TSR to report MSDOS version differently (8.0), in NASM syntax

dosver.asm

bits 16
cpu 8086

org 0100h

section .data
  SInstalled: db "DOSVER is allready installed.",0ah,0dh

section .text

; Jump to main program entry
jmp Main


; ------------------------------------
; The actual interrupt handler routine
; It's at the very beginning to save memory
; -------------------------------------

Int21Handler:
  ; Is it GetVersion?
  cmp ah,30h
  jne GoInt21

  ; Yes, is it installation check?
  cmp bx,'DO'
  jne NotCheck
  cmp cx,'SV'
  jne NoCheck
  cmp dx,'ER'
  jne NotCheck

  ; Installation check, report that we're here
  mov ax,0200h          ; DOSVER version 2.0
  mov bx,'IS'
  mov cx,'HE'
  mov dx,'RE'

NotCheck:
  pushf                 ; Simulate interrupt
  call GoInt21          ; Call original handler
  mov ax,0800h          ; Overwrite with version 8.0
  iret

; Here's some self modifying code magic
GoInt21:
  db 0eah               ; JMP instruction
OldIntOfs:
  dw 0                  ; Patched at runtime
OldIntSeg:
  dw 0                  ; Patched at runtime
FirstUnused:

; ------------------------------------
; Type the string at DS:SI to the screen
; -------------------------------------

TypeStr:
  mov al,[si]
  cmp al,0dh
  je Return
  mov ah,0eh
  mov bh,0
  mov bl,7
  int 10h
  inc si
  jmp TypeStr
Return:
  retn

; ------------------------------------
; Main entry point
; -------------------------------------

Main:
  ; Release environment block
  mov ax,[002ch]
  jz noenv
  mov es,ax
  mov ah,49h
  int 21h

noenv:
  ; Installation check
  mov ah,30h
  mov bx,'DO'
  mov cx,'SV'
  mov dx,'ER'
  int 21h
  cmp bx,'IS'
  jne NotInstalled
  cmp cx,'HE'
  jne NotInstalled
  cmp dx,'RE'
  jne NotInstalled

  ; Allready installed, type the message and exit
  mov si,SInstalled
  jmp TypeStr

; ------------------------------------
; Not installed, so install our handler and terminate (but stay resident)
; -------------------------------------

NotInstalled:
  ; Save old int 21h vector
  mov ax,3521h          ; get interrupt vector 21h
  int 21h
  mov [OldIntOfs],bx    ; store offset
  push es
  pop ax
  mov [OldIntSeg],ax    ; store segment

  ; Install new vector
  mov ax,2521h          ; set interrupt vector 21h
  mov dx,Int21Handler
  int 21h

  ; Terminate and stay resident
  mov dx,FirstUnused    ; We can release the memory after this part
  int 27h