A Technical History of Apple's Operating Systems - Mac OS X Internals

102 Chapter 1 A Technical History of Apple’s Operating Systems
1.8.3. Mach
The sequel to Accent was called Mach, which was conceived as a UNIX-compatible
Accent-inspired system. In retrospect, with respect to the first version (1.0) of
Mach, one could consider Accent and RIG to be Mach versions 0.9 and 0.8 respec-
tively.
When Mach was developed, UNIX had been around for over fifteen years.
Although the designers of Mach subscribed to the importance and usefulness of
UNIX, they noted that UNIX was no longer as simple or as easy to modify as it
once had been. Richard Rashid called the UNIX kernel a “dumping ground for vir-
tually every new feature or facility.” Mach’s design goals were partially a response
to the inexorably increasing complexity of UNIX.
The Mach project started in 1984 with an overall goal of creating a microker-
nel that would be the operating system foundation for other operating systems. The
project’s specific goals included the following.
• Provide full support for multiprocessing.
• Exploit other features of modern hardware architectures that were emerging
at that time. Mach aimed to support diverse architectures, including
shared memory access schemes such as Non-Uniform Memory Access
(NUMA) and No-Remote Memory Access (NORMA).
• Support transparent and seamless distributed operation.
• Reduce the number of features in the kernel to make it less complex,
while giving the programmer a very small number of abstractions to
work with. Nevertheless, the abstractions would be general enough to
allow several operating systems to be implemented on top of Mach.
• Provide compatibility with UNIX.
• Address shortcomings of previous systems such as Accent.
Mach was intended to primarily implement processor and memory manage-
ment, but no file system, networking, or I/O. The “real” operating system was to run