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reactos/freeldr/bootsect/isoboot.asm
Eric Kohl e35e42758a Remove the INSTALL directory of the Boot-CD. 
svn path=/trunk/; revision=4711
2003-05-18 15:21:30 +00:00

1010 lines
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; ****************************************************************************
;
; isolinux.asm
;
; A program to boot Linux kernels off a CD-ROM using the El Torito
; boot standard in "no emulation" mode, making the entire filesystem
; available. It is based on the SYSLINUX boot loader for MS-DOS
; floppies.
;
; Copyright (C) 1994-2001 H. Peter Anvin
;
; 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, Inc., 675 Mass Ave, Cambridge MA 02139,
; USA; either version 2 of the License, or (at your option) any later
; version; incorporated herein by reference.
;
; ****************************************************************************
;
; THIS FILE IS A MODIFIED VERSION OF ISOLINUX.ASM
; MODIFICATION DONE BY MICHAEL K TER LOUW
; LAST UPDATED 3-9-2002
; SEE "COPYING" FOR INFORMATION ABOUT THE LICENSE THAT APPLIES TO THIS RELEASE
;
; ****************************************************************************
;
; This file is a modified version of ISOLINUX.ASM.
; Modification done by Eric Kohl
; Last update 04-25-2002
;
; ****************************************************************************
; Note: The Makefile builds one version with DEBUG_MESSAGES automatically.
;%define DEBUG_MESSAGES ; Uncomment to get debugging messages
; ---------------------------------------------------------------------------
; BEGIN THE BIOS/CODE/DATA SEGMENT
; ---------------------------------------------------------------------------
absolute 0400h
serial_base resw 4 ; Base addresses for 4 serial ports
absolute 0413h
BIOS_fbm resw 1 ; Free Base Memory (kilobytes)
absolute 046Ch
BIOS_timer resw 1 ; Timer ticks
absolute 0472h
BIOS_magic resw 1 ; BIOS reset magic
absolute 0484h
BIOS_vidrows resb 1 ; Number of screen rows
;
; Memory below this point is reserved for the BIOS and the MBR
;
absolute 1000h
trackbuf resb 8192 ; Track buffer goes here
trackbufsize equ $-trackbuf
; trackbuf ends at 3000h
struc open_file_t
file_sector resd 1 ; Sector pointer (0 = structure free)
file_left resd 1 ; Number of sectors left
endstruc
struc dir_t
dir_lba resd 1 ; Directory start (LBA)
dir_len resd 1 ; Length in bytes
dir_clust resd 1 ; Length in clusters
endstruc
MAX_OPEN_LG2 equ 2 ; log2(Max number of open files)
MAX_OPEN equ (1 << MAX_OPEN_LG2)
SECTORSIZE_LG2 equ 11 ; 2048 bytes/sector (El Torito requirement)
SECTORSIZE equ (1 << SECTORSIZE_LG2)
CR equ 13 ; Carriage Return
LF equ 10 ; Line Feed
retry_count equ 6 ; How patient are we with the BIOS?
absolute 5000h ; Here we keep our BSS stuff
DriveNo resb 1 ; CD-ROM BIOS drive number
DiskError resb 1 ; Error code for disk I/O
RetryCount resb 1 ; Used for disk access retries
TimeoutCount resb 1 ; Timeout counter
ISOFlags resb 1 ; Flags for ISO directory search
RootDir resb dir_t_size ; Root directory
CurDir resb dir_t_size ; Current directory
ISOFileName resb 64 ; ISO filename canonicalization buffer
ISOFileNameEnd equ $
alignb open_file_t_size
Files resb MAX_OPEN*open_file_t_size
section .text
org 7000h
start:
cli ; Disable interrupts
xor ax, ax ; ax = segment zero
mov ss, ax ; Initialize stack segment
mov sp, start ; Set up stack
mov ds, ax ; Initialize other segment registers
mov es, ax
mov fs, ax
mov gs, ax
sti ; Enable interrupts
cld ; Increment pointers
mov cx, 2048 >> 2 ; Copy the bootsector
mov si, 0x7C00 ; from 0000:7C00
mov di, 0x7000 ; to 0000:7000
rep movsd ; copy the program
jmp 0:relocate ; jump into relocated code
relocate:
; Display the banner and copyright
%ifdef DEBUG_MESSAGES
mov si, isolinux_banner ; si points to hello message
call writestr ; display the message
mov si,copyright_str
call writestr
%endif
; Make sure the keyboard buffer is empty
%ifdef WAIT_FOR_KEY
.kbd_buffer_test:
call pollchar
jz .kbd_buffer_empty
call getchar
jmp .kbd_buffer_test
.kbd_buffer_empty:
; Display the 'Press key' message and wait for a maximum of 5 seconds
call crlf
mov si, presskey_msg ; si points to 'Press key' message
call writestr ; display the message
mov byte [TimeoutCount], 5
.next_second:
mov eax, [BIOS_timer] ; load current tick counter
add eax, 19 ;
.poll_again:
call pollchar
jnz .boot_cdrom
mov ebx, [BIOS_timer]
cmp eax, ebx
jnz .poll_again
mov si, dot_msg ; print '.'
