I should probably start releasing this old code re-licensed under BSD license, but for educational purposes you can have it as-is.
This is a bootsector (FYI I've never tried it as an MBR replacement) to load a file with a given name from the root of a FAT32 disk. It then relocates and runs it as a regular DOS .EXE program if it finds the MZ signature or as a regular DOS .COM program otherwise.
Populate the BPB(s) with the proper values (just take them from your formatted FAT32 disk, where you want this bootsector installed).
Assemble with "nasm -f bin".
Code: Select all
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; ;;
;; "BootProg" Loader v 1.4 by Alexei A. Frounze (c) 2000-2005 ;;
;; ;;
;; ;;
;; Contact Information: ;;
;; ~~~~~~~~~~~~~~~~~~~~ ;;
;; E-Mail: [email protected] ;;
;; Homepage: http://alexfru.chat.ru ;;
;; Mirror: http://alexfru.narod.ru ;;
;; ;;
;; ;;
;; Thanks: ;;
;; ~~~~~~~ ;;
;; Thanks Thomas Kjoernes (aka NowhereMan) for his excelent idea ;;
;; ;;
;; ;;
;; Features: ;;
;; ~~~~~~~~~ ;;
;; - FAT32 supported using BIOS int 13h function 42h (i.e. it will only ;;
;; work with modern BIOSes supporting HDDs bigger than 8 GB) ;;
;; ;;
;; - Loads particular COM or EXE file placed to the root directory of a disk;;
;; ("ProgramName" variable holds name of a file to be loaded) ;;
;; ;;
;; - Provides simple information about errors occured during load process ;;
;; ("E" message for "Read Error" or "file Not Found") ;;
;; ;;
;; ;;
;; Known Limitations: ;;
;; ~~~~~~~~~~~~~~~~~~ ;;
;; - Works only on the 1st MBR partition which must be a PRI DOS partition ;;
;; with FAT32 (File System ID: 0Bh,0Ch) ;;
;; ;;
;; ;;
;; Known Bugs: ;;
;; ~~~~~~~~~~~ ;;
;; - All bugs are fixed as far as I know. The boot sector tested on my ;;
;; HDD. ;;
;; ;;
;; ;;
;; Memory Map: ;;
;; ~~~~~~~~~~~ ;;
;; ┌────────────────────────┐ ;;
;; │ Interrupt Vector Table │ 0000 ;;
;; ├────────────────────────┤ ;;
;; │ BIOS Data Area │ 0040 ;;
;; ├────────────────────────┤ ;;
;; │ PrtScr Status / Unused │ 0050 ;;
;; ├────────────────────────┤ ;;
;; │ Image Load Address │ 0060 ;;
;; ├────────────────────────┤ ;;
;; │ Available Memory │ nnnn ;;
;; ├────────────────────────┤ ;;
;; │ 2KB Boot Stack │ A000 - 512 - 2KB ;;
;; ├────────────────────────┤ ;;
;; │ Boot Sector │ A000 - 512 ;;
;; ├────────────────────────┤ ;;
;; A000 ;;
;; ;;
;; ;;
;; Boot Image Startup (register values): ;;
;; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ;;
;; dl = boot drive number ;;
;; cs:ip = program entry point ;;
;; ss:sp = program stack (don't confuse with boot sector's stack) ;;
;; COM program defaults: cs = ds = es = ss = 50h, sp = 0, ip = 100h ;;
;; EXE program defaults: ds = es = 50h, other stuff depends on EXE header ;;
;; ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
[BITS 16]
? equ 0
ImageLoadSeg equ 60h ; <=07Fh because of "push byte ImageLoadSeg" instructions
[SECTION .text]
[ORG 0]
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Boot sector starts here ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
jmp short start
nop
bsOemName DB "BootProg" ; 0x03
;;;;;;;;;;;;;;;;;;;;;;
;; BPB1 starts here ;;
;;;;;;;;;;;;;;;;;;;;;;
bpbBytesPerSector DW ? ; 0x0B
bpbSectorsPerCluster DB ? ; 0x0D
bpbReservedSectors DW ? ; 0x0E
bpbNumberOfFATs DB ? ; 0x10
bpbRootEntries DW ? ; 0x11
bpbTotalSectors DW ? ; 0x13
bpbMedia DB ? ; 0x15
bpbSectorsPerFAT DW ? ; 0x16
bpbSectorsPerTrack DW ? ; 0x18
bpbHeadsPerCylinder DW ? ; 0x1A
