25 Facts About Mach kernel

Mach is a kernel developed at Carnegie Mellon University by Richard Rashid and Avie Tevanian to support operating system research, primarily distributed and parallel computing.

The port was successful, but the resulting Accent Mach kernel was of limited practical use because it did not run existing software.

Major change between these experimental kernels and Mach was the decision to make a version of the existing 4.

Such a Mach kernel would be binary compatible with existing BSD software, making the system immediately useful for everyday use while still being a useful experimental platform.

Additionally, the new Mach kernel would be designed from the start to support multiple processor architectures, even allowing heterogeneous clusters to be constructed.

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Mach kernel started largely as an effort to produce a cleanly defined, UNIX-based, highly portable Accent.

Mach kernel developed on Accent's IPC concepts, but made the system much more UNIX-like in nature, even able to run UNIX programs with little or no modification.

Additionally, Mach kernel allowed any program to handle privileges that would normally be given to the operating system only, in order to allow user space programs to handle things like interacting with hardware.

The Mach kernel's job was reduced from essentially being the operating system to maintaining the "utilities" and scheduling their access to hardware.

Physically copying the message would be too slow, so Mach kernel relies on the machine's memory management unit to quickly map the data from one program to another.

Messages were checked for validity by the Mach kernel, to avoid bad data crashing one of the many programs making up the system.

Mach kernel was initially hosted as additional code written directly into the existing 4.

Mach kernel was originally intended to be a replacement for classical monolithic UNIX, and for this reason contained many UNIX-like ideas.

For instance, Mach kernel used a permissioning and security system patterned on UNIX's file system.

Since the Mach kernel was privileged over other OS servers and software, it was possible for malfunctioning or malicious programs to send it commands that would cause damage to the system, and for this reason the Mach kernel checked every message for validity.

For instance, Mach kernel was able to support multi-processor machines with ease.

Mach kernel included a server that could forward messages not just between programs, but even over the network, which was an area of intense development in the late 1980s and early 1990s.

Mach kernel 3 attempted to address this problem by providing a simple pager, relying on user-space pagers for better specialization.

The Chorus microMach kernel made this a feature of the basic system, allowing servers to be raised into the Mach kernel space using built-in mechanisms.

Mach kernel 4 attempted to address these problems, this time with a more radical set of upgrades.

In XNU, the file systems, networking stacks, and process and memory management functions are implemented in the kernel; and file system, networking, and some process and memory management functions are invoked from user mode via ordinary system calls rather than message passing; XNU's Mach messages are used for communication between user-mode processes, and for some requests from user-mode code to the kernel and from the kernel to user-mode servers.

Rest, the majority of the actual problem, was due to the Mach kernel performing tasks such as checking the message for port access rights.

For instance, a single-user operating system running a cell phone or robot might not need any of these features, and this is exactly the sort of system where Mach kernel's pick-and-choose operating system would be most valuable.

Likewise Mach kernel caused problems when memory had been moved by the operating system, another task that only really makes sense if the system has more than one address space.

For instance, the L4 kernel includes only seven system calls and uses 12k of memory, whereas Mach 3 includes about 140 functions and uses about 330k of memory.

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