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+/****************************************************************\
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+
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+hdprefix.S Copyright (C) 2005 Per Dalgas Jakobsen
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+
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+This code has been inspired/derived by the OSLoader by Vladislav Aleksandrov.
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+http://www.programmersheaven.com/zone5/cat469/40546.htm.
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+
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+This software may be used and distributed according to the terms
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+of the GNU Public License (GPL), incorporated herein by reference.
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+
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+hdprefix.S is loaded at 0x0000:0x7c00 by the bios-startup routines.
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+
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+Actions performed by hdprefix:
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+1) Load the MBR to LOADSEG:0
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+2) Check which partition is active (or try first partition if none active)
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+3) Check wether LBA is supported.
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+3a) LBA
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+3a1) Load PAYLOAD_SECTS sectors from chosen partition to LOADSEG:0
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+3b) CHS (standard)
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+3b1) Load PAYLOAD_SECTS sectors from chosen partition to LOADSEG:0
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+4) Check loaded bootsector for BOOTMAGIC code.
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+5) Jump to payload LOADSEG:ENTRYPOINT.
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+
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+Output with failure points (!#):
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+---
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+Loading (!1)partition #
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+Std. BIOS(!2) | Ext. BIOS(!3)
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+Booting...(!4)
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+(!5)
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+---
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+
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+!1: Failed to load MBR with Int13,ah=2.
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+!2: Failed to load bootrecord+payload with Int13,ah=2.
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+!3: Failed to load bootrecord+payload with Int13,ah=42.
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+!4: Invalid BOOTMAGIC in loaded bootrecord.
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+!5: Jumping to payload.
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+
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+\*****************************************************************/
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+
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+.equ BOOTSEG, 0x07c0
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+.equ LOADSEG, 0x1000
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+.equ ENTRYPOINT, _start
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+
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+.equ BOOTMAGIC, 0x0aa55
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+
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+.equ partition_table, 0x1be
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+.equ partition_rec_size, 0x10
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+
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+.equ boot_ind, 0 /* 80h=active */
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+.equ start_head, 1
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+.equ start_sector, 2 /* bits 0-5 */
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+.equ start_cyl, 3 /* bits 8,9 in bits 6,7 of sector */
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+.equ os_ind, 4 /* os indicator */
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+.equ end_head, 5
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+.equ end_sector, 6 /* bits 0-5 */
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+.equ end_track, 7 /* bits 8,9 in bits 6,7 of sector */
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+.equ nsect, 8 /* sectors preceding partition */
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+.equ lenght, 0x0c /* length of partition in sectors */
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+
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+/-------------------------------------------------------------
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+
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+ .arch i386
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+ .text
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+ .section ".prefix", "ax", @progbits
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+ .code16
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+
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+bootstart:
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+ jmp $BOOTSEG,$_go /* reload cs:ip */
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+
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+
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+/****************************************************************/
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+/* support routines. */
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+/*--------------------------------------------------------------*/
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+_failed:
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+ movw $BOOTSEG,%ax
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+ movw %ax,%ds
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+ movw $_failed_msg_end-_failed_msg,%cx
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+ movw $_failed_msg,%si
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+ call _print_str
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+
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+ /* stop execution - should probably have option to auto-reboot after delay. */
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+_failed_loop:
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+ jmp _failed_loop
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+
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+/*--------------------------------------------------------------*/
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+_print_str:
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+ /* cx = count, ds:si = string. */
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+ movw $0x0007,%bx
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+ movb $0x0e,%ah
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+_print_loop:
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+ lodsb
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+ int $0x10
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+ loop _print_loop
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+ ret
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+
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+/*--------------------------------------------------------------*/
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+_print_char:
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+ /* al = char. */
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+ movw $0x0007,%bx
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+ movb $0x0e,%ah
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+ int $0x10
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+ ret
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+
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+/*--------------------------------------------------------------*/
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+_print_nl:
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+ /* - */
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+ movb $0x0d,%al
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+ call _print_char
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+ movb $0x0a,%al
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+ call _print_char
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+ ret
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+
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+/*--------------------------------------------------------------*/
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+_print_hex:
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+ /* dx = value */
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+ movb $0x0e,%ah /* write char, tty mode */
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+ movw $0x0007,%bx /* page 0, attribute 7 (normal) */
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+ call _print_digit
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+ call _print_digit
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+ call _print_digit
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+ /* fall through */
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+_print_digit:
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+ rolw $4,%dx /* rotate so that lowest 4 bits are used */
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+ movb $0x0f,%al /* mask for nibble */
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+ andb %dl,%al
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+ addb $0x90,%al /* convert al to ascii hex (four instructions) */
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+ daa
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+ adcb $0x40,%al
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+ daa
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+ int $0x10
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+ ret
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+
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+/****************************************************************/
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+
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+
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+_go:
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+ cli
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+ movw $BOOTSEG,%ax
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+ movw %ax,%ds
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+ movw %ax,%ss
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+ movw $0x2000,%sp /* good large stack. */
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+ sti
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+ cld
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+ movw $LOADSEG,%ax
