PDP-11/20
Introduction
The PDP-11/20 was the first PDP-11. It was introduced in 1970 at a price of $10,800.00 USD. It was a follow-on from the popular PDP-8 12-bit minicomputer line. The PDP-11/20s KA11 CPU was built from a set of quad width extended-length cards carrying SSI TTL chips, along with a few dual width cards. It introduced the UNIBUS as a universal path to connect together the CPU, main memory and devices. It was the only PDP-11 to use discrete logic for its control circuits, as opposed to microcode (this was because the cheap, fast ROMs needed for economical implementation of microcode did not exist at the time it was designed).
The CPU consisted of two and a half quad system unit backplanes (the two otherwise-unused slots were SPC slots, available for any peripheral) wire-wrapped into a single unit (see here for details). They, and the KY11-A front panel, were mounted in a BA11-C Mounting Box.
The PDP-11/20 did not have, as standard capabilities, many things that eventually became included on later models, such as hardware multiply and divide, floating point, and memory management. DEC provided optional accessories that provided some of these capabilities. One was a co-processor for the multiply/divide functions, the KE11-A Extended Arithmetic Element; rather than additional instructions in the CPU, it was implemented as a device on the bus, making this model somewhat incompatible with the rest of the PDP-11s. The KT11-B Paging option was logically an addition to the CPU, interposed between the KA11 and the UNIBUS, but also tied into the CPU; it allowed hardware-based time-sharing, and use of up to the full 248 KBytes of memory addressable by the UNIBUS.
The PDP-11/20 UNIBUS
The UNIBUS had an addressing range of 18 bits, but only 16 bits were used, which led to the 32 KWord memory limit. As with the other members of the PDP-11 family, the top 4 KW of the memory is reserved to devices connected to the CPU. Most PDP-11/20s were shipped with only 12 KWords of core memory. DEC's Computer Special Systems Division developed two options to increase and manage an extended memory space. The MX11 memory extension option enabled the use of 128 KWords of memory by using 18-bit addressing. The KS11 option provided hardware memory protection, which was lacking on the PDP-11/20.
Operating System
The original operating system for the PDP-11/20 was DOS/BATCH. One of the early versions of UNIX was also developed on the PDP-11/20.
Personal experience: Concordia University in Montreal used a PDP-11/20 well into the 1990s to analyze electromagnetic distribution for the ideal placement of aircraft antennae as part of a grant. A fellow named Mike ran the lab and helped me with my own PDP-11 hobby. I don't recall his last name. I donated an 11/83 processor to the University through Mike, which I could not sell due to a bit of rust on the board. I hoped this provided improved processing performance for the lab. The lab also ran the TSX multi-user operating system on another PDP-11. TSX was equivalent to a multi-user RT-11.
PDP-11/20
Source: computerhistory.org
Sources:- Adapted from: Gunkies.org