2 # Copyright (c) 1999 Robert Nordier
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
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
18 # A 512 byte MBR boot manager that simply boots the active partition.
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 .set FL_PACKET,0x80 # Flag: try EDD
26 .set NHRDRV,0x475 # Number of hard drives
28 .globl start # Entry point
32 # Setup the segment registers for flat addressing and setup the stack.
34 start: cld # String ops inc
36 movw %ax,%es # Address
39 movw $LOAD,%sp # stack
41 # Relocate ourself to a lower address so that we are out of the way when
42 # we load in the bootstrap from the partition to boot.
44 movw $main-EXEC+LOAD,%si # Source
45 movw $main,%di # Destination
46 movw $0x200-(main-start),%cx # Byte count
50 # Jump to the relocated code.
52 jmp main-LOAD+EXEC # To relocated code
54 # Scan the partition table looking for an active entry. Note that %ch is
55 # zero from the repeated string instruction above. We save the offset of
56 # the active partition in %si and scan the entire table to ensure that only
57 # one partition is marked active.
59 main: xorw %si,%si # No active partition
60 movw $partbl,%bx # Partition table
61 movb $0x4,%cl # Number of entries
62 main.1: cmpb %ch,(%bx) # Null entry?
64 jg err_pt # If 0x1..0x7f
65 testw %si,%si # Active already found?
67 movw %bx,%si # Point to active
68 main.2: addb $0x10,%bl # Till
70 testw %si,%si # Active found?
72 int $0x18 # BIOS: Diskless boot
74 # Ok, we've found a possible active partition. Check to see that the drive
75 # is a valid hard drive number.
77 main.3: cmpb $0x80,%dl # Drive valid?
79 movb NHRDRV,%dh # Calculate the highest
80 addb $0x80,%dh # drive number available
81 cmpb %dh,%dl # Within range?
83 main.4: movb (%si),%dl # Load drive
85 # Ok, now that we have a valid drive and partition entry, load the CHS from
86 # the partition entry and read the sector from the disk.
88 main.5: movw %sp,%di # Save stack pointer
89 movb 0x1(%si),%dh # Load head
90 movw 0x2(%si),%cx # Load cylinder:sector
91 movw $LOAD,%bx # Transfer buffer
92 testb $FL_PACKET,flags # Try EDD?
96 movw $0x55aa,%bx # Magic
97 movb $0x41,%ah # BIOS: EDD extensions
100 cmpw $0xaa55,%bx # Magic ok?
102 testb $0x1,%cl # Packet mode present?
104 popw %bx # Restore %bx
105 pushl $0x0 # Set the LBA
106 pushl 0x8(%si) # address
107 pushw %es # Set the address of
108 pushw %bx # the transfer buffer
109 pushw $0x1 # Read 1 sector
110 pushw $0x10 # Packet length
111 movw %sp,%si # Packer pointer
112 movw $0x4200,%ax # BIOS: LBA Read from disk
113 jmp main.8 # Skip the CHS setup
114 main.6: popw %bx # Restore %bx
115 popw %cx # Restore %cx
116 main.7: movw $0x201,%ax # BIOS: Read from disk
117 main.8: int $0x13 # Call the BIOS
118 movw %di,%sp # Restore stack
121 # Now that we've loaded the bootstrap, check for the 0xaa55 signature. If it
122 # is present, execute the bootstrap we just loaded.
124 cmpw $MAGIC,0x1fe(%bx) # Bootable?
126 jmp *%bx # Invoke bootstrap
128 # Various error message entry points.
130 err_pt: movw $msg_pt,%si # "Invalid partition
133 err_rd: movw $msg_rd,%si # "Error loading
134 jmp putstr # operating system"
136 err_os: movw $msg_os,%si # "Missing operating
139 # Output an ASCIZ string to the console via the BIOS.
141 putstr.0: movw $0x7,%bx # Page:attribute
142 movb $0xe,%ah # BIOS: Display
143 int $0x10 # character
144 putstr: lodsb # Get character
145 testb %al,%al # End of string?
147 putstr.1: jmp putstr.1 # Await reset
149 msg_pt: .asciz "Invalid partition table"
150 msg_rd: .asciz "Error loading operating system"
151 msg_os: .asciz "Missing operating system"
154 flags: .byte FLAGS # Flags
156 partbl: .fill 0x10,0x4,0x0 # Partition table
157 .word MAGIC # Magic number