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/*-
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 * Copyright (c) 2005, Kohsuke Ohtani
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 * All rights reserved.
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 *
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 * Redistribution and use in source and binary forms, with or without
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 * modification, are permitted provided that the following conditions
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 * are met:
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 * 1. Redistributions of source code must retain the above copyright
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 *    notice, this list of conditions and the following disclaimer.
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 * 2. Redistributions in binary form must reproduce the above copyright
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 *    notice, this list of conditions and the following disclaimer in the
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 *    documentation and/or other materials provided with the distribution.
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 * 3. Neither the name of the author nor the names of any co-contributors
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 *    may be used to endorse or promote products derived from this software
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 *    without specific prior written permission.
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 *
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 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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 * SUCH DAMAGE.
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 */
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/**
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 * bootsect.S - Boot sector for FAT
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 *
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 * The boot sector is 512 byte code to load the OS image. It is loaded
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 * to address 0:7c00 by POST BIOS.
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 * The boot sector searches the target file within the root directory of
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 * FAT file system, and loads it to predefined memory address. Then, it
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 * jumps to the first byte of the loaded image.
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 *
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 * All disk access are done by using BIOS Int13h interface. The BIOS
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 * parameter block (BPB) has the disk/FAT information, and it exists
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 * in the first portion of the FAT boot sector. It must be filled by the
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 * FAT format utility, or the prex kernel install utility (mkboot.com).
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 * This program assumes that correct BPB is stored in the boot sector.
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 *
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 * Limitation:
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 *  - Support only FAT12/16. FAT32 is not supported.
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 *
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 * Memory usage:
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 *  > 5000 - 6FFF ... Disk work area
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 *  > 7000 - 7BFF ... Stack
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 *  > 7C00 - 7DFF ... This boot sector
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 *  >10000 -      ... FAT cache
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 *  >20000 -      ... Data cache
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 *  >30000 -      ... Image load address
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 */
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.code16
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.text
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.align 1
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# Memory locations
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#define BOOT_STACK		0x7c00
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#define LOAD_ADDR		0x30000
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#define ENTRY_SEG		0x3000
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#define ENTRY_OFF		0x0000
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#define WORK_AREA		0x5000
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#define FAT_SEG			0x1000
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#define DATA_SEG		0x2000
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#define LOAD_MAX		0xA0000
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# FAT Directory entry
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#define F_NAME			0
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#define F_ATTR			11
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#define F_RESERVED		12
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#define F_TIME			22
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#define F_DATA			24
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#define F_CLUSTER		26
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#define F_SIZE			28
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#define DIR_SIZE		32
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#define DIRENT_PER_SECTOR	16
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# BIOS parameter block (BPB) location (%bp points to 0x7c00)
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#define OEM_ID			0x03(%bp)
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#define BYTE_PER_SECTOR		0x0b(%bp)
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#define SECT_PER_CLUSTER	0x0d(%bp)
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#define RESERVED_SECTORS	0x0e(%bp)
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#define NUM_OF_FATS		0x10(%bp)
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#define ROOT_ENTRIES		0x11(%bp)
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#define TOTAL_SECTORS		0x13(%bp)
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#define MEDIA_DESCRIPTOR	0x15(%bp)
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#define SECTORS_PER_FAT		0x16(%bp)
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#define SECTORS_PER_TRACK	0x18(%bp)
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#define HEADS			0x1a(%bp)
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#define HIDDEN_SECTORS		0x1c(%bp)
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#define BIG_TOTAL_SECTORS	0x20(%bp)
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#define PHYSICAL_DRIVE		0x24(%bp)
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#define EXT_BOOT_SIGNATURE	0x26(%bp)
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#define SERIAL_NO		0x27(%bp)
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#define VOLUME_ID		0x2b(%bp)
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#define FILE_SYS_ID		0x36(%bp)
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#define FILE_SYS_ID_NUM		0x3a(%bp)
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# Local data area (Note: These data will overlap the existing code)
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#define FAT_START		0x40(%bp)
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#define DATA_START		0x44(%bp)
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.global _boot
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#
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# Boot the system
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#
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_boot:
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	jmp	start				# Skip BPB
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	nop					# Nop is for DOS compatibility
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#
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# BPB
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#
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	.ascii	"PREX1.00"
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.fill 0x33, 1, 0				# Drive parameter must be
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						# filled by intaller
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#
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# Setup stack and segment registers
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#
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start:
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	cli
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	cld					# Clear direction flag
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	xorl	%eax, %eax			# Set EAX to zero
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	movw	%ax, %ds
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	movw	%ax, %es
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	movw	%ax, %ss
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	movw	$(BOOT_STACK), %sp
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	movw	%sp, %bp			# EBP = Bios Parameter Block
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	sti
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#
