1. Introduction

GNU Mach is the microkernel of the GNU Project. It is the base of the operating system, and provides its functionality to the Hurd servers, the GNU C Library and all user applications. The microkernel itself does not provide much functionality of the system, just enough to make it possible for the Hurd servers and the C library to implement the missing features you would expect from a POSIX compatible operating system.

1.1 Audience

This manual is designed to be useful to everybody who is interested in using, administering, or programming the Mach microkernel.

If you are an end-user and you are looking for help on running the Mach kernel, the first few chapters of this manual describe the essential parts of installing and using the kernel in the GNU operating system.

The rest of this manual is a technical discussion of the Mach programming interface and its implementation, and would not be helpful until you want to learn how to extend the system or modify the kernel.

This manual is organized according to the subsystems of Mach, and each chapter begins with descriptions of conceptual ideas that are related to that subsystem. If you are a programmer and want to learn more about, say, the Mach IPC subsystem, you can skip to the IPC chapter (see section 4. Inter Process Communication), and read about the related concepts and interface definitions.

1.2 Features

GNU Mach is not the most advanced microkernel known to the planet, nor is it the fastest or smallest, but it has a rich set of interfaces and some features which make it useful as the base of the Hurd system.

it's free software

Anybody can use, modify, and redistribute it under the terms of the GNU General Public License (see section A. GNU GENERAL PUBLIC LICENSE). GNU Mach is part of the GNU system, which is a complete operating system licensed under the GPL.

it's built to survive

As a microkernel, GNU Mach doesn't implement a lot of the features commonly found in an operating system, but only the bare minimum that is required to implement a full operating system on top of it. This means that a lot of the operating system code is maintained outside of GNU Mach, and while this code may go through a complete redesign, the code of the microkernel can remain comparatively stable.

it's scalable

Mach is particularly well suited for SMP and network cluster techniques. Thread support is provided at the kernel level, and the kernel itself takes advantage of that. Network transparency at the IPC level makes resources of the system available across machine boundaries (with NORMA IPC, currently not available in GNU Mach).

it exists

The Mach microkernel is real software that works Right Now. It is not a research or a proposal. You don't have to wait at all before you can start using and developing it. Mach has been used in many operating systems in the past, usually as the base for a single UNIX server. In the GNU system, Mach is the base of a functional multi-server operating system, the Hurd.

1.3 Overview

An operating system kernel provides a framework for programs to share a computer's hardware resources securely and efficiently. This requires that the programs are seperated and protected from each other. To make running multiple programs in parallel useful, there also needs to be a facility for programs to exchange information by communication.

The Mach microkernel provides abstractions of the underlying hardware resources like devices and memory. It organizes the running programs into tasks and threads (points of execution in the tasks). In addition, Mach provides a rich interface for inter-process communication.

What Mach does not provide is a POSIX compatible programming interface. In fact, it has no understanding of file systems, POSIX process semantics, network protocols and many more. All this is implemented in tasks running on top of the microkernel. In the GNU operating system, the Hurd servers and the C library share the responsibility to implement the POSIX interface, and the additional interfaces which are specific to the GNU system.

1.4 History

XXX A few lines about the history of Mach here.