Editing Mach (kernel) (section)

===Development===
Mach was initially hosted as additional code written directly into the existing 4.2BSD kernel, allowing the team to work on the system long before it was complete. Work started with the already functional Accent IPC/port system, and moved on to the other key portions of the OS: tasks, threads, and virtual memory. As portions were completed various parts of the BSD system were re-written to call into Mach, and a change to 4.3BSD was also made during this process.

By 1986 the system was complete to the point of being able to run on its own on the [[DEC VAX]]. Although doing little of any practical value, the goal of making a microkernel was realized. This was soon followed by versions on the [[IBM RT PC]] and for [[Sun Microsystems]] [[68030]]-based workstations, proving the system's portability. By 1987 the list included the Encore Multimax and [[Sequent Computer Systems|Sequent Balance]] machines, testing Mach's ability to run on multiprocessor systems. A public Release 1 was made that year, and Release 2 followed the next year.

Throughout this time the promise of a "true" microkernel had not yet been delivered. These early Mach versions included the majority of 4.3BSD in the kernel, a system known as a POE Server, resulting in a kernel that was actually larger than the UNIX it was based on. The idea, however, was to move the UNIX layer out of the kernel into user-space, where it could be more easily worked on and even replaced outright. Unfortunately performance proved to be a major problem, and a number of architectural changes were made in order to solve this problem. Unwieldy UNIX licensing issues also plagued researchers, so this early effort to provide a non-licensed UNIX-like system environment continued to find use, well into the further development of Mach.

The resulting Mach 3 was released in 1990, and generated intense interest. A small team had built Mach and ported it to a number of platforms, including complex multiprocessor systems which were causing serious problems for older-style kernels. This generated considerable interest in the commercial market, where a number of companies were considering changing hardware platforms. If the existing system could be ported to run on Mach, it seemed it would then be easy to change the platform underneath.

Mach received a major boost in visibility when the [[Open Software Foundation]] (OSF) announced they would be hosting future versions of [[OSF/1]] on Mach 2.5, and were investigating Mach 3 as well. Mach 2.5 was also selected for the [[NeXTSTEP]] system and a number of commercial multiprocessor vendors. Mach 3 led to a number of efforts to port other operating systems parts for the microkernel, including [[IBM]]'s [[Workplace OS]] and several efforts by [[Apple Inc.|Apple]] to build a cross-platform version of the [[classic Mac OS]].<ref name=":5" /> Support for running DOS applications in a Mach 3.0 environment was demonstrated by researchers, following on from earlier work running the classic Mac OS and [[MultiFinder]] under Mach 2.5.<ref name="usenixmach1991_malan">{{ cite journal | url=https://archive.org/details/1991-proceedings-mach-symposium-monterey/page/27/mode/1up | title=DOS as a Mach 3.0 Application | journal=Proceedings of the Usenix Mach Symposium | publisher=Usenix Association | date=November 1991 | access-date=January 19, 2024 | first1=Gerald | last1=Malan | first2=Richard | last2=Rashid | first3=David | last3=Golub | first4=Robert | last4=Baron | pages=27–40 }}</ref> A research project at [[Digital Equipment Corporation]] investigated the feasibility of hosting [[OpenVMS]] on top of the Mach 3 kernel, and created a proof of concept with a subset of VMS' features.<ref name="wiecek1992">{{ cite journal | url=https://archive.org/details/1992-proceedings-microkernels-seattle/page/187/mode/1up | title=A Model and Prototype of VMS Using the Mach 3.0 Kernel | journal=Proceedings of the USENIX Workshop on Micro-Kernels and Other Kernel Architectures | last1=Wiecek | first1=Cheryl A. | last2=Kaler | first2=Christopher G. | last3=Fiorelli | first3=Stephen | last4=Davenport, Jr. | first4=William C. | last5=Chen | first5=Robert C. | date=April 1992 | access-date=September 20, 2021 | pages=187–203 }}</ref>