bpbHiddenSectors DD ? ; 0x1C
bpbTotalSectorsBig DD ? ; 0x20
;;;;;;;;;;;;;;;;;;;;
;; BPB1 ends here ;;
;;;;;;;;;;;;;;;;;;;;
;;;;;;;;;;;;;;;;;;;;;;
;; BPB2 starts here ;;
;;;;;;;;;;;;;;;;;;;;;;
bsSectorsPerFAT32 DD ? ; 0x24
bsExtendedFlags DW ? ; 0x28
bsFSVersion DW ? ; 0x2A
bsRootDirectoryClusterNo DD ? ; 0x2C
bsFSInfoSectorNo DW ? ; 0x30
bsBackupBootSectorNo DW ? ; 0x32
bsreserved times 12 DB ? ; 0x34
bsDriveNumber DB ? ; 0x40
bsreserved1 DB ? ; 0x41
bsExtendedBootSignature DB ? ; 0x42
bsVolumeSerialNumber DD ? ; 0x43
bsVolumeLabel DB "NO NAME " ; 0x47
bsFileSystemName DB "FAT32 " ; 0x52
;;;;;;;;;;;;;;;;;;;;
;; BPB2 ends here ;;
;;;;;;;;;;;;;;;;;;;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Boot sector code starts here ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
start:
cld
;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; How much RAM is there? ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;
int 12h ; get conventional memory size (in KBs)
shl ax, 6 ; and convert it to paragraphs
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Reserve some memory for the boot sector and the stack ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
sub ax, 512 / 16 ; reserve 512 bytes for the boot sector code
mov es, ax ; es:0 -> top - 512
sub ax, 2048 / 16 ; reserve 2048 bytes for the stack
mov ss, ax ; ss:0 -> top - 512 - 2048
mov sp, 2048 ; 2048 bytes for the stack
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Copy ourself to top of memory ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
mov cx, 256
mov si, 7C00h
xor di, di
mov ds, di
rep movsw
;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Jump to relocated code ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;
push es
push byte main
retf
main:
push cs
pop ds
mov [bsDriveNumber], dl ; store boot drive number
and byte [bsRootDirectoryClusterNo+3], 0Fh ; mask cluster value
mov esi, [bsRootDirectoryClusterNo] ; esi=cluster # of root dir
RootDirReadContinue:
push byte ImageLoadSeg
pop es
xor bx, bx
call ReadCluster ; read one cluster of root dir
push esi ; save esi=next cluster # of root dir
pushf ; save carry="not last cluster" flag
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Look for a COM/EXE program to be load and run ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
push byte ImageLoadSeg
pop es
xor di, di ; es:di -> root entries array
mov si, ProgramName ; ds:si -> program name
mov al, [bpbSectorsPerCluster]
cbw
mul word [bpbBytesPerSector]; ax = bytes per cluster
shr ax, 5
mov dx, ax ; dx = # of dir entries to search in
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Looks for a file with particular name ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Input: DS:SI -> file name (11 chars) ;;
;; ES:DI -> root directory array ;;
;; DX = number of root entries ;;
;; Output: ESI = cluster number ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
FindName:
mov cx, 11
FindNameCycle:
cmp byte [es:di], ch
jne FindNameNotEnd
jmp ErrFind ; end of root directory (NULL entry found)
FindNameNotEnd:
pusha
repe cmpsb
popa
je FindNameFound
add di, 32
dec dx
jnz FindNameCycle ; next root entry
popf ; restore carry="not last cluster" flag
pop esi ; restore esi=next cluster # of root dir
jc RootDirReadContinue ; continue to the next root dir cluster
jmp ErrFind ; end of root directory (dir end reached)
FindNameFound:
push word [es:di+14h]
push word [es:di+1Ah]
pop esi ; si = cluster no.
