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/********************************************************************************/
/* Rufus - The Reliable USB Formatting Utility, bootable MBR with user prompt */
/* */
/* Copyright (c) 2012 Pete Batard <pete@akeo.ie> */
/* */
/* This program is free software; you can redistribute it and/or modify it */
/* under the terms of the GNU General Public License as published by the Free */
/* Software Foundation, either version 3 of the License, or (at your option) */
/* any later version. */
/* */
/* This program is distributed in the hope that it will be useful, but WITHOUT */
/* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or */
/* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for */
/* more details. */
/* */
/* You should have received a copy of the GNU General Public License along with */
/* this program; if not, see <http://www.gnu.org/licenses/>. */
/* */
/********************************************************************************/
/********************************************************************************/
/* GNU Assembler Settings: */
/********************************************************************************/
.intel_syntax noprefix # Use Intel assembler syntax (same as IDA Pro)
.code16 # MBR code is executed in x86 CPU real/16 bit mode
/********************************************************************************/
/********************************************************************************/
/* Constants: */
/********************************************************************************/
DOT_TIMEOUT = 0x12 # Number of RTC interrupts for ~ 1s
DOT_NUMBER = 0x04 # Number of dots to be printed before timeout
MBR_ADDR = 0x7c00 # Same address for both original MBR and copy
MBR_SIZE = 0x200
MBR_RESERVED = 0x1b8 # Start of the reserved section (partition table, etc.)
PT_MAX = 0x04 # Number of partition entries in the partition table
PT_ENTRY_SIZE = 0x10 # Size of a partition entry in the partition table
INT_RTC = 0x08
INT_DSK = 0x13
DEBUG = 0 # Set to 1 to debug INT13h (shows AH and DL values)
/********************************************************************************/
/* MBR: this section must reside at 0x00007c00, and be exactly 512 bytes */
/********************************************************************************/
.section main, "ax"
.globl mbr # label must be declared global for the linker
mbr:
# We must be able to reuse the original memory location that the BIOS copied
# the MBR to (0x00007c000), so our first task is to copy our MBR data to another
# location, and run our code from there
inc cx # Register fixup
dec bx
inc bp
dec di
cld
xor ax, ax
cli # Disable interrupts when fiddling with the stack
mov ss, ax # First order of the day it to set up the stack
mov sp, MBR_ADDR # This places the stack right before the original MBR
sti
mov si, sp # While we have that value handy
mov di, sp
push ds # Now that we have a stack...
push es
mov ds, ax # Original MBR is in segment 0
mov bx, 0x0413 # McAfee thinks we are a virus if we use 413 directly...
mov ax,[bx]
dec ax # Remove 1KB from the RAM for our copy
mov [bx],ax
shl ax, 6 # Convert to segment address
sub ax, MBR_ADDR>>4 # For convenience of disassembly, so that irrespective
mov es, ax # of the segment, our base address remains MBR_ADDR
mov cx, MBR_SIZE # Move our code out of the way
rep movsb
push ax # Push the CS:IP for our retf
push offset 0f
retf
# ---------------------------------------------------------------------------
# From this point forward, we are running the copy at the same base but different segment
0: mov ds, ax # AX = ES = CS, only DS points back to old seg => fix this
xor ebx, ebx # Sector #1 in 64 bit address mode (#0)
