mbr.s revision 62146
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 62146 2000-06-26 23:58:04Z 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 NDRIVE,0x8 # Drives to support 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 cli # Disable interrupts 38 movw %ax,%ss # Set up 39 movw $LOAD,%sp # stack 40 sti # Enable interrupts 41# 42# Relocate ourself to a lower address so that we are out of the way when 43# we load in the bootstrap from the partition to boot. 44# 45 movw $main-EXEC+LOAD,%si # Source 46 movw $main,%di # Destination 47 movw $0x200-(main-start),%cx # Byte count 48 rep # Relocate 49 movsb # code 50# 51# Jump to the relocated code. 52# 53 jmp main-LOAD+EXEC # To relocated code 54# 55# Scan the partition table looking for an active entry. Note that %ch is 56# zero from the repeated string instruction above. We save the offset of 57# the active partition in %si and scan the entire table to ensure that only 58# one partition is marked active. 59# 60main: xorw %si,%si # No active partition 61 movw $partbl,%bx # Partition table 62 movb $0x4,%cl # Number of entries 63main.1: cmpb %ch,(%bx) # Null entry? 64 je main.2 # Yes 65 jg err_pt # If 0x1..0x7f 66 testw %si,%si # Active already found? 67 jnz err_pt # Yes 68 movw %bx,%si # Point to active 69main.2: addb $0x10,%bl # Till 70 loop main.1 # done 71 testw %si,%si # Active found? 72 jnz main.3 # Yes 73 int $0x18 # BIOS: Diskless boot 74# 75# Ok, we've found a possible active partition. Check to see that the drive 76# is a valid hard drive number. XXX - We assume that there are up to 8 valid 77# hard drives, regardless of how many are actually installed. Yuck. 78# 79main.3: cmpb $0x80,%dl # Drive valid? 80 jb main.4 # No 81 cmpb $0x80+NDRIVE,%dl # Within range? 82 jb main.5 # Yes 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.4: movb (%si),%dl # Load drive 88main.5: movb 0x1(%si),%dh # Load head 89 movw 0x2(%si),%cx # Load cylinder:sector 90 movw $LOAD,%bx # Transfer buffer 91 movw $0x201,%ax # BIOS: Read from 92 int $0x13 # disk 93 jc err_rd # If error 94# 95# Now that we've loaded the bootstrap, check for the 0xaa55 signature. If it 96# is present, execute the bootstrap we just loaded. 97# 98 cmpw $MAGIC,0x1fe(%bx) # Bootable? 99 jne err_os # No 100 jmp *%bx # Invoke bootstrap 101# 102# Various error message entry points. 103# 104err_pt: movw $msg_pt,%si # "Invalid partition 105 jmp putstr # table" 106 107err_rd: movw $msg_rd,%si # "Error loading 108 jmp putstr # operating system" 109 110err_os: movw $msg_os,%si # "Missing operating 111 jmp putstr # system" 112# 113# Output an ASCIZ string to the console via the BIOS. 114# 115putstr.0: movw $0x7,%bx # Page:attribute 116 movb $0xe,%ah # BIOS: Display 117 int $0x10 # character 118putstr: lodsb # Get character 119 testb %al,%al # End of string? 120 jnz putstr.0 # No 121putstr.1: jmp putstr.1 # Await reset 122 123msg_pt: .asciz "Invalid partition table" 124msg_rd: .asciz "Error loading operating system" 125msg_os: .asciz "Missing operating system" 126 127 .org PT_OFF 128 129partbl: .fill 0x10,0x4,0x0 # Partition table 130 .word MAGIC # Magic number 131