call writestr
dec byte [TimeoutCount] ; decrement timeout counter
jz .boot_harddisk
jmp .next_second
.boot_harddisk:
; Boot first harddisk (drive 0x80)
mov ax, 0201h
mov dx, 0080h
mov cx, 0001h
mov bx, 7C00h
int 13h
jnc .go_hd
jmp kaboom
.go_hd:
mov ax, cs
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
mov dx, 0080h
jmp 0:0x7C00
%endif
.boot_cdrom:
; Save and display the boot drive number
mov [DriveNo], dl
%ifdef DEBUG_MESSAGES
mov si, startup_msg
call writemsg
mov al, dl
call writehex2
call crlf
%endif
; Now figure out what we're actually doing
; Note: use passed-in DL value rather than 7Fh because
; at least some BIOSes will get the wrong value otherwise
mov ax, 4B01h ; Get disk emulation status
mov dl, [DriveNo]
mov si, spec_packet
int 13h
jc near spec_query_failed ; Shouldn't happen (BIOS bug)
mov dl, [DriveNo]
cmp [sp_drive], dl ; Should contain the drive number
jne near spec_query_failed
%ifdef DEBUG_MESSAGES
mov si, spec_ok_msg
call writemsg
mov al, byte [sp_drive]
call writehex2
call crlf
%endif
found_drive:
; Get drive information
mov ah, 48h
mov dl, [DriveNo]
mov si, drive_params
int 13h
jnc params_ok
mov si, nosecsize_msg
call writemsg
params_ok:
; Check for the sector size (should be 2048, but
; some BIOSes apparently think we're 512-byte media)
;
; FIX: We need to check what the proper behaviour
; is for getlinsec when the BIOS thinks the sector
; size is 512!!! For that, we need such a BIOS, though...
%ifdef DEBUG_MESSAGES
mov si, secsize_msg
call writemsg
mov ax, [dp_secsize]
call writehex4
call crlf
%endif
;
; Clear Files structures
;
mov di, Files
mov cx, (MAX_OPEN*open_file_t_size)/4
xor eax, eax
rep stosd
;
; Now, we need to sniff out the actual filesystem data structures.
; mkisofs gave us a pointer to the primary volume descriptor
; (which will be at 16 only for a single-session disk!); from the PVD
; we should be able to find the rest of what we need to know.
;
get_fs_structures:
mov eax, 16 ; Primary Volume Descriptor (sector 16)
mov bx, trackbuf
call getonesec
mov eax, [trackbuf+156+2]
mov [RootDir+dir_lba],eax
mov [CurDir+dir_lba],eax
%ifdef DEBUG_MESSAGES
mov si, rootloc_msg
call writemsg
call writehex8
call crlf
%endif
mov eax,[trackbuf+156+10]
mov [RootDir+dir_len],eax
mov [CurDir+dir_len],eax
%ifdef DEBUG_MESSAGES
mov si, rootlen_msg
call writemsg
call writehex8
call crlf
%endif
add eax,SECTORSIZE-1
shr eax,SECTORSIZE_LG2
mov [RootDir+dir_clust],eax
mov [CurDir+dir_clust],eax
%ifdef DEBUG_MESSAGES
mov si, rootsect_msg
call writemsg
call writehex8
call crlf
%endif
; Look for the "REACTOS" directory, and if found,
; make it the current directory instead of the root
; directory.