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+ movw %ax,%es
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+
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+ movw $_load_msg_end-_load_msg,%cx
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+ movw $_load_msg,%si
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+ call _print_str
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+
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+/*--- load MBR so we can use its partition table. ---*/
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+ xorw %bx,%bx
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+ movw $0x0001,%cx /* chs: 0,0,1 */
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+ movb %bh,%dh /* - */
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+ movb $0x80,%dl
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+ movw $0x0201,%ax /* read one sector (MBR) */
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+ int $0x13
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+ jc _failed
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+
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+/*--- find the active partition ---*/
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+ movw $_part_msg_end-_part_msg,%cx
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+ movw $_part_msg,%si
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+ call _print_str
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+
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+ movw $partition_table,%di
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+ movw $4,%cx
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+_partition_loop:
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+ cmpb $0x80,%es:(%di) /* active? */
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+ je _partition_found
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+ addw $partition_rec_size,%di
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+ loop _partition_loop
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+
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+ /*- no partitions marked active - use 1. partition. */
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+ movw $partition_table,%di
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+ movw $4,%cx
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+
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+_partition_found:
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+ movb $'5',%al /* convert to ascii */
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+ subb %cl,%al
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+ call _print_char
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+ call _print_nl
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+
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+/*--- check for lba support ---*/
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+ movw $0x55aa,%bx
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+ movb $0x80,%dl
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+ movb $0x41,%ah
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+ int $0x13
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+ jc __bios
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+ cmpw $0x0aa55,%bx
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+ jnz __bios
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+ testb $1,%cl
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+ jz __bios
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+
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+/*--- use lba bios calls to read sectors ---*/
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+_lba:
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+ movw $_extbios_msg_end-_extbios_msg,%cx
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+ movw $_extbios_msg,%si
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+ call _print_str
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+
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+ movw %es:nsect(%di),%ax
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+ movw %ax,_bios_lba_low
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+ movw %es:nsect+2(%di),%ax
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+ movw %ax,_bios_lba_high
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+ movb $0x80,%dl
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+ movw $_disk_address_packet,%si
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+ movw $0x4200,%ax /* read */
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+ int $0x13
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+ jc _failed
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+ jmp __loaded
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+
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+/*--- use standard bios calls to read sectors ---*/
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+__bios:
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+ movw $_stdbios_msg_end-_stdbios_msg,%cx
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+ movw $_stdbios_msg,%si
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+ call _print_str
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+
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+ movw _disk_address_packet+2(,1),%ax /* only low byte is used. */
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+ xorw %bx,%bx
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+ movw %es:start_sector(%di),%cx
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+ movb %es:start_head(%di),%dh
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+ movb $0x80,%dl
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+ movb $0x02,%ah
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+ int $0x13
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+ jc _failed
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+
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+__loaded:
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+ movw $_boot_msg_end-_boot_msg,%cx
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+ movw $_boot_msg,%si
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+ call _print_str
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+
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+ /* check if it has a valid bootrecord. */
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+ cmpw $BOOTMAGIC,%es:510(,1)
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+ jne _failed
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+ call _print_nl
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+
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+ /* call the payload. */
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+ pushl $0 /* No parameters to preserve for exit path */
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+ pushw $0 /* Use prefix exit path mechanism */
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+ jmp $LOADSEG,$ENTRYPOINT
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+
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+ .section ".text16", "ax", @progbits
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+ .globl prefix_exit
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+prefix_exit:
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+ int $0x19 /* should try to boot machine */
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+ .globl prefix_exit_end
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+prefix_exit_end:
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+ .previous
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+
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+
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+/*--------------------------------------------------------------*/
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+
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+_load_msg: .ascii "Loading "
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+_load_msg_end:
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+_part_msg: .ascii "partition "
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+_part_msg_end:
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+_boot_msg: .ascii "Booting..."
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+_boot_msg_end:
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+_stdbios_msg: .ascii "Std. BIOS\r\n"
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+_stdbios_msg_end:
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+_extbios_msg: .ascii "Ext. BIOS\r\n"
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+_extbios_msg_end:
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+_failed_msg: .ascii "FAILED!!!\r\n"
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+_failed_msg_end:
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+
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+
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+/*--------------------------------------------------------------*/
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+
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+_disk_address_packet:
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+ .byte 0x10 /* size of the packet */
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+ .byte 0 /* reserved */
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+ .word _verbatim_size_sct /* number of sectors to read */
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+ .word 0x0000 /* offset */
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+ .word LOADSEG /* segment of buffer */
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+_bios_lba_low: .word 0
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+_bios_lba_high: .word 0
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+ .word 0
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+ .word 0
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+
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+ .rept 32
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+ .byte 0
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+ .endr
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+
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+
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+/*--- Partition table ------------------------------------------*/
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+
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+ .org 446, 0
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+ .rept 64
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+ .byte 0
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+ .endr
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+
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+
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+/*--- Magic code -----------------------------------------------*/
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+ .org 510, 0
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+ .word BOOTMAGIC
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+
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+/*** END ********************************************************/
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