RealXV6

A Port of the UNIX Version 6 (V6) Kernel to the 8086 PC in Real Mode.

Overview

This project is a port of the classic UNIX Version 6 (V6) kernel to the Intel 8086/8088 architecture, targeting the original IBM PC and its compatibles. The kernel code is based on the version famously analyzed in the Lions' Commentary on UNIX 6th Edition.

The userspace is a hybrid, containing programs from the original V6 distribution as well as some from the xv6 project.

Philosophy: A Port, Not a Rewrite

Unlike projects like xv6 which are modern rewrites inspired by V6, RealXV6 aims to be a faithful port. The goal is to preserve the original structure, algorithms, and spirit of the V6 kernel as much as possible. This makes it an excellent resource for studying the original design on a more accessible platform for modern study, typically through emulation.

Key aspects of the original design are intentionally retained:

• Original Scheduler Logic: The logic at the famous /* You are not expected to understand this */ comment in the scheduler retains its original meaning. As a direct result, the original V6 process swapping mechanism is also fully preserved.
• Process Switching: The original PDP-11 savu/retu assembly primitives relied on the C compiler saving the environment on every function call. In this port, the savu/retu routines are still used, but their role is now limited to managing the switch of the user area (u area). Since the 8086 architecture lacks an MMU, this switch is handled by memcpy within these routines. The actual process context switch is handled by the more portable save/resume functions from UNIX V7.

Architecture

The original V6 was designed for the 16-bit PDP-11. By targeting the native 16-bit architecture of the 8086, this port minimizes the architectural changes required, keeping the code as close to the original as possible.

While understanding the PDP-11 architecture is key to studying the original V6 source, the 8086 architecture is more widely understood. Furthermore, the widespread availability of mature 8086 PC emulators makes this platform highly accessible for development and study. For developers, knowledge of how a typical PC bootloader operates is sufficient to understand the low-level aspects of RealXV6.

The project uses the simple .COM executable format. This choice simplifies development as .COM files are memory-relocatable by design, which makes it easier to manage processes in memory and to implement swapping them to disk.

Building the System

The project is built using the Open Watcom C compiler and toolchain. Open Watcom is a powerful cross-platform suite that can run on modern operating systems like Windows, Linux, and macOS, while still targeting the 8086's 16-bit architecture.

To build the kernel, please refer to the included Makefile.

Code Structure

The source code is organized into several key directories:

• /ken: Named for Ken Thompson, this directory contains the main, machine-independent kernel code (e.g., system calls, process management, file system).
• /dmr: Named for Dennis M. Ritchie, this directory contains machine-dependent drivers and low-level code for the PC architecture (e.g., keyboard, IDE, TTY).
• /h: Contains all kernel header files, defining the core data structures and constants.