The Unofficial (and no longer maintained) GNU Hurd FAQ, Version 0.13 Contributions by: Michael I. Bushnell Len Tower Trent Fisher jlr@usoft.spb.su Remy Card Louis-Dominique Dubeau Original Document by: Derek Upham ============================== Contents: Q4. What's all this about Mach 3.0 (and Mach 4.0)? ============================== Q4. What's all this about Mach 3.0 (and Mach 4.0)? As mentioned above, Mach is a micro-kernel, written at Carnegie Mellon University. A more descriptive term might be a greatest-common-factor kernel, since it provides facilities common to all ``real'' operating systems, such as memory management, inter-process communication, processes, and a bunch of other stuff. Unfortunately, the system calls used to access these facilities are only vaguely related to the familiar and cherished Unix system calls. There are no "fork", "wait", or "sleep" system-calls, no SIGHUPs, nothing like that. All this makes it rather difficult to, say, port GNU Emacs to a Mach box. The trick is, of course, to write an emulation library. Unix programs can then use (what they think are) POSIX system calls and facilities while they are really using Mach system calls and facilities. The simplest way of going about this is to take an ordinary Unix kernel, open it up, and rip out all the machine-specific guts; any time the Unix kernel talks to the machine, replace the code with calls to the Mach micro-kernel. Run this fake kernel on a Mach machine and you end up with something that looks and acts just like Unix (even to GNU Emacs). Note that the Unix kernel we have implemented is just one Really Big Mach program (called a single-server). The Hurd, on the other hand, breaks the giant Unix kernel down into various Mach programs running as daemons. Working in concert with facilities placed in the C library, these daemons provide all of the POSIX system-calls and features; from the outside they look just like a standard Unix kernel. This means that, for practical purposes, anything that you can port to Linux will also port to the Hurd. Of course, if a user wishes to run his own daemons, he can do that as well.... Mach 4.0 is an enhanced version of Mach 3.0, put out by the people at the University of Utah. They are working on another free operating system, and part of it includes an enhanced, more flexible version of Mach. The Hurd has moved to Mach 4.0, which is good, because it is a lot easier to build than 3.0 was. You can find more information on Mach by browsing the Hurd pages given in the next answer, or by looking at the Project Mach and Flux homepages at: Carnegie Mellon University (for Mach versions before 4.0): http://www.cs.cmu.edu/afs/cs.cmu.edu/project/mach/public/www/mach.html the University of Utah (for Mach 4.0): http://www.cs.utah.edu/projects/flux/mach4/html/ ============================== Footnotes: [[1]] Yes, I know that ``micro-kernel'' is about as apt a description as ``Reduced Instruction Set Chip'', but we're stuck with it.