|Working state||UNIX-RTR/3B21D version still in use.|
|Marketing target||Real-time computing applications|
|Platforms||PDP-11, 3B20D, 3B21D|
|Kernel type||Microkernel RTOS|
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.
- Kernel: resource allocation of memory, CPU time and interrupts
- Kernel-mode processes including input/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 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 (
/bin/kpkill) rather than (
/bin/kill), that is used to send signals to kernel processes. It is likely there are two different system calls also (
kpkill(2), 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
/bin/kpkill, 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
ls -l output is a bit different than expected. Namely, there are two completely new bits in the
drwxr-xr-x field. They both take place in the first column, and are
C (contiguous) and
x (extents). Both of these have to do with contiguous data, however one may be to do with inodes and the other with non-metadata.
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):
FALLOC(1) 5ESS UNIX FALLOC(1) NAME falloc - allocate a contiguous file SYNOPSIS falloc filename size DESCRIPTION A contiguous file of the specified filename is allocated to be of 'size' (512 byte) blocks. DIAGNOSTICS The command complains a needed directory is not searchable, the final directory is not writable, the file already exists or there is not enough space for the file.
UNIX-RTR includes an atomic file swap command (atomsw, manual page below):
ATOMSW(1) 5ESS UNIX ATOMSW(1) NAME atomsw - Atomic switch files SYNOPSIS atomsw file1 file2 DESCRIPTION Atomic switch of two files. The contents, permissions, and owners of two files are switched in a single operation. In case of a system fault during the operation of this command, file2 will either have its original contents, permissions and owner, or will have file1's contents, permissions and owner. Thus, file2 is considered precious. File1 may be truncated in case of a system fault. RESTRICTIONS Both files must exist. Both files must reside on the same file system. Neither file may be a "special device" (for example, a TTY port). To enter this command from the craft shell, switching file "/tmp/abc" with file "/tmp/xyz", enter for MML: EXC:ENVIR:UPROC,FN="/bin/atomsw",ARGS="/tmp/abc"-"/tmp/xyz"; For PDS enter: EXC:ENVIR:UPROC,FN"/bin/atomsw",ARGS("/tmp/abc","/tmp/xyz")! NOTE File 1 may be lost during a system fault. FILES /bin/atomsw
- Bayer, D. L.; Lycklama, H. (1975). MERT: a multi-environment real-time operating system. Fifth ACM Symposium on Operating Systems Principles. Austin, Texas. doi:10.1145/800213.806519. Retrieved 2008-08-18.
- Lycklama, H.; Bayer, D. L. (July–August 1978). "The MERT Operating System". Bell System Technical Journal. 57 (6): 2049–2086. doi:10.1002/j.1538-7305.1978.tb02142.x. S2CID 8711402.
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- Kane, J. R.; Anderson, R. E.; McCabe, P. S. (January 1983). "The 3B20D Processor & DMERT Operating System: Overview, Architecture, and Performance of DMERT". Bell System Technical Journal. 62 (1): 291–301. doi:10.1002/j.1538-7305.1983.tb04396.x. S2CID 31828139.
- Grzelakowski, M. E.; Campbell, J. H.; Dubman, M. R. (January 1983). "The 3B20D Processor & DMERT Operating System: DMERT Operating System". Bell System Technical Journal. 62 (1): 303–322. doi:10.1002/j.1538-7305.1983.tb04397.x. S2CID 12901173.
- Wallace, John J.; Barnes, Walter W. (August 1984). "Designing for Ultrahigh Availability: The Unix RTR Operating System" (PDF). IEEE Computer. IEEE. 17 (8): 31–39. doi:10.1109/MC.1984.1659215. S2CID 17689432.
- Ritchie, Dennis M. (1977). The Unix Time-sharing System: A retrospective. Tenth Hawaii International Conference on the System Sciences. Archived from the original on 5 February 2015.