; Sample TSR Code

org 0x100   ; Skip first 0x100 bytes of .COM file
   JMP Install 

; ----- Variables -----
   OldR DD ? ; Original function

MyIntN:
   ; ----- Call existing interrupt routine -----
   Pushf        ; Push flags (for CALL to old procedure)
   Call CS:OldR ; (CS override ensures far call)

   ; ----- Body of new ISR function -----
   ; your code here ..

   IRET         ; Restore flags and finish interrupt

Install:  
   ; Set DS=CS for tiny .COM programs 
   MOV AX,CS
   MOV DS,AX

   ; ----- Store original vector in OldR -----
   MOV AL, N  ; N=vector number
   MOV AH,35h ; Read current vector
   INT 21h    
   MOV WORD PTR OldR+0, BX ; Offset
   MOV WORD PTR OldR+2, ES ; Segment

   ; ----- Store New Vector in table -----
   MOV AL, N   ; N=vector number
   MOV AH,25h  ; Set current vector
   MOV DX,offset MyIntN ; DS:CS = new function ptr
   INT 21h   

   ; ----- Terminate and Stay Resident ----- 
   MOV DX, offset Install 
              ; All data & code up to install
   MOV CL,4    
   SHR DX,CL  ; Specify memory in 16-byte paragraphs 
   ADD DX,1   ; Round up (eg: 17/16+1=2 paragraphs)
   MOV AH,31h ; Exit, but keep in memory
   MOV AL,0   ; No error code
   INT 21h