This is an older version of the code I posted in another of your threads -- perhaps it can provide something useful.
;---------------------------------------------------------------
; This file is the source for a boot loader (boot sector) that
; moves itself up to the top of conventional memory, locates
; and reads a file from the boot diskette, loads the contents
; of the file into memory starting at absolute address 500h,
; and then does a far jump to that address. The code includes
; the components necessary for the system (DOS/Windows) to
; recognize it as a valid boot sector for a 3.5 inch 1.44MB
; diskette, and this in turn allows the system to access the
; diskette file system normally.
;
; Normal structure of a 3.5 inch 1.44MB diskette:
;
; Boot sector (512 bytes:c=0,h=0,s=1)
; FATs (2*9*512 bytes:c=0,h=0,s=2 to c=0,h=1,s=1)
; Root directory (224*32 bytes:c=0,h=1,s=2 to c=0,h=1,s=15)
; File and directory space (starts at c=0,h=1,s=16)
;
; Diskettes use a 12-bit FAT. Because the first two FAT entries
; (000 and 001) are reserved, the entry that corresponds to
; cluster 0 starts at the fourth byte. The remaining FAT
; entries should be interpreted is as follows:
;
; 000h = free cluster
; 001h = unused code
; FF0-FF6h = reserved
; FF7h = bad cluster
; FF8-FFFh = last cluster of file
; any other value = link to next cluster in file
;
; For example, assuming the file occupies three clusters and
; starts at the third entry (002):
;
; reserved [003] [FFF]
; | | || | || |
; F0 FF FF 03 40 00 FF 6F
; | ||
; [004]
;
; Although somewhat difficult to follow because each entry
; is 1.5 bytes, the third entry (002) points to the fourth
; (003), the fourth (003) to the fifth (004), and the fifth
; (004) contains FFFh indicating that the associated cluster
; is the last cluster of the file.
;---------------------------------------------------------------
; Declare a structure for a boot sector header that includes
; all but the 3-byte bsJump field of the BOOTSECTOR structure
; from the Microsoft MS-DOS Programmers Reference (versions
; 5 and 6). The initializers for the essential fields were
; taken from a 3.5 inch 1.44MB diskette formatted under
; Windows 98 SE, but they should be valid for any of the
; later versions of MS-DOS and AFAIK Windows 9x/NT/2000/XP.
;
; Per the MS-DOS Programmers Reference, the structure must
; appear at the beginning of the first sector on the disk.
; The 3-byte bsJump field was left out of the header
; structure so it could be coded separately.
;
BSHEADER struct
bsOemName db "XXXXXXXX" ; must be 8 bytes
bsBytesPerSector dw 512 ; start of BPB
bsSectorsPerCluster db 1
bsReservedSectors dw 1 ; boot sector only
bsFats db 2
bsRootDirEntries dw 224
bsSectors dw 2880
bsMedia db 0f0h
bsFatSectors dw 9
bsSectorsPerTrack dw 18
bsHeads dw 2
bsHiddenSectors dd 0
bsHugeSectors dd 0 ; end of BPB
bsDriveNumber db 0
bsReserved db 0
bsBootSignature db 29h
bsVolumeId dd 12345678h
bsVolumeLabel db 'JUST A TEST' ; must be 11 bytes
bsFileSystemType db 'FAT12 ' ; must be 8 bytes
BSHEADER ENDS
; This structure, included here to serve only as a template,
; is from the MS-DOS Programmers Reference.
;
; Directory entries are 32 bytes each.
;
DIRENTRY STRUCT
deName db 8 dup(?)
deExtension db 3 dup(?)
deAttribute db ?
deReserved db 10 dup(?)
deTime dw ?
deDate dw ?
deStartCluster dw ?
deFileSize dd ?
DIRENTRY ENDS
.386
; Specify USE16 to force address and operand size to 16 bits.
;
_TEXT SEGMENT USE16
; For MASM 6+ ASSUME CS in not necessary.
;
ASSUME ds:_TEXT,es:_TEXT,ss:_TEXT
; The BIOS will load the boot sector at address 0000:7C00.
;
ORG 7C00h
entry:
; Use 'NEAR PTR' to force MASM to encode a 3-byte jump.
;
jmp NEAR PTR init
; Define the header.
;
bsh BSHEADER <>
; Define the data.
;
dest dw OFFSET entry2, 97dfh
filename db 'STARTUP BIN'
db ' not found, system halted', 13, 10, 0
readerror db 'Error reading diskette', 13, 10, 0
ok db 'ok', 13, 10, 0
init:
; Copy the entire boot sector to address 97DFh:7C00h
; (absolute address 9F9F0h) near the top of conventional
; memory, avoiding the top 1024KB just in case the BIOS
; is using it.