mov di,isolinux_dir
mov al,02h ; Search for a directory
call searchdir_iso
jnz .dir_found
mov si,no_dir_msg
call writemsg
jmp kaboom
.dir_found:
mov [CurDir+dir_len],eax
mov eax,[si+file_left]
mov [CurDir+dir_clust],eax
xor eax,eax ; Free this file pointer entry
xchg eax,[si+file_sector]
mov [CurDir+dir_lba],eax
mov di, isolinux_bin ; di points to Isolinux filename
call searchdir ; look for the file
jnz .isolinux_opened ; got the file
mov si, no_isolinux_msg ; si points to error message
call writemsg ; display the message
jmp kaboom ; fail boot
.isolinux_opened:
mov di, si ; save file pointer
%ifdef DEBUG_MESSAGES
mov si, filelen_msg
call writemsg
call writehex8
call crlf
%endif
mov ecx, eax ; calculate sector count
shr ecx, 11
test eax, 0x7FF
jz .full_sector
inc ecx
.full_sector:
%ifdef DEBUG_MESSAGES
mov eax, ecx
mov si, filesect_msg
call writemsg
call writehex8
call crlf
%endif
mov bx, 0x8000 ; bx = load address
mov si, di ; restore file pointer
mov cx, 0xFFFF ; load the whole file
call getfssec ; get the whole file
%ifdef DEBUG_MESSAGES
mov si, startldr_msg
call writemsg
call crlf
%endif
mov dl, [DriveNo] ; dl = boot drive
mov dh, 0 ; dh = boot partition
jmp 0:0x8000 ; jump into OSLoader
;
; searchdir:
;
; Open a file
;
; On entry:
; DS:DI = filename
; If successful:
; ZF clear
; SI = file pointer
; DX:AX or EAX = file length in bytes
; If unsuccessful
; ZF set
;
;
; searchdir_iso is a special entry point for ISOLINUX only. In addition
; to the above, searchdir_iso passes a file flag mask in AL. This is useful
; for searching for directories.
;
alloc_failure:
xor ax,ax ; ZF <- 1
ret
searchdir:
xor al,al
searchdir_iso:
mov [ISOFlags],al
call allocate_file ; Temporary file structure for directory
jnz alloc_failure
push es
push ds
pop es ; ES = DS
mov si,CurDir
cmp byte [di],'\' ; If filename begins with slash
jne .not_rooted
inc di ; Skip leading slash
mov si,RootDir ; Reference root directory instead
.not_rooted:
mov eax,[si+dir_clust]
mov [bx+file_left],eax
mov eax,[si+dir_lba]
mov [bx+file_sector],eax
mov edx,[si+dir_len]
.look_for_slash:
mov ax,di
.scan:
mov cl,[di]
inc di
and cl,cl
jz .isfile
cmp cl,'\'
jne .scan
mov [di-1],byte 0 ; Terminate at directory name
mov cl,02h ; Search for directory
xchg cl,[ISOFlags]
push di
push cx
push word .resume ; Where to "return" to
push es
.isfile:
xchg ax,di
.getsome:
; Get a chunk of the directory
mov si,trackbuf
pushad
xchg bx,si
mov cx,1 ; load one sector
call getfssec
popad
.compare:
movzx eax, byte [si] ; Length of directory entry
cmp al, 33
jb .next_sector
mov cl, [si+25]
xor cl, [ISOFlags]
test cl, byte 8Eh ; Unwanted file attributes!
jnz .not_file
pusha
movzx cx, byte [si+32] ; File identifier length
add si, byte 33 ; File identifier offset
call iso_compare_names
popa
je .success
.not_file:
sub edx, eax ; Decrease bytes left
jbe .failure
add si, ax ; Advance pointer
.check_overrun:
; Did we finish the buffer?
