Multi-Environment Real-Time (MERT), later renamedUNIX Real-Time (UNIX-RT),^[3] is a hybridtime-sharing andreal-time operating system developed in the 1970s atBell Labs for use inembeddedminicomputers (especiallyPDP-11s). A version namedDuplex Multi Environment Real Time (DMERT) was the operating system for theAT&T 3B20Dtelephone switching minicomputer, designed forhigh availability;^[4][5][6] DMERT was later renamedUnix RTR (Real-Time Reliable).^[6]

Ageneralization of Bell Labs' time-sharing operating systemUnix,^[7]MERT featured a redesigned, modularkernel that was able to run Unix programs and privilegedreal-time computing processes. These processes' data structures were isolated from other processes withmessage passing being the preferred form ofinterprocess communication (IPC), althoughshared memory was also implemented. MERT also had a customfile system with special support for large, contiguous, statically sized files, as used in real-timedatabase applications. The design of MERT was influenced by Dijkstra'sTHE, Hansen'sMonitor, and IBM'sCP-67.^[2]

The MERT operating system was a four-layer design, in decreasing order ofprotection:^[2]

• Kernel: resource allocation of memory, CPU time andinterrupts
• Kernel-mode processes includinginput/output (I/O)device drivers, file manager,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 extendedsystem call interface andshell that enabled the use of MERT's custom IPC mechanisms, although anRSX-11 emulator also existed.^[2]

• Real-time operating systems
• Bell Labs Unices
• Microkernel-based operating systems
• Microkernels

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