;
xor ax, ax
mov ds, ax
mov si, 7c00h
mov di, si
mov es, dest+2
mov cx, 512
cld
rep movsb
; Do a far jump to the copy of the boot sector, bypassing
; the instructions that precede entry2.
;
jmp DWORD PTR dest
entry2:
; Set DS, ES, and SS to the new segment address.
;
push cs
pop ds
push cs
pop es
push cs
pop ss
; This initial SP value will cause the stack to use
; memory immediately below the boot sector code.
;
mov sp, 7C00h
; We need 224*32 = 7,168 bytes of memory to store the
; root directory. Allowing a generous 512 bytes for
; the stack, we can use offset address 5E00h for the
; base of the storage buffer.
;
BUFFER_BASE EQU 5E00h
; Load the root directory into the storage buffer.
;
mov al, 14 ; number of sectors
mov ch, 0 ; cylinder number
mov cl, 2 ; starting sector number
mov dh, 1 ; head number
call readdisk
jnc @F
mov si, OFFSET readerror
call print
jmp $ ; dynamic halt
@@:
; Scan the directory for the target file.
;
mov bx, BUFFER_BASE
mov cx, 224
dirloop:
push cx
mov cx, 11
mov di, bx
mov si, OFFSET filename
repz cmpsb
pop cx
jz match
add bx, 32
loop dirloop
; Not found, display and halt.
;
mov si, OFFSET filename
call print
jmp $
match:
mov si, OFFSET ok
call print
jmp $
;mov ax,[bx].deStartCluster
;ASSUME bx:NOTHING
;---------------------------------------------------------------
; This code uses the BIOS Write Teletype function to display
; an ASCIIZ string.
;
; Parameters:
; ds:si = address of string
; Uses:
; ax bx si
; Returns:
; nothing
;---------------------------------------------------------------
;
print:
mov ah, 0Eh
xor bx, bx
@@:
lodsb
or al, al
jz @F
int 10h
jmp @B
@@:
ret
;---------------------------------------------------------------
; This code uses the BIOS Read Disk Sectors function to read
; sectors from the first diskette drive into memory. The drive
; number and destination address (ES:BUFFER_BASE) are hard
; coded, and the read cannot span tracks.
;
; Parameters:
; al = number of sectors to read (cannot span tracks)
; ch = cylinder number (0-79)
; cl = starting sector number (1-18)
; dh = head number (0-1)
; Uses:
; ax bx cx dx si
; Returns:
; CF set on error
;---------------------------------------------------------------
;
readdisk:
mov si, 4 ; maximum of 4 tries
xor dl, dl ; drive = 0
mov bx, BUFFER_BASE
@@:
mov ah, 2 ; read disk sectors
int 13h
jnc @F ; finished if no error
xor ah, ah ; reset diskette drive and controller
int 13h
jc @F ; finished if error
dec si ; dec does not affect carry flag
jnz @B
@@:
ret
comment ^
AH = 02h
AL = number of sectors to read (must be nonzero)
CH = low eight bits of cylinder number
CL = sector number 1-63 (bits 0-5)
high two bits of cylinder (bits 6-7, hard disk only)
DH = head number
DL = drive number (bit 7 set for hard disk)
ES:BX -> data buffer
AH = 00h
DL = drive (if bit 7 is set both hard disks and floppy disks reset)
Return: AH = status (see #00234)
CF clear if successful (returned AH=00h)
CF set on error
^
;---------------------------------------------------------------
; This code is from the MS-DOS Encyclopedia, Ray Duncan (ed.),
; Microsoft Press, 1988.
;
; Obtains the next link number from a 12-bit FAT.
;
; Parameters:
; ax = current entry (cluster) number
; ds:bx = address of FAT (must be contiguous)
; Uses:
; bx cx
; Returns:
; ax = next link number
;---------------------------------------------------------------
;
next12:
add bx,ax ; ds:bx = partial index
shr ax,1 ; ax = offset/2
; carry = shift needed
pushf ; save carry
add bx,ax ; ds:bx = next cluster number index
mov ax,[bx] ; ax = next cluster number
popf ; carry = shift needed
jc shift ; skip if using top 12 bits
and ax,0fffh ; ax = lower 12 bits
ret
shift:
mov cx,4 ; cx = shift count
shr ax,cl ; ax = top 12 bits in lower 12 bits
ret
;---------------------------------------------------------------
; MAKEIT.BAT generates a listing file that can be used to
; determine exactly what MASM assembled, and how much of
; the 512 bytes allocated for a boot sector has been filled
; with code and data. For this source, there are 365 free
; bytes between the above RET instruction and the signature.
;
org 7C00h + 510
db 55h,0AAh
_TEXT ENDS
end entry