mov es, bx # ES remains set to segment 0 from here on
inc cx # Sector #1 in CHS address mode (#1) (and CX = 0 from rep movsb)
mov dx, 0x0081 # drive number (DL), track 0 (DH)
call read_sector
jb boot_usb # If we couldn't get data => just boot USB
read_success:
mov bx, offset partition_table
mov cx, PT_MAX
check_table: # check the partition table for an active (bootable) partition
cmpb es:[bx], 0x00
jl found_active # 0x80 or greater means the partition is active
jnz invalid_table # anything between 0x01 and 0x7f is invalid
add bx, PT_ENTRY_SIZE # next partition
loop check_table
invalid_table:
jmp boot_usb
found_active:
mov si, offset prompt_string
call print_string # Prompt the user
call flush_keyboard
set_rtc_int_vect: # Set the interrupt vector for CMOS real-time clock
mov dx, offset rtc_interrupt
mov si, offset rtc_interrupt_org
call set_int_vect
wait_for_keyboard:
mov ah, 0x01
int 0x16 # KEYBOARD - CHECK BUFFER, DO NOT CLEAR
jnz boot_usb_rem_rtc # Z is clear when characters are present in the buffer
mov ah, 0x02
int 0x16 # KEYBOARD - GET SHIFT STATUS
and al, 0x04 # AL = shift status bits
jnz boot_usb
cmpb ds:counter_dot, 0x00
jg check_timeout
print_dot: # Every so often, we print a dot
mov si, offset dot_string
call print_string
movb ds:counter_dot, DOT_TIMEOUT
check_timeout:
cmpb ds:counter_timeout, 0x00
jnz wait_for_keyboard
boot_fixed_disk: # Timeout occurred => boot second bootable disk (non USB)
call restore_rtc_vect # Remove our RTC override
movb ds:partition_table, 0x81 # target we want to swap with 0x80
push 0x0080
boot_drive:
mov dx, offset dsk_interrupt # Set interrupt override for int13h
mov si, offset dsk_interrupt_org
call set_int_vect
pop dx # retrieve disk index to feed BR
pop es
pop ds
jmp 0:MBR_ADDR
# ---------------------------------------------------------------------------
boot_usb_rem_rtc: # Boot USB drive (0x80)
call restore_rtc_vect # Remove our RTC override
boot_usb:
call flush_keyboard # Make sure the keyboard buffer is clear
mov bx, offset partition_table
mov dx, ds:[bx]
push dx
mov dl, 0x80 # Override disk number, as we're not using our int yet
mov cx, ds:[bx+2]
mov ebx, ds:[bx+8] # Must come last since it modifies BX
call read_sector
jnb boot_drive
pop es # failed to read PBR from USB - exit back to BIOS
pop ds
retf
/********************************************************************************/
/* Subroutines */
/********************************************************************************/
read_sector: # Read a single sector in either CHS or LBA mode
# EBX = LBA sector address (32 bit), CH = track
# CL = sector, DL = drive number, DH = head
pusha # save all registers
mov ah, 0x41
mov bx, 0x55aa
int 0x13
jb no_ext # failure to get ext
cmp bx, 0xaa55
jnz no_ext
test cx, 1 # is packet access supported?
jz no_ext
ext: # http://en.wikipedia.org/wiki/INT_13H#INT_13h_AH.3D42h:_Extended_Read_Sectors_From_Drive
popa
push ds
xor eax, eax
mov ds, ax # We'll use the stack for DAP, which is in seg 0
push eax # bits 32-63 of sector address (set to 0)
push ebx # bits 0-31 of sector address (EBX)
push ax # destination segment
push MBR_ADDR # destination address
inc ax
push ax # number of sectors to be read (1)
push 0x0010 # size of DAP struct
mov si, sp # DAP address (= stack)
mov ah, 0x42 # Extended Read Sectors From Drive
int 0x13
lahf
add sp,0x10
sahf
pop ds
ret
no_ext: # http://en.wikipedia.org/wiki/INT_13H#INT_13h_AH.3D02h:_Read_Sectors_From_Drive
popa
mov bx, MBR_ADDR # CL, CH, DL, DH, ES are already set
mov ax, 0x0201
int 0x13
ret
# ---------------------------------------------------------------------------
set_int_vect: # Set the interrupt vector
cli # SI = pointer to backup vector (must contain the interrupt #)
mov bx, ds:[si]
mov eax, es:[bx] # Backup the original vector (ES = 0)
mov ds:[si], eax
