When we talk about free software, we usually refer to the free software licenses. We also need relief from software patents, so our freedom is not restricted by them. But there is a third type of freedom we need, and that's user freedom.
????????????? Expert users don't take a system as it is. They like to change the configuration, and they want to run the software that works best for them. That includes window managers as well as your favourite text editor. But even on a GNU/Linux system consisting only of free software, you can not easily use the filesystem format, network protocol or binary format you want without special privileges. In traditional Unix systems, user freedom is severly restricted by the system administrator.
????????????? The Hurd is built on top of CMU's Mach 3.0 kernel and uses Mach's virtual memory management and message-passing facilities. The GNU C Library will provide the Unix system call interface, and will call the Hurd for needed services it can't provide itself. The design and implementation of the Hurd is being lead by Michael Bushnell, with assistance from Richard Stallman, Roland McGrath, Jan Brittenson, and others.
????????????? The fundamental purpose of an operating system (OS) is to enable a variety of programs to share a single computer efficiently and productively. This demands memory protection, preemptively scheduled timesharing, coordinated access to I/O peripherals, and other services. In addition, an OS can allow several users to share a computer. In this case, efficiency demands services that protect users from harming each other, enable them to share without prior arrangement, and mediate access to physical devices.
On today's computer systems, programmers usually implement these goals through a large program called the kernel. Since this program must be accessible to all user programs, it is the natural place to add functionality to the system. Since the only model for process interaction is that of specific, individual services provided by the kernel, no one creates other places to add functionality. As time goes by, more and more is added to the kernel.
A traditional system allows users to add components to a kernel only if they both understand most of it and have a privileged status within the system. Testing new components requires a much more painful edit-compile-debug cycle than testing other programs. It cannot be done while others are using the system. Bugs usually cause fatal system crashes, further disrupting others' use of the system. The entire kernel is usually non-pageable. (There are systems with pageable kernels, but deciding what can be paged is difficult and error prone. Usually the mechanisms are complex, making them difficult to use even when adding simple extensions.)
Because of these restrictions, functionality which properly belongs behind the wall of a traditional kernel is usually left out of systems unless it is absolutely mandatory. Many good ideas, best done with an open/read/write interface cannot be implemented because of the problems inherent in the monolithic nature of a traditional system. Further, even among those with the endurance to implement new ideas, only those who are privileged users of their computers can do so. The software copyright system darkens the mire by preventing unlicensed people from even reading the kernel source The Hurd removes these restrictions from the user. It provides an user extensible system framework without giving up POSIX compatibility and the unix security model.
????????????? When Richard Stallman founded the GNU project in 1983, he wanted to write an operating system consisting only of free software. Very soon, a lot of the essential tools were implemented, and released under the GPL. However, one critical piece was missing: The kernel. After considering several alternatives, it was decided not to write a new kernel from scratch, but to start with the Mach microkernel. This was in 1988, and it was not before 1991 that Mach was released under a license allowing the GNU project to distribute it as a part of the system.
????????????? Microkernels were very popular in the scientific world around that time. They don't implement a full operating system, but only the infrastructure needed to enable other tasks to implement most features. In contrast, monolithical kernels like Linux contain program code of device drivers, network protocols, process management, authentication, file systems, POSIX compatible interfaces and much more.
So what are the basic facilities a microkernel provides? In general, this is resource management and message passing. Resource management, because the kernel task needs to run in a special privileged mode of the processor, to be able to manipulate the memory management unit and perform context switches (also to manage interrupts). Message passing, because without a basic communication facility the other tasks could not interact to provide the system services. Some rudimentary hardware device support is often necessary to bootstrap the system. So the basic jobs of a microkernel are enforcing the paging policy (the actual paging can be done by an external pager task), scheduling, message passing and probably basic hardware device support.
Mach was the obvious choice back then, as it provides a rich set of interfaces to get the job done. Beside a rather brain-dead device interface, it provides tasks and threads, a messaging system allowing synchronous and asynchronous operation and a complex interface for external pagers. The GNU project maintains its own version of Mach, called GNU Mach, which is based on Mach 4.0. In addition to the features contained in Mach 4.0, the GNU version contains many of the Linux 2.0 block device and network card drivers.
?????????? Small changes can have far reaching side effects Because the system is split up into several components, clean interfaces have to be developed, and the responsibilities of each part of the system must be clear.
Once a microkernel is written, it can be used as the base for several different operating systems. Those can even run in parallel which makes debugging easier. When porting, most of the hardware dependant code is in the kernel.
Much of the code that doesn't need to run in the special kernel mode of the processor is not part of the kernel, so stability increases because there is simply less code to break. New features are not added to the kernel, so there is no need to hold the barrier high for new operating system features. Compare this to a monolithical kernel, where you either suffer from creeping featuritis or you are intolerant of new features (we see both in the Linux kernel).
????????????? Because in a monolithical kernel, all parts of the kernel can access all data structures in other parts, it is more likely that short cuts are used to avoid the overhead of a clean interface. This leads to a simple speed up of the kernel, but also makes it less comprehensible and more error prone. A small change in one part of the kernel can break remote other parts.
A single-server system is comparable to a monolithic kernel system. It has similar advantages and disadvantages.
There exist a couple of operating systems based on Mach, but they all have the same disadvantages as a monolithical kernel, because those operating systems are implemented in one single process running on top of the kernel. This process provides all the services a monolithical kernel would provide. This doesn't make a whole lot of sense (the only advantage is that you can probably run several of such isolated single servers on the same machine). Those systems are also called single-server systems. The Hurd is the only usable multi-server system on top of Mach. In the Hurd, there are many server programs, each one responsible for a unique service provided by the operating system. These servers run as Mach tasks, and communicate using the Mach message passing facilities. One of them does only provide a small part of the functionality of the system, but together they build up a complete and functional POSIX compatible operating system.
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