Editing 3B series computers (section)

==3B high-availability processors==
The original series of 3B computers includes the models 3B20C, 3B20D [[superminicomputer]],<ref name=3Bseries.NYT840328/> 3B21D, and 3B21E.

These systems are 32-bit [[microcode|microprogrammed]] duplex ([[redundancy (engineering)|redundant]]) [[high availability]] processor units running a [[real-time operating system]]. They were first produced in the late 1970s at the [[Western Electric]] factory in [[Lisle, Illinois]], for telecommunications applications including the [[No. 4 Electronic Switching System|4ESS]] and [[5ESS Switching System|5ESS]] systems.

They use the Duplex Multi Environment Real Time ([[DMERT]]) operating system which was renamed [[UNIX-RTR]] (Real Time Reliable) in 1982. The Data Manipulation Unit (DMU) provides arithmetic and logic operations on 32-bit words using eight [[AMD Am2900|AMD 2901]] 4-bit-slice [[Arithmetic logic unit|ALU]]s.<ref>J. O. Becker, The 3B20D PROCESSOR and DMERT Operating System (The Bell System Technical Journal, January 1983, Vol. 62, No. 1, Part 1), Page 193</ref> The first 3B20D is called the Model 1. Each processor's control unit consists of two frames of circuit packs. The whole duplex system requires seven-foot frames of circuit packs plus at least one tape drive frame (most telephone companies at that time wrote billing data on [[magnetic tapes]]), and many [[washing machine]]-sized disk drives. For training and lab purposes, a 3B20D can be divided into two "half-duplex" systems. A 3B20S consists of most of the same hardware as a half-duplex but uses a completely different operating system.

The 3B20C was briefly available as a high-availability fault tolerant [[multiprocessing]] general-purpose computer in the commercial market in 1984. The 3B20E was created to provide a cost-reduced 3B20D for small offices that did not expect such high availability. It consists of a virtual "emulated" 3B20D environment running on a stand-alone general purpose computer; the system was ported to many computers, but primarily runs on the [[Sun Microsystems]] [[Solaris (operating system)|Solaris]] environment.

There were improvements to the 3B20D UNIX-RTR system in both software and hardware in the 1980s, 1990s, 2000s, and 2010s. Innovations included disk independent operation (DIOP: the ability to continue essential software processing such as telecommunications after duplex failure of redundant essential disks); off-line boot (the ability to split in half and boot the out-of-service half, typically on a new software release) and switch forward (switch processing to the previously out-of-service half); upgrading the disks to [[solid-state drive]] (SSD); and upgrading the tape unit to [[CompactFlash]].

The processor was re-engineered and renamed in 1992 as the 3B21D. It is still in use {{asof|2023|lc=on}} as a component of [[Nokia]] products such as the [[2STP]] [[Signal Transfer Point|signal transfer point]] and the 4ESS and 5ESS switches, which Nokia inherited from AT&T spin-off [[Lucent Technologies]].