PDP-11/20

PDP-11/20
Manufacturer:	Digital Equipment Corporation
Bus Architecture:	UNIBUS

PDP11/20

The PDP-11/20 was the first PDP-11 from DEC; its 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 that approach did not exist yet 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.

Extensions

The -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 a series of 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-11's.

The KS11 was apparently DEC's first attempt at an add-on to provide memory management to the -11/20. Other than a funny story, nothing is known of it. The extremely rare KT11-B Paging Option was logically an addition to the CPU, interposed between the KA11 and the UNIBUS (but also having a few ties into the CPU); it allowed hardware-based time-sharing, and use of up to the full 248 KBytes of memory addressable by the UNIBUS.

Software

The -11/20 was the machine that RSTS (called 'RSTS-11' when it was on the -11/20) ran on. Many other DEC PDP-11 operating systemd (e.g. DOS-11) started out on the -11/20.

It was the first PDP-11 to run UNIX, albeit a very early version.

hampage.hu

Quoted: The first of the -11's, born in June 1970, featuring the KA11 processor.

The PDP-11/20 is a general-purpose computer with a wordlength of 16 bits. It features 8 general registers, and one interconnect for all system components (CPU, memory, peripheral devices), the UNIBUS (which is not the successor of the PDP8 OMNIBUS).

This bus had an addressing range of 18 bits, but only 16 were used, which led to the 32 Kword (as mentioned, one word was 16 bits, so this is equal 64 Kbyte) memory limit. As with the other members of the -11 family, the top 4 KW of the memory is reserved to the system (the upper memory addresses refer to devices connected to the CPU). It is also interesting to know that most machines were shipped with only 12 KW of magnetic core memory.

On the picture above we can see some typical I/O devices: 4 ASR33 teletype terminals and DECtape drives over the fan-fold papertape puncher/reader. The front panel had lights and switches for address and data (the lights were not LED's).

The original operating system was DOS/BATCH, but one of the early versions of UNIX was also developed on the /20.

Interesting options: MX11 - Memory Extension Option: this enabled the usage of 128 KW memory (18-bit addressing range); KS11: this option provided hardware memory protection, which the plain /20 lacked. Both options were developed by the Digital CSS (Computer Special Systems).

Trivia: The machine was set to be named "PDP-11/30" originally, but it was renamed to 11/20 because of the similar designation of the IBM 1130.

External links

• 11/20 - documentation at Bitsavers
  • PDP-11/20 system manual (DEC-11-HR1B-D)
  • PDP-11/20 Engineering Drawings
• pdp11 handbook - PDP-11/20 Processor Handbook