Sample program in assembly language using interrupt 21h or int 21h.?

Question

Please provide me atleast three sample program of assembly language using interrupt 21h or int 21h.

Answer 1

Well if they're using int 21h they're probably on a PC writing for DOS. int 21h calls most the basic DOS functions.

But it does so many I would like to know what they would like to do?

Answer 2

Not knowing which CPU nor assembler you use, you can't get a "sample": assembly is chip and manufacturer dependant.
In your code, you would have something like this:

; VECTORS
; ------------------------------------------
CSEG
ORG 0h ; reset
ajmp Start
ORG 21h ; Interrupt vector
ajmp Interrupt
; ------------------------------------------
Start:
mov sp, #080h ; initialise stack pointer
....
Interrupt: ; your interrupt routine
jnb Txflag, pcint1 ; currently sending to PC
clr BitA
...
RETI ; return from interrupt.
; -------------------------
Google the chip manufacturer (i.e. above was for ATMEL micro series).
Get data sheets of the chip from manufacturer
Often, they have samples of applications, in ASM.
Good luck!