Let's build a DOS .EXE like it's 1992

05 Jan 2025

Introduction

In this post, we will build a simple DOS .EXE file from scratch. We will use the NASM assembler and the OpenWatcom C compiler to create a simple program that prints “Hello, World!” to the console.

This post is part of series where by I am trying to recreate the environment that I used in 1990s - stay tuned for more posts on this topic.

Prerequisites

You will need the following tools to follow along with this post:

We’ll use NASM to write the assembly code for our program, OpenWatcom to link the assembly code with the C runtime library, and FreeDOS to run the resulting .EXE file. (this was not as easily done in 1992, or 2025)

Setting up the environment

The above software is available on the FreeDOS bonus CD, which you can download from the FreeDOS website. You can install the addition softwareusing FDIMPLES or FDUPDATE commands in FreeDOS. (nice and easy)

Building the program

First, create a new file called hello.asm and add the following code:


section .text
    global _start

_start:
    mov ah, 9
    mov dx, msg
    int 21h
    mov ah, 4ch
    int 21h

section .data
    msg db 'Hello, World!', 0

NOTE: ChatGPT was terrible at this, I had to fix the code, compile and linking commands to get it to work. Most of the time it will build the code, but not function correctly. With misaligned data segments and the like, it was a hot mess.

Next we need to compile the assembly code using NASM:

nasm -f obj hello.asm

the we need to link the object file with the OpenWatcom C runtime library:

wlink sys dos name hello.exe start=_start file hello.obj

Breakng this down a bit, the sys dos option tells the linker to create a DOS executable file. The name hello.exe option specifies the name of the output file. The start=_start option tells the linker to use the _start symbol as the entry point of the program. The file hello.obj option tells the linker to link the hello.obj file with the C runtime library.

Running the program

To run the program, simply run it from the command line. You should see the message “Hello, World!” printed to the console.

Conclusion

In this post, we built a simple DOS .EXE file from scratch using the NASM assembler and the OpenWatcom C compiler. We used FreeDOS to run the resulting .EXE file and saw the message “Hello, World!” printed to the console. This demonstrates how to create a simple DOS program using tools that were commonly used in 1992.

I hope you found this post helpful. If you have any questions or comments, please feel free to contact me.

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