Thursday, 9 January 2020

Digital (DEC) PDP-11

Well-appointed PDP-11 at TNMOC, Bletchley
Launched January 1970

The Digital PDP-11 is a computer you may not have heard of, but it was hugely influential in terms of hardware and even more so in the software that it helped to create.

Digital Equipment Corporation (typically known as DEC or Digital) was founded in 1957, first tinkering with electronics for laboratory environments and then producing a full computer system in 1959 with the PDP-1 minicomputer. Other models followed and DEC grew quickly through the 1960s with a wide range of new products including the very successful PDP-8.

The term “minicomputer” is rarely used these days, and by modern standard there was nothing mini about them. Often house in racks and sometimes filling a small room, minicomputers were shrunk down versions of the huge mainframe computers that tended to require their own building. Even as microcomputers became popular, minicomputers were much more powerful and made it easier for people to work collaboratively, these days their modern descendant would be a server. Typically you would access a minicomputer with a terminal such as a VT52.

Time and technology move on and by the late 1960s the computer industry started to settle on 8, 16 and 32 bit architecture (based on an 8-bit word size) where Digital was mostly producing 12, 18 and 36 bit machines (based on a 6-bit word size). In part this change happened because the computer industry was starting to standardise on the 7-bit ASCII character set.

In January 1970, DEC launched its first 16-bit minicomputer – the PDP-11. Combining the extensive experience of the company from the previous decade (both good and bad), the PDP-11 was a high usable and expandable system. A key feature of the PDP-11 was that it was relatively easy to program, especially when it came to using peripherals (initially on the Unibus bus and later Q-Bus). And peripherals were available from DEC in abundance, including disk drives, tapes drives, printers and terminals.

From the outset the PDP-11 was a huge success, starting with the original 11/20 and 11/15 models in 1970 and then developing along with advancing technologies to become smaller and more powerful, ending with the 11/93 and 11/94 in 1990 (which were in production until 1997). But PDP-11 systems ended up being squeezed into other “smart” peripherals too such as robot arms and when added to a terminal such as the VT100 they could make a compact desktop version (such as the VT103). DEC even tried to make a PDP-11 to compete with the IBM PC with the DEC Professional range.

Perhaps confusingly DEC had many operating systems for the PDP-11, notably RT-11. However the most famous OS that the PDP-11 is famous for is Unix – a platform that was developed at Bell Labs in response to the complex Multics OS. In fact, Unix was tied to the PDP-11 platform until 1978 when it was finally ported to a fairly obscure system called the Interdata 8/32.

Unix became an enormous success – it took a while – and today descendants and variants of that OS power smartphones, servers and personal computers worldwide. But the PDP-11 hardware too was hugely influential, directly inspiring 1970s processors such as the Motorola 68000 and Intel 8086.

DEC sold hundreds of thousands of PDP-11s while it was in production, making it possibly the most popular minicomputer ever made. The 32-bit DEC VAX launched in 1977 was meant to be the next logical step, however both the PDP-11 and VAX ended up being sold in parallel.

In terms of both software and hardware the PDP-11 was a hugely significant device, even if most people may never have seen one. Surprisingly it seems some are still in use, and there’s a brisk trade in parts and components on the second hard market.

Image credit: Loz Pycock via Flickr

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