DOS Syscall Example

Minimal DOS program demonstrating direct INT 21h system calls using the Open Watcom C/C++ compiler. Supports both 16-bit real mode and 32-bit DOS/4GW protected mode targets.

Building

Requires Open Watcom and CMake 3.21+.

cmake --preset dos32-debug --fresh
cmake --build --preset dos32-debug

Available presets: dos32-debug, dos32-release, dos16-debug, dos16-release.

Output: build/dossyscl.exe

Overview

Two syscall techniques are demonstrated in src/dossyscall.c:

• Library-based (dos_get_version) -- uses int86() / int386() to invoke INT 21h/AH=30h.
• Inline assembly (dos_get_time) -- uses Watcom #pragma aux to emit inline INT 21h/AH=2Ch with explicit register bindings.

Both paths compile for 16-bit and 32-bit targets via #if defined(_M_I86).