cmp si, trackbuf+trackbufsize
jb .compare ; No, keep going
jmp short .getsome ; Get some more directory
.next_sector:
; Advance to the beginning of next sector
lea ax, [si+SECTORSIZE-1]
and ax, ~(SECTORSIZE-1)
sub ax, si
jmp short .not_file ; We still need to do length checks
.failure:
%ifdef DEBUG_MESSAGES
mov si, findfail_msg
call writemsg
call crlf
%endif
xor eax, eax ; ZF = 1
mov [bx+file_sector], eax
pop es
ret
.success:
mov eax, [si+2] ; Location of extent
mov [bx+file_sector], eax
mov eax, [si+10] ; Data length
push eax
add eax, SECTORSIZE-1
shr eax, SECTORSIZE_LG2
mov [bx+file_left], eax
pop eax
mov edx, eax
shr edx, 16
and bx, bx ; ZF = 0
mov si, bx
pop es
ret
.resume:
; We get here if we were only doing part of a lookup
; This relies on the fact that .success returns bx == si
xchg edx, eax ; Directory length in edx
pop cx ; Old ISOFlags
pop di ; Next filename pointer
mov byte [di-1], '\' ; restore the backslash in the filename
mov [ISOFlags], cl ; Restore the flags
jz .failure ; Did we fail? If so fail for real!
jmp .look_for_slash ; Otherwise, next level
;
; allocate_file: Allocate a file structure
;
; If successful:
; ZF set
; BX = file pointer
; In unsuccessful:
; ZF clear
;
allocate_file:
push cx
mov bx, Files
mov cx, MAX_OPEN
.check:
cmp dword [bx], byte 0
je .found
add bx, open_file_t_size ; ZF = 0
loop .check
; ZF = 0 if we fell out of the loop
.found:
pop cx
ret
;
; iso_compare_names:
; Compare the names DS:SI and DS:DI and report if they are
; equal from an ISO 9660 perspective. SI is the name from
; the filesystem; CX indicates its length, and ';' terminates.
; DI is expected to end with a null.
;
; Note: clobbers AX, CX, SI, DI; assumes DS == ES == base segment
;
iso_compare_names:
; First, terminate and canonicalize input filename
push di
mov di, ISOFileName
.canon_loop:
jcxz .canon_end
lodsb
dec cx
cmp al, ';'
je .canon_end
and al, al
je .canon_end
stosb
cmp di, ISOFileNameEnd-1 ; Guard against buffer overrun
jb .canon_loop
.canon_end:
cmp di, ISOFileName
jbe .canon_done
cmp byte [di-1], '.' ; Remove terminal dots
jne .canon_done
dec di
jmp short .canon_end
.canon_done:
mov [di], byte 0 ; Null-terminate string
pop di
mov si, ISOFileName
.compare:
lodsb
mov ah, [di]
inc di
and ax, ax
jz .success ; End of string for both
and al, al ; Is either one end of string?
jz .failure ; If so, failure
and ah, ah
jz .failure
or ax, 2020h ; Convert to lower case
cmp al, ah
je .compare
.failure:
and ax, ax ; ZF = 0 (at least one will be nonzero)
.success:
ret
;
; getfssec: Get multiple clusters from a file, given the file pointer.
;
; On entry:
; ES:BX -> Buffer
; SI -> File pointer
; CX -> Cluster count; 0FFFFh = until end of file
; On exit:
; SI -> File pointer (or 0 on EOF)
; CF = 1 -> Hit EOF
;
getfssec:
cmp cx, [si+file_left]
jna .ok_size
mov cx, [si+file_left]
.ok_size:
mov bp, cx
push cx
push si
mov eax, [si+file_sector]
call getlinsec
xor ecx, ecx
pop si
pop cx
add [si+file_sector], ecx
sub [si+file_left], ecx
ja .not_eof ; CF = 0
xor ecx, ecx
mov [si+file_sector], ecx ; Mark as unused
xor si,si
stc
.not_eof:
ret
; INT 13h, AX=4B01h, DL=<passed in value> failed.
; Try to scan the entire 80h-FFh from the end.
spec_query_failed:
mov si,spec_err_msg
call writemsg
mov dl, 0FFh
.test_loop:
pusha
mov ax, 4B01h
mov si, spec_packet
mov byte [si], 13 ; Size of buffer
int 13h
popa
jc .still_broken
mov si, maybe_msg
call writemsg
mov al, dl
call writehex2
call crlf
cmp byte [sp_drive], dl
jne .maybe_broken
; Okay, good enough...
mov si, alright_msg
call writemsg
mov [DriveNo], dl
.found_drive:
jmp found_drive
; Award BIOS 4.51 apparently passes garbage in sp_drive,
; but if this was the drive number originally passed in
; DL then consider it "good enough"
.maybe_broken:
cmp byte [DriveNo], dl
je .found_drive
.still_broken:
dec dx
cmp dl, 80h
jnb .test_loop
fatal_error:
mov si, nothing_msg
call writemsg
.norge:
jmp short .norge
; Information message (DS:SI) output
; Prefix with "isolinux: "
;
writemsg:
push ax
push si
mov si, isolinux_str
call writestr
pop si
call writestr
pop ax
ret
;
; crlf: Print a newline
;
crlf:
mov si, crlf_msg
; Fall through
;
; writestr: write a null-terminated string to the console, saving
; registers on entry.
