View source for Multi-Environment Real-Time

{{Short description|Unix variant with real-time operating system capabilities}}
{{Distinguish|Penn MERT}}
{{Infobox OS
| name = Multi-Environment Real-Time
| developer = [[Bell Labs]]<ref name="acm">{{cite conference
  |last1=Bayer
  |first1=D. L.
  |last2=Lycklama
  |first2=H.
  |url=http://portal.acm.org/citation.cfm?id=806519
  |title=MERT: a multi-environment real-time operating system
  |conference=Fifth ACM Symposium on Operating Systems Principles
  |location=Austin, Texas
  |year=1975
  |doi=10.1145/800213.806519
  |access-date=2008-08-18
 |doi-access=free
  }}</ref>
| family = [[Unix-like]]
| kernel_type = [[Microkernel]]<ref name="acm"/> [[Real-time operating system|RTOS]]
| supported_platforms = [[PDP-11]],<ref name="acm"/> [[3B20D]], [[3B21D]]
| marketing_target = [[Real-time computing]] applications
| programmed_in = [[C (programming language)|C]]<ref name="bstj78"/>
| language = English
| working_state = UNIX-RTR/3B21D version still in use.
}}
'''Multi-Environment Real-Time''' ('''MERT'''), later renamed '''UNIX Real-Time''' ('''UNIX-RT'''),<ref name="porting">{{Cite journal |last1=Bodenstab |first1=D. E. |last2=Houghton |first2=T. F. |last3=Kelleman |first3=K. A. |last4=Ronkin |first4=G. |last5=Schan |first5=E. P. |year=1984 |title=UNIX Operating System Porting Experiences |journal=AT&T Bell Laboratories Technical Journal |volume=63 |issue=8 |pages=1769–1790 |doi=10.1002/j.1538-7305.1984.tb00064.x |s2cid=35326182}}</ref> is a hybrid [[time-sharing]] and [[real-time operating system]] developed in the 1970s at [[Bell Labs]] for use in [[embedded system|embedded]] [[minicomputer]]s (especially [[PDP-11]]s). A version named '''Duplex Multi Environment Real Time''' ('''DMERT''') was the operating system for the [[3B series computers|AT&T 3B20D]] [[telephone switching]] minicomputer, designed for [[high availability]];<ref>{{cite journal |last1=Kane |first1=J. R. |last2=Anderson |first2=R. E. |last3=McCabe |first3=P. S. |date=January 1983 |title=The 3B20D Processor & DMERT Operating System: Overview, Architecture, and Performance of DMERT |journal=[[Bell System Technical Journal]] |volume=62 |issue=1 |pages=291–301 |doi=10.1002/j.1538-7305.1983.tb04396.x|s2cid=31828139}}</ref><ref>{{cite journal |last1=Grzelakowski |first1=M. E. |last2=Campbell |first2=J. H. |last3=Dubman |first3=M. R. |date=January 1983 |title=The 3B20D Processor & DMERT Operating System: DMERT Operating System |journal=[[Bell System Technical Journal]] |volume=62 |issue=1 |pages=303–322 |doi=10.1002/j.1538-7305.1983.tb04397.x |s2cid=12901173}}</ref><ref name="unix-rtr">{{cite journal |last1=Wallace |first1=John J. |last2=Barnes |first2=Walter W. |date=August 1984 |url=http://www.computer.org/csdl/mags/co/1984/08/01659215.pdf |title=Designing for Ultrahigh Availability: The Unix RTR Operating System |journal=[[IEEE Computer]] |volume=17 |issue=8 |pages=31–39 |publisher=[[IEEE]] |doi=10.1109/MC.1984.1659215 |s2cid=17689432}}</ref> DMERT was later renamed '''Unix RTR''' (Real-Time Reliable).<ref name="unix-rtr"/>

A ''generalization'' of Bell Labs' time-sharing operating system [[Unix]],<ref>{{cite conference |last=Ritchie |first=Dennis M. |author-link=Dennis Ritchie |year=1977 |title=The Unix Time-sharing System: A retrospective |conference=Tenth Hawaii International Conference on the System Sciences |url=http://cm.bell-labs.com/cm/cs/who/dmr/retro.html |archive-url=https://web.archive.org/web/20150205030215/http://cm.bell-labs.com/cm/cs/who/dmr/retro.html |archive-date=5 February 2015}}</ref>
MERT featured a redesigned, modular [[Kernel (operating system)|kernel]] that was able to run Unix programs and privileged [[real-time computing]] processes. These processes' data structures were isolated from other processes with [[message passing]] being the preferred form of [[interprocess communication]] (IPC), although [[shared memory]] was also implemented. MERT also had a custom [[file system]] with special support for large, contiguous, statically sized files, as used in real-time [[database]] applications. The design of MERT was influenced by Dijkstra's [[THE multiprogramming system|THE]], Hansen's [[RC 4000 multiprogramming system|Monitor]], and IBM's [[CP-67]].<ref name="bstj78">{{cite journal |last1=Lycklama |first1=H. |last2=Bayer |first2=D. L. |title=The MERT Operating System |journal=[[Bell System Technical Journal]] |volume=57 |issue=6 |pages=2049–2086 |date=July–August 1978 |doi=10.1002/j.1538-7305.1978.tb02142.x|s2cid=8711402}}</ref>

The MERT operating system was a four-layer design, in decreasing order of [[protection ring|protection]]:<ref name="bstj78"/>
* Kernel: resource allocation of memory, CPU time and [[hardware interrupt|interrupts]]
* Kernel-mode processes including [[input/output]] (I/O) [[device driver]]s, file manager, [[Paging#History|swap]] manager, ''root process'' that connects the file manager to the disk (usually combined with the swap manager)
* Operating system supervisor
* User processes

The standard supervisor was MERT/UNIX, a Unix emulator with an extended [[system call]] interface and [[Unix shell|shell]] that enabled the use of MERT's custom IPC mechanisms, although an [[RSX-11]] emulator also existed.<ref name="bstj78"/>

==References==
{{Reflist}}

{{Bell Unix}}
{{Real-time operating systems}}
{{Microkernel}}

[[Category:Real-time operating systems]]
[[Category:Bell Labs Unices]]
[[Category:Microkernel-based operating systems]]
[[Category:Microkernels]]