/hello

A 23-byte “hello, world” program assembled with DEBUG.EXE in MS-DOS

Primary LanguageAssemblyMIT LicenseMIT

Programming "Hello, World" in MS-DOS

The program HELLO.COM was developed on MS-DOS Version 6.22 using the DOS program named DEBUG.EXE. It is exactly 23 bytes in length. It can be used to print the string "hello, world" followed by newline to standard output.

Assemble

Here is the complete DEBUG.EXE session that shows how this program was written:

C:\>debug
-A
1165:0100 MOV AH, 9
1165:0102 MOV DX, 108
1165:0105 INT 21
1165:0107 RET
1165:0108 DB 'hello, world', D, A, '$'
1165:0117
-G
hello, world

Program terminated normally
-N HELLO.COM
-R CX
CX 0000
:17
-W
Writing 00017 bytes
-Q

C:\>HELLO
hello, world

C:\>

Note that the N (name) command specifies the name of the file where we write the binary machine code to. Also, note that the W (write) command expects the registers BX and CX to contain the number of bytes to be written to the file. When DEBUG.EXE starts, it already initializes BX to 0 automatically, so we only set the register CX to 1A (decimal 26) with the R CX command above.

The debugger session inputs are archived in the file named HELLO.TXT, so the binary file named HELLO.COM can also be created by running the following DOS command:

DEBUG < HELLO.TXT

The binary executable file can be created on a Unix or Linux system using the printf command as follows:

printf '\xB4\x09\xBA\x08\x01\xCD\x21\xC3\x68\x65\x6C\x6C\x6F\x2C\x20\x77\x6F\x72\x6C\x64\x0D\x0A\x24' > HELLO.COM

Unassemble

Here is a disassembly of HELLO.COM to confirm that it has been written correctly:

C:\>DEBUG
-N HELLO.COM
-L
-U 100 116
117C:0100 B409          MOV     AH,09
117C:0102 BA0801        MOV     DX,0108
117C:0105 CD21          INT     21
117C:0107 C3            RET
117C:0108 68            DB      68
117C:0109 65            DB      65
117C:010A 6C            DB      6C
117C:010B 6C            DB      6C
117C:010C 6F            DB      6F
117C:010D 2C20          SUB     AL,20
117C:010F 776F          JA      0180
117C:0111 726C          JB      017F
117C:0113 64            DB      64
117C:0114 0D0A24        OR      AX,240A
-D 100 116
117C:0100  B4 09 BA 08 01 CD 21 C3-68 65 6C 6C 6F 2C 20 77   ......!.hello, w
117C:0110  6F 72 6C 64 0D 0A 24                              orld..$

Run

To run this program on MS-DOS, simply enter the following command at the command prompt:

HELLO

Credits

  • userbinator for suggesting that MOV AH, 0 and INT 21 instructions to terminate the program can be replaced with RET thus saving 3 bytes.
  • colejohnson66 for suggesting that moving the string from top to bottom avoids a JMP instruction thus saving 2 bytes.

License

This is free and open source software. You can use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of it, under the terms of the MIT License. See LICENSE.md for details.

This software is provided "AS IS", WITHOUT WARRANTY OF ANY KIND, express or implied. See LICENSE.md for details.

More

If you liked this repository, see reboot.