Simple "Hello World!" in Turbo Assembler

hello.asm

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;;;;;;;;;;     HELLO.ASM: "Hello World" for DOS/Turbo Assembler     ;;;;;;;;;;
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;; A comparatively simple "Hello, World" program in DOS assembly language.
;; It prompts the user for a yes/no response to "is it after noon?" and then
;; displays a message based on the result. If the user answers with something
;; other than 'Y' or 'N' (case-insensitively) it displays an error message and
;; prompts again.
;;
;; Written with Borland Turbo Assembler 4.0 mnemonics. Assemble with:
;;
;; Normal assembly:
;;    C> TASM /s /ml hello
;;    C> TLINK hello
;;
;; Assemble and generate list file and xref file:
;;
;;    C> TASM /s /c /la /ml /z /zd hello,,hello,hello
;;    C> TLINK hello
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;; Modified from the "hello.asm" on page 9 of the Turbo Assember 4.0 User's
;; Guide by Tammy Cravit <[email protected]>
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; This program is equivalent to the following Python code:
;;
;;      import sys
;;      from getch import getch
;;
;;      input_char = ""
;;      while not input_char in ['y', 'n']:
;;         print("Is it after noon (Y/N)? ")
;;         input_char = str(getch()).lower()[0]
;;         if not input_char in ['y', 'n']:
;;             print("Invalid input!")
;;             input_char = ''
;;
;;     if input_char == 'y':
;;         print("Good afternoon, world!")
;;         sys.exit(1)
;;     elif input_char == 'n':
;;         print("Good morning, world!")
;;         sys.exit(2)
;;
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.8086                                                             ; Disable 286/386 instructions
.MODEL SMALL                                                      ; Small memory model
                                                                  ; NEAR code, NEAR data
                                                                  ; CS=_text, DS=SS=dgroup
.STACK 200h                                                       ; Allocate 1KB for stack segment
VERSION T400                                                      ; TASM 4.00 mode

.DATA                                                             ; Begin the Data Segment

;; DOS API functions are called by putting the "function index" into the
;; AH register, the arguments to the functions in the other CPU registers,
;; and then invoking the "interrupt handler" for interrupt 33 (21h). These
;; constants define the DOS API functions we're using.

    __getchar          DB 1h                                      ; DOS "get character" function index
    __printstr         DB 9h                                      ; DOS "print string" function index
    __exitapp          DB 4ch                                     ; DOS "exit program" function index

    EOS                DB '$'                                     ; End of string marker
    CRLF               DB 13, 10                                  ; Newline character (CR + LF)

    ExitMorning        DB 01h                                     ; Return code if it's morning
    ExitAfternoon      DB 02h                                     ; Return code if it's afternoon

    ;; Prompt strings

    TimePrompt         DB 'Is it after noon (Y/N)?', EOS
    MorningMessage     DB 'Good morning, world!', CRLF, EOS
    AfternoonMessage   DB 'Good afternoon, world!', CRLF, EOS
    InvalidCharMessage DB 'Invalid response!', CRLF, CRLF, EOS

.CODE                                               ; Begin the Code Segment

    start:     
               mov ax, @data                        ; Set up the data segment address
               mov ds, ax

    promptuser:
               mov dx, OFFSET TimePrompt            ; Display the time prompt
               mov ah, __printstr
               mov 21h
               mov ah, __getchar                    ; Read a character
               int 21h

    parseinput:
               or  al, 20h                          ; Convert character to lowercase
               cmp al, 'y'                          ; If the character is 'y', go to 'goodmorn'
               je  goodmorn
               cmp al, 'n'                          ; If the character is 'n', go to 'goodaft'
               je  goodaft
               jmp invalid                          ; Otherwise, prompt again

    goodmorn:                                       ; Display the Good Morning message
               mov dx, OFFSET MorningMessage
               mov ah, _printstr
               int 21h
               mov al, ExitMorning                  ; Get ready to exit and return a status
               jmp do_exit                          ; code to DOS.

    goodaft:                                        ; Display the Good Afternoon message
               mov dx, OFFSET AfternoonMessage
               mov ah, _printstr
               int 21h
               mov al, ExitAfternoon                ; Get ready to exit and return a status
               jmp do_exit                          ; code to DOS.

    invalid:   
               mov dx, OFFSET InvalidCharMessage    ; Display the "invalid" message.
               mov ah, _printstr
               int 21h
               jmp promptuser                       ; And prompt the user again.

    do_exit:                                        ; Exit to DOS. We've already loaded
               mov ah, __exitapp                    ; the return code into AL
               int 21h

END start