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# Display boot message
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#
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	movw	$load_msg, %si
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	movw	$21, %cx
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	call	puts
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#
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# Store disk information
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#
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	movl	HIDDEN_SECTORS, %ebx		# Get hidden sector
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	movw	RESERVED_SECTORS, %ax		# Add reserved sector
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	addl	%eax, %ebx			# High 16 bit of EAX is 0
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	movl	%ebx, FAT_START			# FAT start = hidden + reserved
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	movzbw	NUM_OF_FATS, %ax		# Normally 2
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	mulw	SECTORS_PER_FAT			# AX = Num of sector of FATs
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	addl	%ebx, %eax			# EAX = Start of root directory
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	movw	ROOT_ENTRIES, %bx
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	shrw	$4, %bx				# / 16 = DIRENT_PER_SECTOR
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	movw	%bx, %cx			# CX = Num of sectors for root directory
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	addl	%eax, %ebx			# DATA start = FAT start + root
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	movl	%ebx, DATA_START		# Start sector of data area
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#
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# Find the OS image in the root directory
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#
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						# EAX = Start sector of root
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next_sector:
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	pushw	%cx
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	movl	$(WORK_AREA), %ebx
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	pushw	%bx
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	call	read_sector			# Read 1 sector in root
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	popw	%di				# DI = dir_entry
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	movw	$(DIRENT_PER_SECTOR), %cx	# CX = directory count
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next_entry:
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	cmpb	$0, (%di)			# End of dir entry ?
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	je	error				# Not found
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	testb	$0x18, F_ATTR(%di)		# Subdir or Volume ?
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	jnz	not_file			# Skip it
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	pusha
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	movw	$11, %cx			# File name + ext = 11 byte
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	movw	$image_name, %si
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	repe					# Compare file name
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	cmpsb
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	popa
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	je	found_file
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not_file:
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	addw	$(DIR_SIZE), %di		# Check next directory entry
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	loop	next_entry
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	popw	%cx
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	loop	next_sector
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						# Fall through
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#
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# Error case
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#
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error:
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	movw	$err_msg, %si
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	movw	$5, %cx
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	call	puts
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hang:
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	hlt
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	jmp	hang				# Stop here
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#
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# Load image
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#
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found_file:
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	movzwl	F_CLUSTER(%di), %eax		# EAX = 1st cluster of loader
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	movl	$(LOAD_ADDR), %ebx		# EBX = 32bit load address
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load_next:
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	call	read_cluster			# Read cluster of loader
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	call	next_cluster			# Get next cluster# in EAX
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	jb	load_next			# EOF ?
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#
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# Turn fdd motor off
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#
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	movw	$0x3f2, %dx
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	xorb	%al, %al
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	outb	%al, %dx
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#
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# Jump to loaded image
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#
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	ljmp	$0x3000, $0x0
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#
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# Puts - Print string
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#
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# Entry:
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#   SI - Pointer to message string
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#   CX - Number of character
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#
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puts:
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	lodsb
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	movb	$0x0e, %ah
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	movw	$0x0007, %bx
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	int	$0x10
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	loop	puts
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	ret
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#
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# next_cluster - Return next cluster
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#
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# Entry:
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#   EAX - Current cluter#
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#
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# Exit:
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#   AF - End of cluster
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#   EAX - Next cluter#
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#
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# Modified:
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#   Flags,CX,EDX,SI,DI
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#
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next_cluster:
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	pushl	%ebx
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	movw	%ax, %di			# Save cluster# in DI
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	movw	$0xfff8, %si			# Set default EOF to FAT16
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	movl	%eax, %ecx
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	shll	$1, %eax			# * 2
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	cmpb	$0x36, FILE_SYS_ID_NUM		# ID is 'FAT16' ?
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	je	fat_16
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	addl	%ecx, %eax			# * 3
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	shrl	$1, %eax			# / 2
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	movw	$0xff8, %si			# EOF for FAT12
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fat_16:
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						# EAX - Offset of FAT entry
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	xorw	%dx, %dx
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	divw	BYTE_PER_SECTOR
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	addl	FAT_START, %eax			# EAX = Sector# for FAT
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						# DX = Offset in sector
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	movl	$(WORK_AREA), %ebx
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	pushw	%bx
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	call	read_sector			# Read 2 sector for border
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	call	read_sector			# data
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	popw	%bx
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	addw	%dx, %bx
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	movw	(%bx), %ax
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	cmpw	$0xfff8, %si			# FAT16 ?
