Multi-Environment Real-Time

Multi-Environment Real-Time (MERT), later renamed UNIX Real-Time (UNIX-RT), is a hybrid time-sharing and real-time operating system developed in the 1970s at Bell Labs for use in embedded minicomputers (especially PDP-11s). A version named Duplex Multi Environment Real Time (DMERT) was the operating system for the AT&T 3B20D telephone switching minicomputer, designed for high availability;
DMERT was later renamed Unix RTR (Real-Time Reliable).
A "generalization" of Bell Labs' time-sharing operating system Unix,
MERT featured a redesigned, modular 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, Hansen's Monitor, and IBM's CP-67.
The MERT operating system was a four-layer design, in decreasing order of protection:
The standard supervisor was MERT/UNIX, a Unix emulator with an extended system call interface and shell that enabled the use of MERT's custom IPC mechanisms, although an RSX-11 emulator also existed.
Kernel and non-kernel processes.
One interesting feature that DMERT – UNIX-RTR introduced was the notion of "kernel" processes. This is connected with its microkernelish architecture roots. In support, there is a separate command (codice_1) rather than (codice_2), that is used to send signals to kernel processes. It is likely there are two different system calls also (codice_3 and codice_4, the first to end a user process and the second to end a kernel process). It is unknown how much of the normal userland "signaling" mechanism is in place in codice_1, assuming there is a system call for it, it is not known if one can send various signals or simply send one. Also unknown is whether the kernel process has a way of catching the signals that are delivered to it. It may be that the UNIX-RTR developers implemented an entire signal and messaging application programming interface (API) for kernel processes.
File system bits.
If one has root on a UNIX-RTR system, they will surely soon find that their codice_6 output is a bit different than expected. Namely, there are two completely new bits in the codice_7 field. They both take place in the first column, and are codice_8 (contiguous) and codice_9 (extents). Both of these have to do with contiguous data, however one may be to do with inodes and the other with non-metadata.
Example codice_6 (which does not include group names, as codice_6 did not used to print them).
 drwxr-xr-x root 64 Sun Dec 4 2003 /cft
 xrwxr-xr-x root 64 Mon Dec 11 2013 /no5text
 Crwxr-xr-x root 256 Tue Dec 12 2014 /no5data
Lucent emulator and VCDX.
AT&T, then Lucent, and now Alcatel-Lucent, are the vendor of the SPARC-based and Solaris-OEM package ATT3bem (which lives on Solaris SPARC in /opt/ATT3bem). This is a full 3B21D emulator (known as the 3B21E, the system behind the Very Compact Digital eXchange, or VCDX) which is meant to provide a production environment to the Administrative Module (AM) portion of the 5ESS switch. There are parts of the 5ESS that are not part of the 3B21D microcomputer at all: SMs and CMs. Under the emulator the workstation is referred to as the 'AW' (Administrative Workstation). The emulator installs with Solaris 2.6/SPARC and also comes with Solstice X.25 9.1 (SUNWconn), formerly known as SunLink X.25. The reason for packaging the X.25 stack with the 3B21D emulator is because the Bell System, regional Bell operating companies, and ILECs still use X.25 networks for their most critical of systems (telephone switches may live on X.25 or Datakit VCS II, a similar network developed at Bell Labs), but they do not have TCP/IP stacks).
The AT&T/Alcatel-Lucent emulator is not an easy program to get working correctly, even if one manages to have an image from a pulled working 5ESS hard disk 'dd' output file. 
First, there are quite a few bugs the user must navigate around in the installation process. Once this is done, there is a configuration file which connects peripherals to emulated peripherals. But there is scant documentation on the CD which describes this. The name of this file is em_devmap for SS5s, and em_devmap.ultra for Ultra60s.
In addition, one of the bugs mentioned in the install process is a broken script to fdisk and image hard disks correctly: certain things need to be written to certain offsets, because the /opt/ATT3bem/bin/3bem process expects, or seems to need, these hard-coded locations.
The emulator runs on SPARCstation-5s and UltraSPARC-60s. It is likely that the 3B21D is emulated faster on a modern SPARC than a 3B21D microcomputer's processor actually runs as measured in MIPS. The most difficult thing about having the emulator is acquiring a DMERT/UNIX-RTR hdd image to actually run. The operating system for the 5ESS is restricted to a few people, employees and customers of the vendor, who either work on it or write the code for it. Having an image of a running system, which can be obtained on eBay, pulled from a working 3B21D, and imaged to a file or put into an Ultra60 or SPARCstation-5, provides the resources to attempt to run the UNIX-RTR system.
The uname -a output of the Bourne shell running UNIX-RTR (Real-time Reliable) is:
 # uname -a
 <3B21D> <3B21D>
Though on 3B20D systems it will print 20 instead of 21, though 3B20Ds are rare, nowadays most non-VCDX 5ESSs are 3B21D hardware, not 3B20D (although they will run the software fine).
The 3B20D uses the WE32000 processor while the 21 uses the WE32100. There may be some other differences, as well. One thing unusual about the processor is the direction the stack grows: up.
Manual page for falloc (which may be responsible for Contiguous or eXtent file space allocation):
UNIX-RTR includes an atomic file swap command (atomsw, manual page below):