Editing Mach (kernel) (section)

===New concepts===
Unix pipes offered a conceptual system that could be used to build arbitrarily complex solutions out of small cooperating programs. These smaller programs were easier to develop and maintain, and had well-defined interfaces that simplified programming and debugging. These qualities are even more valuable for device drivers, where small size and bug-free performance was extremely important. There was a strong desire to model the kernel on the same basis of small cooperating programs.{{Citation needed|date=February 2025}}

One of the first systems to use a pipe-like system underpinning the operating system was the [[Aleph kernel]] developed at the [[University of Rochester]]. This introduced the concept of ports, which were essentially a [[shared memory]] implementation. In Aleph, the kernel was reduced to providing access to the hardware, including memory and the ports, while conventional programs using the ports system implemented all behavior, from device drivers to user programs. This concept greatly reduced the size of the kernel, and permitted users to experiment with different drivers simply by loading them and connecting them together at runtime. This greatly eased the problems when developing new operating system code, which would otherwise require the machine to be restarted. The overall concept of a small kernel and external drivers became known as a microkernel.{{Citation needed|date=February 2025}}

Aleph was implemented on [[Data General Eclipse]] minicomputers and was tightly bound to them. This machine was far from ideal, since it required memory to be copied between programs, which resulted in considerable performance overhead. It was also quite expensive. Nevertheless, Aleph proved that the basic system was sound, and went on to demonstrate [[computer cluster]]ing by copying the memory over an early [[Ethernet]] interface.{{Citation needed|date=February 2025}}

Around this time a new generation of [[Central processing unit|central processors]] (CPUs) were coming to market, offering a 32-bit [[address space]] and (initially optional) support for a [[memory management unit]] (MMU). The MMU handled the instructions needed to implement a [[virtual memory]] system by keeping track of which ''pages'' of memory were in use by various programs. This offered a new solution to the port concept, using the [[copy-on-write]] (COW) mechanism provided by the virtual memory system. Instead of copying data between programs, all that was required was  to instruct the MMU to provide access to that same memory. This system would implement the [[interprocess communication]]s (IPC) system with dramatically higher performance.{{Citation needed|date=February 2025}}

This concept was picked up at Carnegie-Mellon, who adapted Aleph for the [[PERQ|PERQ workstation]] and implemented it using copy-on-write. The port was successful, but the resulting Accent kernel was of limited practical use because it did not run existing software. Moreover, Accent was as tightly tied to PERQ as Aleph was to the Eclipse.{{Citation needed|date=February 2025}}