MERT operating system - Revision history

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'''MERT''' (an acronym for '''Multi-Environment Real Time''') was an influential [[operating system]] from [[Bell Laboratories|Bell Labs]]. It was one of the earliest to be constructed using an organizational concept that later became known as a [[microkernel]] system; it was one of the very earliest successful ones.

MERT was created in the 1970s, and was a spinoff of the [[UNIX]] work; it ran on [[Digital Equipment Corporation|DEC]] [[PDP-11]] computers, models -11/45 and -11/70 (it made use of the 'supervisor' mode provided by the [[memory management]] of those models). The basic kernel (below) mostly written in [[assembly language]]; the rest (file manager, UNIX supervisor, etc) was in [[C]].

Although it was not exactly a microkernel as they are today, it was a definitely not a monolithic [[kernel]]; it was a major step down the road to microkernels. MERT was an operating system which was divided up into several semi-independent components, all of which ran on a lower-level "kernel" (as they described it).

The kernel provided only the lowest-level basic mechanisms (memory management, [[process]] scheduling, etc); the other components needed for an operating system (e.g. a [[file system]]) were constructed as processes which ran on top of the kernel. Inter-process communication was done with messages, event flags, and shared memory and shared [[file]]s.

MERT was also intended to be used in [[real-time]] applications, and had a number of features to meet this goal. Process scheduling had real-time mechanisms, as did the file system.

The [[source code]] of the original PDP-11 MERT sadly seems to have been lost; it was later renamed 'UNIX Real-Time' (UNIX-RT). MERT was later [[software port|ported]] to the [[multi-processor]] 3B20D computer in the form of the '''DMERT operating system''' - Duplex MERT - which was used for many functions in the Bell System; DMERT was later renamed 'Unix RTR' (Real-Time Reliable).

==Internal structure==

MERT's basic overall internal organization has four layers:

* the 'kernel', the lowest: provided all the basic primitives such as inter-process communication, process dispatching; also allocation of all basic system resources ([[main memory]], [[address space]] units, etc); includes initial [[trap]] and [[interrupt]] handlers, etc
* privileged processes: [[device driver]]s, [[file]] manager, memory manager and [[swapping]] manager, scheduler
* supervisor processes: provide the environment for user programs (one is a UNIX environment)
* user programs: applications

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==Further reading==

* D. L. Bayer, H. Lycklama, ''MERT - a multi-environment real-time operating system'', (Fifth ACM Symposium on Operating Systems Principles, Austin, Texas, 1975)

==External links==

* H. Lycklama, D. L. Bayer, [https://www.tuhs.org/Archive/Documentation/Papers/BSTJ/bstj57-6-2049.pdf The MERT Operating System] (The Bell System Technical Journal, July-August 1978, Vol. 57, No. 6, Part 2)
* R.E. Anderson, Jr Kane, P.S. Mccabe, [http://www.vtda.org/pubs/BSTJ/vol62-1983/articles/bstj62-1-291.pdf The 3B20D Processor & DMERT Operating System: Overview, Architecture, and Performance of DMERT] (The Bell System Technical Journal, January 1983, Vol. 62, No. 1)
* M.E. Grzelakowski, J.H. Campbell, M.R. Dubman, [http://www.vtda.org/pubs/BSTJ/vol62-1983/articles/bstj62-1-303.pdf The 3B20D Processor & DMERT Operating System: DMERT Operating System] (The Bell System Technical Journal, January 1983, Vol. 62, No. 1)


[[Category: Operating Systems]]
[[Category: PDP-11 Operating Systems]]
[[Category: Non-DEC Operating Systems]]
[[Category: Unix OS's]]

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 * [https://www.tuhs.org/Archive/Documentation/Manuals/MERT_Release_0/ MERT Release 0] - complete manual at [[The Unix Heritage Society|TUHS]]
 ** [https://www.tuhs.org/Archive/Documentation/Manuals/MERT_Release_0/Pgs%2003-04%20Table%20of%20Contents.pdf Table of Contents]
 * H. Lycklama, D. L. Bayer, [https://www.tuhs.org/Archive/Documentation/Papers/BSTJ/bstj57-6-2049.pdf The MERT Operating System] (The Bell System Technical Journal, July-August 1978, Vol. 57, No. 6, Part 2)
 * R.E. Anderson, Jr Kane, P.S. Mccabe, [http://www.vtda.org/pubs/BSTJ/vol62-1983/articles/bstj62-1-291.pdf The 3B20D Processor & DMERT Operating System: Overview, Architecture, and Performance of DMERT] (The Bell System Technical Journal, January 1983, Vol. 62, No. 1)

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 '''MERT''' (an acronym for '''Multi-Environment Real Time''') was an influential [[operating system]] from [[Bell Laboratories|Bell Labs]]. It was one of the earliest to be constructed using an organizational concept that later became known as a [[microkernel]] system; it was one of the very earliest successful ones.
-MERT was created in the 1970s, and was a spinoff of the [[UNIX]] work; it ran on [[Digital Equipment Corporation|DEC]] [[PDP-11]] computers, models -11/45 and -11/70 (it made use of the 'supervisor' mode provided by the [[memory management]] of those models). The basic kernel (below) mostly written in [[assembly language]]; the rest (file manager, UNIX supervisor, etc) was in [[C]].
+MERT was created in the 1970s, and was a spinoff of the [[UNIX]] work; it ran on [[Digital Equipment Corporation|DEC]] [[PDP-11]] computers, models -11/45 and -11/70 (it made use of the 'supervisor' mode provided by the [[memory management]] of those models). The basic kernel (below) mostly written in [[assembly language]]; the rest (file manager, UNIX supervisor, etc) was in [[C programming language|C]].
 Although it was not exactly a microkernel as they are today, it was a definitely not a monolithic [[kernel]]; it was a major step down the road to microkernels. MERT was an operating system which was divided up into several semi-independent components, all of which ran on a lower-level "kernel" (as they described it).