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	je	chk_end
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	shrw	$1, %di
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	jc	odd_pos
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	andb	$0x0f, %ah
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	jmp	chk_end
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odd_pos:
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	shrw	$4, %ax
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chk_end:
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	cmpw	%si, %ax
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	popl	%ebx
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	ret
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#
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# read_cluster - Read one cluster
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#
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# Entry:
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#   EBX - 32-bit pointer to buffer
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#   EAX - Cluster number
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#
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# Exit:
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#   EBX - Point to next buffer
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#
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# Modified:
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#   flags,ECX,ECX,EDX
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#
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read_cluster:
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	pushl	%eax
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	decw	%ax				# Translate clust# to sec#
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	decw	%ax
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	xorl	%ecx, %ecx
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	movb	SECT_PER_CLUSTER, %cl
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	mull	%ecx
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	addl	DATA_START, %eax		# EAX = Read sec#
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						# CX = Read sector size
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read_loop:
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	call	read_sector
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	cmpl	$(LOAD_MAX), %ebx
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	jae	error
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	loop	read_loop
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	popl	%eax
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	ret
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#
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# read_sector - Read one sector
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#
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# Entry:
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#   EBX - 32-bit pointer to buffer
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#   EAX - Logical sector# to read
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#
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# Exit:
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#   EBX - Pointer to next buffer
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#   EAX - Next sector
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#
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# Modified:
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#   Flags
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#
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read_sector:
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	pushal
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	pushw	%ds
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	pushw	%es
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	movl	%eax, %esi			# ESI = buffer
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	movzwl	SECTORS_PER_TRACK, %ecx		# Get sec/track
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	xorl	%edx, %edx
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	divl	%ecx				# EAX = track#
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						# DX = sec#
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	movw	$(DATA_SEG), %cx		# Check in cache
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	leaw	last_data, %di
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	cmpl	DATA_START, %esi
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	jae	data_reqest
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	movw	$(FAT_SEG), %cx
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	leaw	last_fat, %di
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data_reqest:
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						# CX = Cached segment
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	pushal					# [DI] = Cached track
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	movw	%cx, %es
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	xorw	%bx, %bx			# ES:BX = Cache address
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	cmpl	(%di), %eax			# Last track ?
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	je	hit_cache
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	movl	%eax, (%di)			# Save current track#
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	call	read_track
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hit_cache:
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	popal
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	pushw	%es
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	popw	%ds
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	shlw	$9, %dx				# sec# * 512
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	movw	%dx, %si			# DS:SI = Offset in cache
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	movw	%bx, %di			# [EBX] -> ES:[DI]
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	andw	$0xf, %di
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	shrl	$4, %ebx
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	movw	%bx, %es
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	mov	$512, %cx			# Copy 1 sector
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	rep
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	movsb
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	popw	%es
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	popw	%ds
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	popal
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	addl	$512, %ebx			# Next buffer
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	incl	%eax				# Next sector
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	ret
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#
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# read_track - Read one track
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#
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# Entry:
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#   ES:[BX] - Pointer to buffer
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#   EAX	    - Track number to read
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#
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# Exit:
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#   None
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#
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# Modified:
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#   Flags,EAX,ECX,EDX
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#
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read_track:
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	movzwl	HEADS, %ecx			# Get num of head
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	xorl	%edx, %edx
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	divl	%ecx				# AX = cyl#
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						# DL = head#
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	movb	%al, %ch			# CH = cyl# (low 8 bits)
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	andb	$3, %ah
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	shlb	$6, %ah
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	orb	$1, %ah
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	movb	%ah, %cl			# CL[7:6] = cyl# (high 2 bits)
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						# CL[5:0] = sec# = 1
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	movb	%dl, %dh			# DH = Head#
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	movw	SECTORS_PER_TRACK, %ax		# AL = Num of sectors to read
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	movb	$2, %ah				# AH = 02h (Read Disk Sectors)
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	movb	PHYSICAL_DRIVE, %dl		# DL = Drive#
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	int	$0x13				# Invoke Disk BIOS
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	jc	error
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	ret
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#
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# Local Data
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#
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last_fat:	.long	0xffffffff
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last_data:	.long	0xffffffff
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load_msg:	.ascii	"Loading "
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image_name:	.ascii	"PREXOS     "
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crlf:		.byte	0x0a, 0x0d
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err_msg:	.ascii	"Error"
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.org	510
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		.word	0xaa55