Write to file in Assembly Lanugage

.model small
.stack
.data
buffer db "I'm writing.",'$'
file db "myfile.txt",0


.code
mov ax,@data
mov ds,ax
 
mov dx, offset file
;mov al,0            ;Open file (read-Only)
mov al,1            ; Write only
;mov al,2            ; Read & Write
mov ah,3dh           ; Load File handler and store in ax
int 21h

mov cx,10            ; no. of bytes to be written. I want only first 10 bytes to be written in my already created file
mov bx,ax            ; Move file Handler to bx
mov dx,offset buffer ; load offset of string which is to be written to file
mov ah,40h
int 21h

mov ah,4ch
int 21h
end

Read from file Assembly Language

.model small
.stack
.data
buffer db 5000 dup("$")
file db "myfile.txt",0


.code
mov ax,@data
mov ds,ax
 
mov dx, offset file
mov al,0            ;Open file (read-Only)
;mov al,1            ; Write only
;mov al,2            ; Read & Write
mov ah,3dh           ; Load File handler and store in ax
int 21h

mov bx,ax            ; Move file Handler to bx
mov dx,offset buffer
mov ah,3fh
int 21h

mov ah,9h
int 21h

mov ah,4ch
int 21h
end

3fh not working Assembly Language

Problem: 3fh not working in Assembly Language

Solution: Don't do mov cx,0 before calling interrupt for input.