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.
More Usable Approach To OS Design
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
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 micro kernel.
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