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# A 512 byte MBR boot manager that simply boots the active partition. 17 18 .set LOAD,0x7c00 # Load address 19 .set EXEC,0x600 # Execution address 20 .set PT_OFF,0x1be # Partition table 21 .set MAGIC,0xaa55 # Magic: bootable 22 .set FL_PACKET,0x80 # Flag: try EDD 23 24 .set NHRDRV,0x475 # Number of hard drives 25 26 .globl start # Entry point 27 .code16 28 29# 30# Setup the segment registers for flat addressing and setup the stack. 31# 32start: cld # String ops inc 33 xorw %ax,%ax # Zero 34 movw %ax,%es # Address 35 movw %ax,%ds # data 36 movw %ax,%ss # Set up 37 movw $LOAD,%sp # stack 38# 39# Relocate ourself to a lower address so that we are out of the way when 40# we load in the bootstrap from the partition to boot. 41# 42 movw $main-EXEC+LOAD,%si # Source 43 movw $main,%di # Destination 44 movw $0x200-(main-start),%cx # Byte count 45 rep # Relocate 46 movsb # code 47# 48# Jump to the relocated code. 49# 50 jmp main-LOAD+EXEC # To relocated code 51# 52# Scan the partition table looking for an active entry. Note that %ch is 53# zero from the repeated string instruction above. We save the offset of 54# the active partition in %si and scan the entire table to ensure that only 55# one partition is marked active. 56# 57main: xorw %si,%si # No active partition 58 movw $partbl,%bx # Partition table 59 movb $0x4,%cl # Number of entries 60main.1: cmpb %ch,(%bx) # Null entry? 61 je main.2 # Yes 62 jg err_pt # If 0x1..0x7f 63 testw %si,%si # Active already found? 64 jnz err_pt # Yes 65 movw %bx,%si # Point to active 66main.2: addb $0x10,%bl # Till 67 loop main.1 # done 68 testw %si,%si # Active found? 69 jnz main.3 # Yes 70 int $0x18 # BIOS: Diskless boot 71# 72# Ok, we've found a possible active partition. Check to see that the drive 73# is a valid hard drive number. 74# 75main.3: cmpb $0x80,%dl # Drive valid? 76 jb main.4 # No 77 movb NHRDRV,%dh # Calculate the highest 78 addb $0x80,%dh # drive number available 79 cmpb %dh,%dl # Within range? 80 jb main.5 # Yes 81main.4: movb (%si),%dl # Load drive 82# 83# Ok, now that we have a valid drive and partition entry, load the CHS from 84# the partition entry and read the sector from the disk. 85# 86main.5: movw %sp,%di # Save stack pointer 87 movb 0x1(%si),%dh # Load head 88 movw 0x2(%si),%cx # Load cylinder:sector 89 movw $LOAD,%bx # Transfer buffer 90 testb $FL_PACKET,flags # Try EDD? 91 jz main.7 # No. 92 pushw %cx # Save %cx 93 pushw %bx # Save %bx 94 movw $0x55aa,%bx # Magic 95 movb $0x41,%ah # BIOS: EDD extensions 96 int $0x13 # present? 97 jc main.6 # No. 98 cmpw $0xaa55,%bx # Magic ok? 99 jne main.6 # No. 100 testb $0x1,%cl # Packet mode present? 101 jz main.6 # No. 102 popw %bx # Restore %bx 103 pushl $0x0 # Set the LBA 104 pushl 0x8(%si) # address 105 pushw %es # Set the address of 106 pushw %bx # the transfer buffer 107 pushw $0x1 # Read 1 sector 108 pushw $0x10 # Packet length 109 movw %sp,%si # Packer pointer 110 movw $0x4200,%ax # BIOS: LBA Read from disk 111 jmp main.8 # Skip the CHS setup 112main.6: popw %bx # Restore %bx 113 popw %cx # Restore %cx 114main.7: movw $0x201,%ax # BIOS: Read from disk 115main.8: int $0x13 # Call the BIOS 116 movw %di,%sp # Restore stack 117 jc err_rd # If error 118# 119# Now that we've loaded the bootstrap, check for the 0xaa55 signature. If it 120# is present, execute the bootstrap we just loaded. 121# 122 cmpw $MAGIC,0x1fe(%bx) # Bootable? 123 jne err_os # No 124 jmp *%bx # Invoke bootstrap 125# 126# Various error message entry points. 127# 128err_pt: movw $msg_pt,%si # "Invalid partition 129 jmp putstr # table" 130 131err_rd: movw $msg_rd,%si # "Error loading 132 jmp putstr # operating system" 133 134err_os: movw $msg_os,%si # "Missing operating 135 jmp putstr # system" 136# 137# Output an ASCIZ string to the console via the BIOS. 138# 139putstr.0: movw $0x7,%bx # Page:attribute 140 movb $0xe,%ah # BIOS: Display 141 int $0x10 # character 142putstr: lodsb # Get character 143 testb %al,%al # End of string? 144 jnz putstr.0 # No 145putstr.1: jmp putstr.1 # Await reset 146 147msg_pt: .asciz "Invalid partition table" 148msg_rd: .asciz "Error loading operating system" 149msg_os: .asciz "Missing operating system" 150 151 .org PT_OFF-1,0x90 152flags: .byte FLAGS # Flags 153 154partbl: .fill 0x10,0x4,0x0 # Partition table 155 .word MAGIC # Magic number 156