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Last Post by Garni
;0-10    File name (8 bytes) with extension (3 bytes)
;11      Attribute - a bitvector. Bit 0: read only. Bit 1: hidden.
;        Bit 2: system file. Bit 3: volume label. Bit 4: subdirectory.
;        Bit 5: archive. Bits 6-7: unused.
;12-21   Reserved (see below)
;22-23   Time (5/6/5 bits, for hour/minutes/doubleseconds)
;24-25   Date (7/4/5 bits, for year-since-1980/month/day)
;26-27   Starting cluster (0 for an empty file)
;28-31   Filesize in bytes


text segment word 'code'
assume cs:text, ds:text

org 7C00h

jmp short ldr

boot    proc

db ?                    ;byte not needed for short jump

OEMname         db     'OEM NAME'
SecSize         dw     512
ClustSize       db     1
reserved        dw     1
FATCnt          db     2
RootEntries     dw     224  ;max entries in root directory
unused1         dw     ?
mdb             db     0F0h
FATSize         dw     9
SecsPerTrk      dw     18
heads           dw     2
hidden          dd     10   ;sectors reserved for boot loader (5120 bytes available)
unused2         dd     0
drive           db     0
unused3         db     0
signature       db     29h
SerNum          dd     03C9A71BFh
VolumeLabel     db     'NO NAME   '
                db     'FAT12   '

xor ax,ax
mov cs,ax
mov ds,ax                   ;initialise segment registers
mov ss,ax
mov es,ax
mov sp, 0fffeh              ;set up stack

mov ax, FATSize             ;load sectors per track
mov bh, FATCnt
mul bh                      ;calculate number of FAT sectors
add ax, reserved            ;add on boot sector(s)
inc ax                      ;first sector of root directory
push ax
pop cx                      ;cl = first sector to read, ch = track 0
push cx                     ;save sector address of root directory
xor dh,dh                   ;dh = head 0
mov dl, drive               ;load drive number
mov bx, 8000H               ;es:bx -> buffer on DMA boundary
mov ax, 201H                ;read first sector of root directory into memory
int 13h
jc BadTrk0                  ;jump to error handler if read failed

mov si, 8000H               ;ds:si -> first directory entry
lea di, osldr               ;es:di -> loader's filename
mov cx, 11
repe cmpsb                  ;is loader present?
je ldr_found                ;if so, skip error handler

lea si, errmsg2             ;ds:si -> non system disk message
call disp                   ;display message
lea si, errmsg10
call disp
jmp short reboot            ;reboot computer

mov ax, RootEntries         
shr ax, 4                   ;ax = number of root directory sectors
pop cx                      ;pop seg address of root directory & track number
add cl,al                   ;cl = first sector after root directory
cmp cl, byte ptr SecsPerTrk ;isw cl > number of sectors on track?
jna la1                     ;no - so no adjustment necessary

sub cl, byte ptr SecsPerTrk ;sector number for track 0, head 1
xor dh, 1                   ;toggle head number

call read64k                ;read loader into memory
jnc ldr_read_ok             ;skip error handler if read was successful

lea si, errmsg1             ;ds:si -> error message
call disp                   ;display message
lea si, errmsg10
call disp

xor ah,ah                   ;wait for keystroke
int 16h
mov bx,472H                 ;0:472 = address of boot status word
mov word ptr [ bx ], 1234H  ;skip POST test
int 19h                     ;reboot

jmp ldr_addr                ;jump to osldr

osldr     db 'osldr   sys'

errmsg1   db 'Bad track 0 - Disk unusable.', 0dh, 0ah, 0
errmsg2   db 'Not a system disk', 0dh, 0ah, 0
errmsg10  db 'Strike any key', 0

ldr_addr  dd 8000H

boot endp

disp    proc

xor bh,bh               ;page number
lodsb                   ;load next character
or al,al                ;end of string?
jz EndDisp              ;if so jump out of loop

mov ah, 0Eh             ;display character
int 10h
jmp short nextChar      ;next character


disp    endp

read64k    proc    near
;dl = drive
;dh = head
;cl = sector
;ch = track
;bx -> buffer on DMA boundary

mov si,cx                ;save sector and track numbers
mov cx, 16               ;16 * 4096 byte pages = 64kb

xchg cx,si               ;cx = sector and track numbers, si = loop count
call readPage            ;read 4096 byte page into memory
jc read64kEnd

xchg si,cx               ;si = returned sector and track numbers, cx = loop count
loop nextPage            ;read next page



read64k    endp

readPage    proc    near
;dl = drive
;dh = head
;cl = sector
;ch = track
;bx -> buffer on DMA boundary

push bx
push cx
push dx
mov ax, 8              ;8 sectors = 4096 byte DMA page
push cx                ;save sector and track number
add cl,al              ;calculate end sector number
cmp cl, byte ptr SecsPerTrk
pop cx                 ;past end of track?
jna SameTrk            ;no - so skip special code
mov ax, SecsPerTrk     ;load al with number of sectors per track
sub al,cl
inc ax                 ;ax = number of sectors to read
push ax                
push bx
push cx
push dx
mov ah, 2              ;read sectors into buffer
int 13h
pop dx
pop cx
pop bx
pop ax                 ;ax = number of sectors read
jnc la3                ;skip error handler if carry clear

add sp, 8              ;pop pushed registers
stc                    ;flag error
ret                    ;return to caller

push ax                
shl ax, 9              ;ax = number of bytes read
add bx,ax              ;update buffer pointer ready for next read
pop ax
sub al, 8
neg al                 ;ax = number of remaining sectors to read
mov cl, 1              ;starting with sector 1
xor dh, 1              ;toggle head number
jnz SameTrk            ;jump if not back with head 0

inc ch                 ;otherwise increment track number

mov ah, 2              ;complete read
int 13h
jc la4                 ;return with carry set upon error

pop dx
pop cx
pop bx
add bx, 4096           ;increment bx by number of bytes in page
add cl, 8              ;increment starting sector by eight
cmp cl, byte ptr SecsPerTrk ;pased end of track?
jna la2                ;no - so skip handler

sub cl, byte ptr SecsPerTrk
xor dh, 1              ;toggle head number
jnz la2                ;if head != 0 do not increment track number

inc ch                 ;new track number

clc                    ;no error

readPage    endp

dw 0, 55AAH

text ends


Chances are you wiould need a utility to strip off the first 7C00H bytes after compiling and linking it. (Not yet tested or debugged by the way.)

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