mbr.S

#
# 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.
#

# 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