dos.zig

DOS SDK for Zig

Write and cross-compile DOS programs with the Zig programming language. Programs run in 32-bit protected mode and require a resident DPMI host. CWSDPMI is bundled with the executable for environments that do not have DPMI available.

To comply with the CWSDPMI license, published programs must provide notice to users that they have the right to receive the source code and/or binary updates for CWSDPMI. Distributors should indicate a site for the source in their documentation.

This package is in a primordial state. It is a minimal demonstration of how to create a simple DOS program with Zig. Only basic file/terminal input/output are working, and does not include proper error handling. It will require hacking if you wish to adapt it for your own needs.

Quick Start

Install:

Zig (version 0.11.0)
DOSBox

Setup submodules:

Run the command git submodule init && git submodule update

Add DOSBox to the path:

Mac: Add export PATH="$PATH:/applications/dosbox.app/contents/macos/" to your ~/.zshrc.
Windows: https://stackoverflow.com/questions/9546324/adding-a-directory-to-the-path-environment-variable-in-windows

Run:

zig build run

Design

There are five main components of this package:

DOS API wrappers call the 16-bit real mode interrupt 21 routines (via DPMI) and implement operating system interfaces for the Zig standard library.
DPMI API wrappers manage extended memory blocks and segments.
A custom linker script produces DJGPP COFF executables.
The CWSDPMI stub loader enters protected mode and runs the COFF executable attached to it.
A small demo program exercises all of the above.

Roadmap

Proper error handling.
Parse environment data (command, variables) and hook into standard library abstractions.
Implement mprotect for stack guard and zero pages.
Implement a page_allocator for the standard library.
Add graphical demo program.

Questions and Answers

Can I just target 16-bit real mode rather than require DPMI?

It is technically possible, but not a goal of this package. Zig (via LLVM) can generate “16-bit” code using the code16 ABI target. In reality, this code is often 32-bit instructions with added prefixes that override the address or operand size. Using code16 actually produces larger binaries. Additionally, the oldest CPU that can be targeted is an Intel 80386, which supports 32-bit protected mode. It’s guaranteed to be there and has a lot of advantages, so we might as well use it.

Why not integrate with an existing DOS toolchain like DJGPP?

This was attempted and had mixed results. DJGPP’s object format (COFF) is subtly different from the one produced by modern toolchains such as Zig. Crude conversion scripts had to be used to get them to work together and it was not robust. While it would be nice to leverage DJGPP’s C library, it ultimately felt like more trouble than it was worth. Not relying on a separate toolchain will make things easier in the long run.