#
# Copyright (c) 1999 Robert Nordier
# All rights reserved.
#
# Redistribution and use in source and binary forms are freely
# permitted provided that the above copyright notice and this
# paragraph and the following disclaimer are duplicated in all
# such forms.
#
# This software is provided "AS IS" and without any express or
# implied warranties, including, without limitation, the implied
# warranties of merchantability and fitness for a particular
# purpose.
#
# $FreeBSD$
# A 512 byte MBR boot manager that simply boots the active partition.
.set LOAD,0x7c00 # Load address
.set EXEC,0x600 # Execution address
.set PT_OFF,0x1be # Partition table
.set MAGIC,0xaa55 # Magic: bootable
.set FL_PACKET,0x80 # Flag: try EDD
.set NHRDRV,0x475 # Number of hard drives
.globl start # Entry point
.code16
#
# Setup the segment registers for flat addressing and setup the stack.
#
start: cld # String ops inc
xorw %ax,%ax # Zero
movw %ax,%es # Address
movw %ax,%ds # data
movw %ax,%ss # Set up
movw $LOAD,%sp # stack
#
# Relocate ourself to a lower address so that we are out of the way when
# we load in the bootstrap from the partition to boot.
#
movw $main-EXEC+LOAD,%si # Source
movw $main,%di # Destination
movw $0x200-(main-start),%cx # Byte count
rep # Relocate
movsb # code
#
# Jump to the relocated code.
#
jmp main-LOAD+EXEC # To relocated code
#
# Scan the partition table looking for an active entry. Note that %ch is
# zero from the repeated string instruction above. We save the offset of
# the active partition in %si and scan the entire table to ensure that only
# one partition is marked active.
#
main: xorw %si,%si # No active partition
movw $partbl,%bx # Partition table
movb $0x4,%cl # Number of entries
main.1: cmpb %ch,(%bx) # Null entry?
je main.2 # Yes
jg err_pt # If 0x1..0x7f
testw %si,%si # Active already found?
jnz err_pt # Yes
movw %bx,%si # Point to active
main.2: addb $0x10,%bl # Till
loop main.1 # done
testw %si,%si # Active found?
jnz main.3 # Yes
int $0x18 # BIOS: Diskless boot
#
# Ok, we've found a possible active partition. Check to see that the drive
# is a valid hard drive number.
#
main.3: cmpb $0x80,%dl # Drive valid?
jb main.4 # No
movb NHRDRV,%dh # Calculate the highest
addb $0x80,%dh # drive number available
cmpb %dh,%dl # Within range?
jb main.5 # Yes
main.4: movb (%si),%dl # Load drive
#
# Ok, now that we have a valid drive and partition entry, load the CHS from
# the partition entry and read the sector from the disk.
#
main.5: movw %sp,%di # Save stack pointer
movb 0x1(%si),%dh # Load head
movw 0x2(%si),%cx # Load cylinder:sector
movw $LOAD,%bx # Transfer buffer
testb $FL_PACKET,flags # Try EDD?
jz main.7 # No.
pushw %cx # Save %cx
pushw %bx # Save %bx
movw $0x55aa,%bx # Magic
movb $0x41,%ah # BIOS: EDD extensions
int $0x13 # present?
jc main.6 # No.
cmpw $0xaa55,%bx # Magic ok?
jne main.6 # No.
testb $0x1,%cl # Packet mode present?
jz main.6 # No.
popw %bx # Restore %bx
pushl $0x0 # Set the LBA
pushl 0x8(%si) # address
pushw %es # Set the address of
pushw %bx # the transfer buffer
pushw $0x1 # Read 1 sector
pushw $0x10 # Packet length
movw %sp,%si # Packer pointer
movw $0x4200,%ax # BIOS: LBA Read from disk
jmp main.8 # Skip the CHS setup
main.6: popw %bx # Restore %bx
popw %cx # Restore %cx
main.7: movw $0x201,%ax # BIOS: Read from disk
main.8: int $0x13 # Call the BIOS
movw %di,%sp # Restore stack
jc err_rd # If error
#
# Now that we've loaded the bootstrap, check for the 0xaa55 signature. If it
# is present, execute the bootstrap we just loaded.
#
cmpw $MAGIC,0x1fe(%bx) # Bootable?
jne err_os # No
jmp *%bx # Invoke bootstrap
#
# Various error message entry points.
#
err_pt: movw $msg_pt,%si # "Invalid partition
jmp putstr # table"
err_rd: movw $msg_rd,%si # "Error loading
jmp putstr # operating system"
err_os: movw $msg_os,%si # "Missing operating
jmp putstr # system"
#
# Output an ASCIZ string to the console via the BIOS.
#
putstr.0: movw $0x7,%bx # Page:attribute
movb $0xe,%ah # BIOS: Display
int $0x10 # character
putstr: lodsb # Get character
testb %al,%al # End of string?
jnz putstr.0 # No
putstr.1: jmp putstr.1 # Await reset
msg_pt: .asciz "Invalid partition table"
msg_rd: .asciz "Error loading operating system"
msg_os: .asciz "Missing operating system"
.org PT_OFF-1,0x90
flags: .byte FLAGS # Flags
partbl: .fill 0x10,0x4,0x0 # Partition table
.word MAGIC # Magic number