;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Load entire a program ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;
push byte ImageLoadSeg
pop es
xor bx, bx
FileReadContinue:
call ReadCluster ; read one cluster of root dir
jc FileReadContinue
;;;;;;;;;;;;;;;;;;;
;; Type checking ;;
;;;;;;;;;;;;;;;;;;;
push byte ImageLoadSeg
pop ds
mov ax, ds ; ax=ds=seg the program is loaded to
cmp word [0], 5A4Dh ; "MZ" signature?
je RelocateEXE ; yes, it's an EXE program
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Setup and Run COM program ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
sub ax, 10h ; "org 100h" stuff :)
mov es, ax
mov ds, ax
mov ss, ax
xor sp, sp
push es
push word 100h
jmp short Run
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Relocate, setup and run EXE program ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
RelocateEXE:
add ax, [08h] ; ax = image base
mov cx, [06h] ; cx = reloc items
mov bx, [18h] ; bx = reloc table pointer
jcxz RelocationDone
ReloCycle:
mov di, [bx] ; di = item ofs
mov dx, [bx+2] ; dx = item seg (rel)
add dx, ax ; dx = item seg (abs)
push ds
mov ds, dx ; ds = dx
add [di], ax ; fixup
pop ds
add bx, 4 ; point to next entry
loop ReloCycle
RelocationDone:
mov bx, ax
add bx, [0Eh]
mov ss, bx ; ss for EXE
mov sp, [10h] ; sp for EXE
add ax, [16h] ; cs
push ax
push word [14h] ; ip
Run:
mov dl, [cs:bsDriveNumber] ; let program know boot drive
; set the magic number so the program knows who has loaded it:
mov si, 16381 ; prime number 2**14-3
mov di, 32749 ; prime number 2**15-19
mov bp, 65521 ; prime number 2**16-15
retf
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Reads a FAT32 cluster ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Inout: ES:BX -> buffer ;;
;; ESI = cluster no ;;
;; Output: ESI = next cluster ;;
;; ES:BX -> next addr ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
ReadCluster:
mov ax, [bpbBytesPerSector]
shr ax, 2 ; ax=# of FAT32 entries per sector
cwde
mov ebp, esi ; ebp=esi=cluster #
xchg eax, esi
cdq
div esi ; eax=FAT sector #, edx=entry # in sector
movzx edi, word [bpbReservedSectors]
add edi, [bpbHiddenSectors]
add eax, edi
push dx ; save dx=entry # in sector on stack
mov cx, 1
call ReadSectorLBA ; read 1 FAT32 sector
pop si ; si=entry # in sector
add si, si
add si, si
and byte [es:si+3], 0Fh ; mask cluster value
mov esi, [es:si] ; esi=next cluster #
lea eax, [ebp-2]
movzx ecx, byte [bpbSectorsPerCluster]
mul ecx
mov ebp, eax
movzx eax, byte [bpbNumberOfFATs]
mul dword [bsSectorsPerFAT32]
add eax, ebp
add eax, edi
call ReadSectorLBA
mov ax, [bpbBytesPerSector]
shr ax, 4 ; ax = paragraphs per sector
mul cx ; ax = paragraphs read
mov cx, es
add cx, ax
mov es, cx ; es:bx updated
cmp esi, 0FFFFFF8h ; carry=0 if last cluster, and carry=1 otherwise
ret
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Reads a sector using BIOS Int 13h fn 42h ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Input: EAX = LBA ;;
;; CX = sector count ;;
;; ES:BX -> buffer address ;;
;; Output: CF = 1 if error ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
ReadSectorLBA:
pushad
ReadSectorLBANext:
pusha
push byte 0
push byte 0
push eax
push es
push bx
push byte 1
push byte 16
mov ah, 42h
mov dl, [bsDriveNumber]
mov si, sp
push ss
pop ds
int 13h
push cs
pop ds
add sp, 16
jc short ErrRead
popa
dec cx
jz ReadSectorLBADone2 ; last sector
add bx, [bpbBytesPerSector] ; adjust offset for next sector
add eax, byte 1 ; adjust LBA for next sector
jmp short ReadSectorLBANext
ReadSectorLBADone2:
popad
ret
;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Error Messaging Code ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;
ErrRead:
ErrFind:
mov ax, 0E00h+'E'
mov bx, 7
int 10h
jmp short $
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Fill free space with zeroes ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
times (512-13-($-$$)) db 0
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Name of a program to be load and run ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
ProgramName db "STARTUP BIN" ; name and extension must be padded
; with spaces (11 bytes total)
;;;;;;;;;;;;;;;;;;;;;;;;;;
;; End of the sector ID ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;
dw 0AA55h
It catches the error in this jump here. If you say it works fine on your end, I think I'm going crazy here 
But, of course, there could be a bug or two. I'll take a look.