printable COM file

print.asm

; $ nasm -fbin print.asm -o print.com
; $ cat print.com
; h<|XP- {P_X(%GGG(%GGWZ- sh LI!XI!Hello, DOS!$
;
; C:\>PRINT.COM
; Hello, DOS!

org 100h

    ; di <- replace
    push word replace+0x7b20
    pop  ax
    push ax  ; used below
    sub  ax, 0x7b20
    push ax
    pop  di

    ; restore ah <- 0x7c
    ; 0x49 - 0x7c = 0xcd
    pop  ax
    sub  [di], ah
    inc  di
    inc  di
    inc  di
    sub  [di], ah

    ; dx <- msg
    inc  di
    inc  di
    push di
    pop  dx

    ; ah <- 0x09 (write string)
    sub  ax, 0x7320

    ; prepare ah <- 0x4c (exit)
    push word 0x4c20

replace:
    db 0x49, 0x21  ; becomes int 0x21
    pop  ax
    db 0x49, 0x21  ; becomes int 0x21

msg:
    db "Hello, DOS!$"