The Evolution of the Mach Operating System Part 1.
I wanted to do a quick history lesson on Mach the precursor to the XNU kernel used by Apple. In part 1 we will cover Mach through the 2.5 release which would later become the foundation for the mac, iPhone, Apple Watch, and basically all of Apple's consumer electronics. Part 2 will cover where Mach went wrong and why the kernel wasn't more widely adopted outside of Apple.
None of this article was written by AI.
Genesis
Mach was a third-generation operating system designed at Carnegie Mellon University. Originally Mach was funded as a Defense Advanced Research Projects Agency (DARPA) research project with the goal of advancing distributed computing systems. Mach finds it roots in the Accent operating system itself being derived from the RIG operating system started at the University of Rochester . Signature capabilities of the Mach operating system such as ports and messages started with RIG and can still be found as the main interprocess communication (IPC) mechanism in Mach's successor Apple 's XNU kernel.
Mach was created to address the challenges with Accent. Multiprocessor CPUs and distrusted computing where on the horizon and porting Accent from the hardware it was designed on would be a challenge. Concurrently UNIX was quickly becoming the dominate operating system in universities and the enterprise but Accent compatibly was poor.
RIG and Accent did pioneer a number of features adopted by most modern operating systems. See the below references for a more complete history of RIG and Accent.
Enter Mach
Mach was born at an interesting time in history known as the UNIX wars. By the mid 80's several different companies were selling their own UNIX like operating systems leading to a period of great innovation and even greater fragmentation in operating systems. Most of the UNIX implementations at the time ran on kernels that were the products of evolution as opposed to well thought out design leading to non-portable kernel code with numerous ways to accomplish the same task. To be fair to the developers of the time operating systems were nowhere near as common nor mature as they are now. All of the UNIX variants that exist today (BSD, Linux, QNX, etc.) all have the benefit of learning from what came before.
Most folks today remember Mach as a microkernel but none of the early research papers use that phrase nor imply that today's notion of a micro kernel (a hardware abstraction layer that runs services like filesystems and UNIX in the user space) was a goal of the original project.
In an interview with the Computer History Museum Avie Tevanian one of the original designers of Mach and the creator of the XNU kernel states that the microkernel with user space servers goal came later as a research effort at CMU years after he left the university to implement Mach 2.5 in NextStep Oral History of Avie Tevanian.
All of the Early research papers prior to the notion of Mach 3.0 are consistent in the goals of the project. Mach focused on:
Mach realized the fundamental goals. The kernel has been ported to numerous architectures, ARM, x86, PowerPC to name a few and numerous operating systems have been ported to the kernel such as DOS, BSD, MacOS 9, etc.
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The API
Mach achieved its goals by focusing on providing fundamental abstractions that were reinvented by every kernel. Essentially Mach served as a hardware abstraction layer. Mach's real value proposition was once the kernel was implemented many, and multiple operating system could be implemented on top of the kernel using the Mach API to schedule, allocate memory, and perform interprocess communication.
In practice anyone looking to develop an operating system could start from Mach only needing to implement the assembler needed to get the kernel running on a given architecture if it wasn't already done. This portability made it easy to port the kernel to new architectures and made kernel development easier as operating system developers could focus on the unique value of the API they provide to higher level developers.
Some of the prominent API's provided by Mach are briefly described below.
Task
In Mach a task is the fundamental unit of resource allocation. Tasks have an address space, execution threads, priorities, and ports for IPC assigned to them. Tasks are lower level than processes providing the basic mechanism needed to schedule in the CPU and group resources for what we think of as a process.
Virtual Memory
Mach pioneered the Copy on Write VM-Object. A VM object is a reference to a contiguous bit of memory mapped into a tasks address space. Instead of directly assigning pages of memory Mach maps VM-Objects into a tasks address space abstracting the physical backing of memory and enabling features like copy on write. When processes share memory in Mach VM-Objects are mapped into each process's memory leading to efficiency gains.
Message Passing
Like most micro-kernels processes (Mach tasks) communicate by passing messages between processes. Mach implements this mechanism through the concept of ports. A port is an abstraction over a message queue enabling IPC. Each port has rights associated with that control who can send and receive messages on the given port. In addition to ports Mach includes an interface generator called MIG (Mach Interface Generator) that enables sending complex messages and data structures across ports.
Mach does include other abstractions, but these are the big three that we will cover in more detail in future blog post.
That's it for the brief history of Mach. Given the history of Mach it would be fair for one to wonder why Mach isn't more widely used or talked about.
No viable Mach implementation exists outside of the XNU kernel developed by Apple.
No viable implementation of Mach 3.0 exists which is odd but that is a story for another blog. Stay tuned for part 2.
You can find more in-depth references to the history of Mach in CMU's Mach papers here: https://www.cs.cmu.edu/afs/cs.cmu.edu/project/mach/public/www/doc/documents_top.html