mov es:[bx], dx
mov es:[bx+2], cs
sti
ret
# ---------------------------------------------------------------------------
restore_rtc_vect: # Restore the interrupt vector for RTC
cli
mov bx, INT_RTC*4
mov eax, ds:rtc_interrupt_org
mov es:[bx], eax
sti
ret
# ---------------------------------------------------------------------------
flush_keyboard: # Flush the keyboard buffer
mov ah, 0x01
int 0x16 # KEYBOARD - CHECK BUFFER, DO NOT CLEAR
jz 0f # Z is set if no character in buffer
mov ah, 0x00
int 0x16 # KEYBOARD - READ CHAR FROM BUFFER
loop flush_keyboard
0: ret
# ---------------------------------------------------------------------------
print_string: # Print NUL terminated string in DS:SI to console
lodsb
cmp al, 0x00
jz 0f
mov ah, 0x0e
mov bx, 0x0007 # BH = display page, BL = foreground color
int 0x10 # VIDEO - WRITE CHARACTER AND ADVANCE CURSOR
jmp print_string
0: ret
# ---------------------------------------------------------------------------
disk_swap: # Swap disk according to part table entry
push ax
mov al, cs:partition_table
cmp dl, 0x80
jne 0f
mov dl, al # 0x80 -> cs:pt
jmp 1f
0: cmp dl, al # cs:pt -> 0x80
jne 1f
mov dl, 0x80
1: pop ax
ret
# ---------------------------------------------------------------------------
.if DEBUG
print_hex: # Hex dump of AH,DL
pusha
mov cx, 0x04
mov dh, ah
0: rol dx, 0x04
mov ax, 0xe0f
and al, dl
daa
add al, 0xF0
adc al, 0x40
int 0x10
loop 0b
popa
ret
.endif
/********************************************************************************/
/* Interrupt overrides */
/********************************************************************************/
# RTC (INT 8) interrupt override
rtc_interrupt:
cli
cmpb cs:counter_timeout, 0x00
jz 0f # Don't decrement counters if timeout expired
decb cs:counter_dot
decb cs:counter_timeout
rtc_interrupt_org = .+1 # Same trick used by the LILO mapper
0: jmp 0:INT_RTC*4 # These CS:IP values will be changed at runtime
# ---------------------------------------------------------------------------
# DISK (INT 13h) interrupt override
dsk_interrupt:
pushf
.if DEBUG
call print_hex
.endif
# Some machines (eg. IBM T43p) have a BIOS that reenters INT 13h to issue
# an SCSI passthrough (AH = 50h). Therefore swapping the drive on each call
# would result in failure. To ensure that the disk is only swapped once
# we keep a counter, and swap only if that counter is 0.
# NB: If concurrent INT 13h calls are issued, this approach will break
incb cs:already_mapped
jnz 0f
call disk_swap
dsk_interrupt_org = .+1
0: call 0:INT_DSK*4 # These CS:IP values will be changed at runtime
# NB: subcommands 0x08 and 0x15 (disk props) modify DL, but they only
# do so to return the number of drives => unless your computer has more
# than 128 drives, disk_swap will not touch those values.
pushf # Don't modify the returned flags
decb cs:already_mapped
jns 0f
call disk_swap
0: popf
retf 2
/********************************************************************************/
/* Data section */
/********************************************************************************/
already_mapped: .byte 0xff
counter_timeout:.byte DOT_NUMBER*DOT_TIMEOUT + 1
counter_dot: .byte DOT_TIMEOUT
.if !DEBUG
prompt_string: .string "\r\nPress any key to boot from USB."
.else
prompt_string: .string "USB."
.endif
dot_string = .-2 # Reuse the end of previous string
/********************************************************************************/
/* From offset 0x1b8, the MBR contains the partition table and signature data */
/********************************************************************************/
.org MBR_RESERVED
disk_signature:
.space 0x04
filler:
.space 0x02
partition_table:
.space PT_ENTRY_SIZE * PT_MAX
mbr_signature:
.word 0xAA55
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