open file and read form it to buffer in assembly - file

Question

hey im a beginner in assembly ,, i wont to open a file and read value " integer "
from it and save the integer in buffer to print on the screen this is my code it doesn't work
include inout.asm
.model small,c
.486
.stack
.data
org 100h ; .com memory layout
buf db ?
file db "c:\rtasm\bin\file.txt";the file name in bin
.code
mov dx, offset file ; address of file to dx
mov al,0 ; open file (read-only)
mov ah,3dh
int 21h ; call the interupt
mov bx,ax ; put handler to file in bx
mov ah,40h
mov bx,ax
mov cx,2h ;; how many bytes you want to read
mov dx,offset buf ;; where you want to store that data (see note on Offset above)
int 21h
call putchar,offset buf; print char on the screen
mov ah,3eh
mov bx,ax
int 21h
.exit
END

Answer 1

Int 21h function 3Dh ("OPEN EXISTING FILE") expects a zero-terminated string in ds:dx. The string you're supplying doesn't have a zero terminator. The filename should be declared as file db "c:\rtasm\bin\file.txt",0.
Both function 3Dh and 40h return error codes if they fail. You should check for these and inform the user (yourself in this case) if an error has occurred, rather than assuming that the operations always will succeed.
Another problem is the following code:
mov bx,ax ; put handler to file in bx
mov ah,40h
mov bx,ax <-- gives you a nonsense file handle since ah now is 40h
mov cx,2h ;; how many bytes you want to read
mov dx,offset buf ;; where you want to store that data (see note on Offset above)
int 21h
The second mov bx,ax is unnecessary since bx already contains the file handle. In fact, it's not only unnecessary but also incorrect since you've overwritten the high part of ax (ah) with the value 40h.
There's also the fact that you're reading two bytes into a buffer that only has room for one byte.