XiDOS

A POSIX compatibility layer for MS-DOS / FreeDOS.

XiDOS is to DOS what Cygwin is to Windows: a runtime library plus headers that implement a substantial subset of the POSIX API on top of the DOS kernel. Where Cygwin maps POSIX semantics onto the Win32 API, XiDOS maps them onto the MS-DOS / FreeDOS system-call interface (chiefly INT 21h) and the underlying BIOS.

The goal is to let Unix-flavoured C compile and run on real (or emulated) DOS machines with minimal changes — open/read/write/close, path translation between POSIX (/usr/bin) and DOS (C:\USR\BIN) conventions, errno, process spawning, and a growing slice of the C library that DOS does not provide.

⚠️ Status: early scaffolding. The API surface is being stubbed out. Nothing here is production-ready yet.

Documentation

  • Architecture — the hybrid design, the backend seam, the process/fork() problem, and a survey of prior art (DJGPP, EMX, Cygwin).
  • Building — toolchain setup for the real-mode (Open Watcom) and pmode (DJGPP-in-Docker) backends, plus the DOSBox run/debug harness.

The source lives at github.com/Arawn-Davies/xidos.

Hybrid design in one picture

DOS in real mode caps out at 640K and 16-bit pointers — fine for small tools, but bash and GCC need a flat 32-bit address space. XiDOS resolves this with a backend seam: the whole POSIX layer is written once and machine-independent, and exactly one backend is linked in per build.

  Host OS API translated to Analogy
Cygwin Windows (NT) Win32 full POSIX emulation
MinGW Windows (NT) Win32 (thin) native, minimal layer
DJGPP DOS (32-bit pmode) DPMI + libc GCC port
XiDOS MS-DOS / FreeDOS INT 21h / BIOS POSIX emulation
  • realmode (v0) — 16-bit, services via INT 21h/BIOS. Small POSIX-style utilities on any 8086+. No flat memory, no real fork().
  • pmode (v1) — 32-bit protected mode on top of DJGPP. The target that makes bash / GCC / TCC feasible, plus EMX/Cygwin-style fork() emulation.

See the architecture doc for the full design.

Roadmap

v0 — realmode backend (small utilities)

  • Project scaffold, hybrid build system, backend seam
  • POSIX ↔ DOS path translation (/c/foo ↔ C:\FOO), host-tested
  • fd table mapping POSIX fds to backend handles
  • open/read/write/close/lseek over INT 21h
  • errno mapping from DOS error codes
  • Directory operations (opendir/readdir)
  • Process spawning via INT 21h AH=4Bh (blocking spawn, no fork)
  • Standard streams and basic TTY handling
  • A mount-style table for the virtual filesystem root

v1 — pmode backend (bash / GCC / TCC)

  • DJGPP-hosted backend wiring (forward xbe_* to DJGPP libc)
  • fork() emulation (EMX/Cygwin-style address-space copy)
  • exec* family + environment passing
  • Bring up TCC, then a shell (bash) on top of XiDOS
  • Signals subset, termios subset

License

Distributed under the GNU Lesser General Public License v2.1 (LGPL-2.1), so that programs compiled against the XiDOS runtime are not themselves forced to be GPL — only modifications to XiDOS itself must stay open. This mirrors how glibc and the Cygwin runtime are licensed, but stays in the GPL-2 family.