;
writestr:
pushfd
pushad
.top:
lodsb
and al, al
jz .end
call writechr
jmp short .top
.end:
popad
popfd
ret
;
; writehex[248]: Write a hex number in (AL, AX, EAX) to the console
;
writehex2:
pushfd
pushad
shl eax, 24
mov cx, 2
jmp short writehex_common
writehex4:
pushfd
pushad
shl eax, 16
mov cx, 4
jmp short writehex_common
writehex8:
pushfd
pushad
mov cx, 8
writehex_common:
.loop:
rol eax, 4
push eax
and al, 0Fh
cmp al, 10
jae .high
.low:
add al, '0'
jmp short .ischar
.high:
add al, 'A'-10
.ischar:
call writechr
pop eax
loop .loop
popad
popfd
ret
;
; Write a character to the screen. There is a more "sophisticated"
; version of this in the subsequent code, so we patch the pointer
; when appropriate.
;
writechr:
pushfd
pushad
mov ah, 0Eh
xor bx, bx
int 10h
popad
popfd
ret
;
; Get one sector. Convenience entry point.
;
getonesec:
mov bp, 1
; Fall through to getlinsec
;
; Get linear sectors - EBIOS LBA addressing, 2048-byte sectors.
;
; Note that we can't always do this as a single request, because at least
; Phoenix BIOSes has a 127-sector limit. To be on the safe side, stick
; to 32 sectors (64K) per request.
;
; Input:
; EAX - Linear sector number
; ES:BX - Target buffer
; BP - Sector count
;
getlinsec:
mov si,dapa ; Load up the DAPA
mov [si+4],bx
mov bx,es
mov [si+6],bx
mov [si+8],eax
.loop2:
push bp ; Sectors left
cmp bp,[MaxTransfer]
jbe .bp_ok
mov bp,[MaxTransfer]
.bp_ok:
mov [si+2],bp
push si
mov dl,[DriveNo]
mov ah,42h ; Extended Read
call xint13
pop si
pop bp
movzx eax,word [si+2] ; Sectors we read
add [si+8],eax ; Advance sector pointer
sub bp,ax ; Sectors left
shl ax,SECTORSIZE_LG2-4 ; 2048-byte sectors -> segment
add [si+6],ax ; Advance buffer pointer
and bp,bp
jnz .loop2
mov eax,[si+8] ; Next sector
ret
; INT 13h with retry
xint13:
mov byte [RetryCount], retry_count
.try:
pushad
int 13h
jc .error
add sp, byte 8*4 ; Clean up stack
ret
.error:
mov [DiskError], ah ; Save error code
popad
dec byte [RetryCount]
jz .real_error
push ax
mov al,[RetryCount]
mov ah,[dapa+2] ; Sector transfer count
cmp al,2 ; Only 2 attempts left
ja .nodanger
mov ah,1 ; Drop transfer size to 1
jmp short .setsize
.nodanger:
cmp al,retry_count-2
ja .again ; First time, just try again
shr ah,1 ; Otherwise, try to reduce
adc ah,0 ; the max transfer size, but not to 0
.setsize:
mov [MaxTransfer],ah
mov [dapa+2],ah
.again:
pop ax
jmp .try
.real_error:
mov si, diskerr_msg
call writemsg
mov al, [DiskError]
call writehex2
mov si, ondrive_str
call writestr
mov al, dl
call writehex2
call crlf
; Fall through to kaboom
;
; kaboom: write a message and bail out. Wait for a user keypress,
; then do a hard reboot.
