw $0x55aa,%bx # Magic 98 movb $0x41,%ah # BIOS: EDD extensions 99 int $0x13 # present? 100 jc main.6 # No. 101 cmpw $0xaa55,%bx # Magic ok? 102 jne main.6 # No. 103 testb $0x1,%cl # Packet mode present? 104 jz main.6 # No. 105 popw %bx # Restore %bx 106 pushl $0x0 # Set the LBA 107 pushl 0x8(%si) # address 108 pushw %es # Set the address of 109 pushw %bx # the transfer buffer 110 pushw $0x1 # Read 1 sector 111 pushw $0x10 # Packet length 112 movw %sp,%si # Packer pointer 113 movw $0x4200,%ax # BIOS: LBA Read from disk 114 jmp main.8 # Skip the CHS setup 115main.6: popw %bx # Restore %bx 116 popw %cx # Restore %cx 117main.7: movw $0x201,%ax # BIOS: Read from disk 118main.8: int $0x13 # Call the BIOS 119 movw %di,%sp # Restore stack 120 jc err_rd # If error 121# 122# Now that we've loaded the bootstrap, check for the 0xaa55 signature. If it 123# is present, execute the bootstrap we just loaded. 124# 125 cmpw $MAGIC,0x1fe(%bx) # Bootable? 126 jne err_os # No 127 jmp *%bx # Invoke bootstrap 128# 129# Various error message entry points. 130# 131err_pt: movw $msg_pt,%si # "Invalid partition 132 jmp putstr # table" 133 134err_rd: movw $msg_rd,%si # "Error loading 135 jmp putstr # operating system" 136 137err_os: movw $msg_os,%si # "Missing operating 138 jmp putstr # system" 139# 140# Output an ASCIZ string to the console via the BIOS. 141# 142putstr.0: movw $0x7,%bx # Page:attribute 143 movb $0xe,%ah # BIOS: Display 144 int $0x10 # character 145putstr: lodsb # Get character 146 testb %al,%al # End of string? 147 jnz putstr.0 # No 148putstr.1: jmp putstr.1 # Await reset 149 150msg_pt: .asciz "Invalid partition table" 151msg_rd: .asciz "Error loading operating system" 152msg_os: .asciz "Missing operating system" 153 154 .org PT_OFF 155 156partbl: .fill 0x10,0x4,0x0 # Partition table 157 .word MAGIC # Magic number 158