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[mbr] add LBA sector readout, as required by some BIOSes

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* I'm looking at you, Dell Optiplex!
* also fix boot in case no secondary bootable drive exists
* also adds a readme
This commit is contained in:
Pete Batard 2012-03-20 20:31:54 +00:00
parent 0f57ed1684
commit 1b7f88eb99
4 changed files with 159 additions and 94 deletions
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192
res/mbr/mbr.S
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@ -1,7 +1,7 @@
/********************************************************************************/
/* Rufus - The Reliable USB Formatting Utility, bootable USB MBR */
/* */
/* Copyright (c) 2011 Pete Batard <pete@akeo.ie> */
/* 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 */
@ -26,6 +26,7 @@
.code16 # MBR code is executed in x86 CPU real/16 bit mode
/********************************************************************************/
/********************************************************************************/
/* Constants: */
/********************************************************************************/
@ -34,14 +35,14 @@ 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.)
BUF_SEGMENT = 0x3000 # Segment for the buffer
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
/********************************************************************************/
/* mbr: this section must reside at 0x00007c00, and be exactly 512 bytes */
/* 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
@ -65,9 +66,9 @@ mbr:
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+0]
mov ax,[bx]
dec ax # Remove 1KB from the RAM for our copy
mov [bx+0],ax
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
@ -79,60 +80,33 @@ mbr:
# ---------------------------------------------------------------------------
# From this point forward, we are running the copy at a different segment from seg 0
# 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
push es
xor ax, ax
mov es, ax
mov bx, MBR_ADDR # TODO optimize
mov ax, 0x0201
mov cx, 0x0001
mov dx, 0x0081 # Check next bootable disk
int 0x13 # DISK - READ SECTORS INTO MEMORY
# AL = number of sectors to read, CH = track, CL = sector
# DH = head, DL = drive, ES:BX -> buffer to fill
# Return: CF set on error, AH = status, AL = number of sectors read
jnb read_success
read_failure: # If we couldn't get data for second bootable, just boot USB
pop es
jmp boot_usb_no_rtc
push 0
pop es # ES remains set to segment 0 from here on
xor ebx, ebx # Sector #1 in 64 bit address mode (#0)
mov cx, 0x0001 # Sector #1 in CHS address mode (#1)
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
mov bp, offset partition_table
check_table: # check the partition table for an active (bootable) partition
cmpb es:[bp+0], 0x00
jl active_partition # 0x80 or greater means the partition is active
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 bp, PT_ENTRY_SIZE # next partition
add bx, PT_ENTRY_SIZE # next partition
loop check_table
no_active:
pop es
jmp exit
active_partition:
push bp
check_one_active: # check that all subsequent partitions are zero (only one active)
add bp, PT_ENTRY_SIZE
dec cx
jz valid_active
cmpb es:[bp+0], 0x00
jz check_one_active
pop bp
invalid_table:
pop es
jmp exit
jmp boot_usb
valid_active:
pop bp
pop es
found_active:
mov si, offset prompt_string
call print_string
call print_string # Prompt the user
call flush_keyboard
set_rtc_int_vect: # Set the interrupt vector for CMOS real-time clock
@ -143,21 +117,21 @@ set_rtc_int_vect: # Set the interrupt vector for CMOS real-time clock
wait_for_keyboard:
mov ah, 0x01
int 0x16 # KEYBOARD - CHECK BUFFER, DO NOT CLEAR