;
kaboom:
mov ax, cs
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
sti
mov si, err_bootfailed
call writestr
call getchar
cli
mov word [BIOS_magic], 0 ; Cold reboot
jmp 0F000h:0FFF0h ; Reset vector address
getchar:
.again:
mov ah, 1 ; Poll keyboard
int 16h
jz .again
.kbd:
xor ax, ax ; Get keyboard input
int 16h
.func_key:
ret
;
; pollchar: check if we have an input character pending (ZF = 0)
;
pollchar:
pushad
mov ah,1 ; Poll keyboard
int 16h
popad
ret
isolinux_banner db CR, LF, 'Loading IsoBoot...', CR, LF, 0
copyright_str db ' Copyright (C) 1994-2002 H. Peter Anvin', CR, LF, 0
presskey_msg db 'Press any key to boot from CD', 0
dot_msg db '.',0
%ifdef DEBUG_MESSAGES
startup_msg: db 'Starting up, DL = ', 0
spec_ok_msg: db 'Loaded spec packet OK, drive = ', 0
secsize_msg: db 'Sector size appears to be ', 0
rootloc_msg: db 'Root directory location: ', 0
rootlen_msg: db 'Root directory length: ', 0
rootsect_msg: db 'Root directory length(sectors): ', 0
fileloc_msg: db 'SETUPLDR.SYS location: ', 0
filelen_msg: db 'SETUPLDR.SYS length: ', 0
filesect_msg: db 'SETUPLDR.SYS length(sectors): ', 0
findfail_msg: db 'Failed to find file!', 0
startldr_msg: db 'Starting SETUPLDR.SYS', 0
%endif
nosecsize_msg: db 'Failed to get sector size, assuming 0800', CR, LF, 0
spec_err_msg: db 'Loading spec packet failed, trying to wing it...', CR, LF, 0
maybe_msg: db 'Found something at drive = ', 0
alright_msg: db 'Looks like it might be right, continuing...', CR, LF, 0
nothing_msg: db 'Failed to locate CD-ROM device; boot failed.', CR, LF, 0
isolinux_str db 'IsoBoot: ', 0
crlf_msg db CR, LF, 0
diskerr_msg: db 'Disk error ', 0
ondrive_str: db ', drive ', 0
err_bootfailed db CR, LF, 'Boot failed: press a key to retry...'
isolinux_dir db '\LOADER', 0
no_dir_msg db 'Could not find the LOADER directory.', CR, LF, 0
isolinux_bin db 'SETUPLDR.SYS', 0
no_isolinux_msg db 'Could not find SETUPLDR.SYS.', CR, LF, 0
;
; El Torito spec packet
;
align 8, db 0
spec_packet: db 13h ; Size of packet
sp_media: db 0 ; Media type
sp_drive: db 0 ; Drive number
sp_controller: db 0 ; Controller index
sp_lba: dd 0 ; LBA for emulated disk image
sp_devspec: dw 0 ; IDE/SCSI information
sp_buffer: dw 0 ; User-provided buffer
sp_loadseg: dw 0 ; Load segment
sp_sectors: dw 0 ; Sector count
sp_chs: db 0,0,0 ; Simulated CHS geometry
sp_dummy: db 0 ; Scratch, safe to overwrite
;
; EBIOS drive parameter packet
;
align 8, db 0
drive_params: dw 30 ; Buffer size
dp_flags: dw 0 ; Information flags
dp_cyl: dd 0 ; Physical cylinders
dp_head: dd 0 ; Physical heads
dp_sec: dd 0 ; Physical sectors/track
dp_totalsec: dd 0,0 ; Total sectors
dp_secsize: dw 0 ; Bytes per sector
dp_dpte: dd 0 ; Device Parameter Table
dp_dpi_key: dw 0 ; 0BEDDh if rest valid
dp_dpi_len: db 0 ; DPI len
db 0
dw 0
dp_bus: times 4 db 0 ; Host bus type
dp_interface: times 8 db 0 ; Interface type
db_i_path: dd 0,0 ; Interface path
db_d_path: dd 0,0 ; Device path
db 0
db_dpi_csum: db 0 ; Checksum for DPI info
;
; EBIOS disk address packet
;
align 8, db 0
dapa: dw 16 ; Packet size
.count: dw 0 ; Block count
.off: dw 0 ; Offset of buffer
.seg: dw 0 ; Segment of buffer
.lba: dd 0 ; LBA (LSW)
dd 0 ; LBA (MSW)
alignb 4, db 0
MaxTransfer dw 2 ;32 ; Max sectors per transfer
times 2046-($-$$) db 0 ; Pad to file offset 2046
dw 0aa55h ; BootSector signature
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