jnz boot_usb # Z is clear when characters are present in the buffer
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
cmpw ds:counter_dot, 0x0000
cmpb ds:counter_dot, 0x00
jg short check_timeout
print_dot:
print_dot: # Every so often, we print a dot
mov si, offset dot_string
call print_string
movw ds:counter_dot, DOT_TIMEOUT
movb ds:counter_dot, DOT_TIMEOUT
check_timeout:
cmpw ds:counter_timeout, 0x0000
cmpb ds:counter_timeout, 0x00
jnz wait_for_keyboard
boot_fixed_disk: # Timeout occured => boot second bootable disk (non USB)
@ -169,25 +143,23 @@ boot_fixed_disk: # Timeout occured => boot second bootable disk (non USB)
boot_drive:
pop es
pop ds
mov dl, 0x0080 # In both case, we pretend the disk is the first bootable
mov dx, 0x0080 # In both case, we pretend the disk is the first bootable
jmp 0:MBR_ADDR
# ---------------------------------------------------------------------------
boot_usb: # Boot USB drive (0x80)
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
boot_usb_no_rtc:
mov bx, offset partition_table
mov dx, es:[bx]
mov cx, es:[bx+2]
xor ax, ax
mov es, ax # make sure ES is set to seg 0 for copy
mov ax, 0x0201
mov bx, MBR_ADDR
int 0x13
mov dx, ds:[bx]
mov cx, ds:[bx+2]
mov ebx, ds:[bx+8] # Must come last since it modifies BX
call read_sector
jnb boot_drive
exit:
exit: # failed to read PBR from USB - exit back to BIOS
pop es
pop ds
retf
@ -197,17 +169,56 @@ exit:
/* 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 #)
push es # DX = pointer to interrupt
xor ax, ax
mov es, ax
mov bx, ds:[si]
mov eax, es:[bx] # Backup the original vector
mov ds:[si], eax
mov es:[bx], dx
mov es:[bx+2], cs
pop es
sti
ret
@ -215,13 +226,9 @@ set_int_vect: # Set the interrupt vector
restore_rtc_vect: # Restore the interrupt vector for RTC
cli
push es
xor ax, ax
mov es, ax
mov bx, INT_RTC*4
mov eax, ds:rtc_interrupt_org
mov es:[bx], eax
pop es
sti
ret
@ -238,23 +245,15 @@ flush_keyboard: # Flush the keyboard buffer
# ---------------------------------------------------------------------------
print_string: # Print a NUL terminated string to console
push ax
push bx
ps_putchar:
print_string: # Print NUL terminated string in DS:SI to console
lodsb
cmp al, 0x00
jz ps_exit
jz 0f
mov ah, 0x0e
mov bx, 0x0007 # BH = display page, BL = foreground color
int 0x10 # VIDEO - WRITE CHARACTER AND ADVANCE CURSOR
jmp ps_putchar
ps_exit:
pop bx
pop ax
ret
jmp print_string
0: ret
# ---------------------------------------------------------------------------
@ -267,6 +266,23 @@ disk_swap: # Swap disks 0x80 and 0x81
xor dl, 0x01
0: ret
# ---------------------------------------------------------------------------
.if 0
print_hex: # Hex dump of the word at address ES:BX
mov cx, 0x04
mov dx, es:[bx]
0: rol dx, 0x04
mov ax, 0xe0f
and al, dl
daa
add al, 0xF0
adc al, 0x40
int 0x10
loop 0b
ret
.endif
/********************************************************************************/
/* Interrupt overrides */
@ -276,10 +292,10 @@ disk_swap: # Swap disks 0x80 and 0x81
rtc_interrupt:
pushf
cli
cmpw cs:counter_timeout, 0x0000
cmpb cs:counter_timeout, 0x00
jz rtc_exec_org
decw cs:counter_dot
decw cs:counter_timeout
decb cs:counter_dot
decb cs:counter_timeout
rtc_exec_org:
rtc_interrupt_org = .+1 # Same trick used by the LILO mapper
@ -315,8 +331,8 @@ dsk_interrupt_org = .+1
int13_cmd: .byte 0x00
prompt_string: .string "\r\nPress any key to boot from USB."
dot_string = .-2 # Reuse the end of previous string
counter_timeout:.word DOT_NUMBER*DOT_TIMEOUT + 1
counter_dot: .word DOT_TIMEOUT
counter_timeout:.byte DOT_NUMBER*DOT_TIMEOUT + 1
counter_dot: .byte DOT_TIMEOUT
/********************************************************************************/