Harold McFarland (PDP-11)

Harold McFarland (1945-2018)
Harold McFarland (1945-2018)

After the famous PDP-1 in 1960, DEC established a successful business and during the middle 1960s launched its remarkable 12-bit PDP-8 series of minicomputers. During the late 1960s however, the time had come for the introduction of a 16-bit machine to replace the PDP-8. In 1967, one of DEC’s key design engineers, the 28 years old Edson de Castro was assigned to design a 16-bit machine, with the code name PDP-X. In the spring of 1968 however, the project was canceled and Ed de Castro and some of his friends went off to form another company, Data General, building the initial success of that company on his 16-bit NOVA computer. It seems however that the design by de Castro PDP-X has little relationship to DEC’s eventual 16-bit architecture, the PDP-11.

Gordon Bell, the vice president of research in DEC, was involved with recommending what became the PDP-11, while the actual design was made by Harold McFarland, who just graduated from the Electrical Engineering Department of Carnegie-Mellon University and was a protege of Gordon Bell, who was a professor at the University. McFarland had been working on several computer architectures while at University, and one of these architectures was essentially what became the PDP-11. McFarland joined DEC in September 1968 and was appointed chief architect of the project, the prototype of PDP-11 was ready in 1969, and in the spring of 1970, the computer was launched to the market.

The PDP-11 (Programmed Data Processor 11) was one the (if not the) most successful computers of all time. It began its career as a minicomputer and ended up as a micro or supermicro/supermini. It was manufactured from 1970 until the early 1990s. Members of its line were sold in very high numbers, more than 600000 computers were sold, thanks to the growing OEM industry. The VAX line of DEC began its life as an enhancement to the PDP-11 architecture, the first VAX computers are sometimes mentioned as PDP11-7xx, in contrast to the official label VAX-11/7xx.

The venerable PDP-11 is still spry to this day, powering a GE nuclear power-plant robotic applications — and will do so until 2050! That’s right!


The first model of PDP-11, (priced at 20000 USD), named PDP-11/20, was shipped in the spring of 1970. It had a word length of 16 bits, a speed of 800 nanoseconds, the cycle time was 1.5 microseconds, and the access time, 0.75 microseconds. The CPU had eight 16-bit registers, six general purposed, the stack pointer, and the program counter. Primary memory was magnetic core, 56 Kbytes (28 KWords) maximum (some documentation referred to 32k max memory, but the top 4k was reserved for the I/O space). Initial software included a symbolic editor, debugger, and utilities. PAL Architecture was UNIBUS. The console is TTY ASR33. The typical I/O was a paper-tape reader/punch.

PDP-11 was the first system to run an AT&T UNIX which was written in C. It was also the PDP-11 on which BSD UNIX was first developed.

One of the reasons, which made the PDP-11 so successful, was that it was designed to be produced in a factory by semiskilled labor. All of the dimensions of its pieces were relatively non-critical.

The PDP-11, like the PDP-8 before it, was cloned and copied extensively behind the so-called Iron Curtain, i.e. in Eastern European Socialist countries. A number of plants produced PDP-11 compatible systems in the Soviet Union (СМ-4, СМ-1420/xxx, СМ-1600/xxx, Електроника-xxx, etc.), Bulgaria (СМ-4, СМ-1420/xxx), DDR (SM-1420/xxx), Poland (Mera-xxx), Hungary (SM-4). Nobody knows how many of these clones were issued by many of the plants in the socialist countries, but some believe that the total amounts of units should be counted by tens of thousands.

Biography of Harold McFarland

Harold L. McFarland Jr. was born on 27 December 1945 in Portsmouth, Ohio, to Harold McFarland, Sr. (1922-1984) and Marilyn Frances Spence McFarland (1923-1984). In 1963 he enrolled at Carnegie Mellon University, where he graduated in 1968 with a degree in electrical engineering. McFarland worked for DEC from 1968 until the end of 1971, when he left to join Advanced Retail Systems (Litton). In 1979 McFarland became a member of the founding group (together with the electrical engineer Thampy Thomas) of Elxsi Corporation, a minicomputer manufacturing company established in Silicon Valley, USA. In 1986 McFarland and Thomas founded the semiconductor company NexGen Microsystems, best known for its unique implementation of the x86 architecture in its processors. McFarland worked for NexGen until his retirement in 1997. He was a holder of quite a few patents in the area of data processing and communications, e.g.: patent Nr. 3614741 (DATA PROCESSING SYSTEM) from 19 Oct. 1971, patent Nr. 4736124 (HIGH-SPEED DATABUS STRUCTURE) from 5 Apr. 1988, and patent Nr. 5414820 (CROSSING TRANSFERS FOR MAXIMIZING THE EFFECTIVE BANDWITH) from 9 May 1995, patent Nr. 5781753 (SEMI-AUTONOMOUS RISC PIPELINES) from 14 July 1998, and others.

Harold McFarland was married to Rita McFarland. They had three daughters: Sandi, Laurel, and Alana.

Harold L. “Mac” McFarland, who had a passion for knowledge and computer innovations throughout his life, passed away at age 72 on 14 July 2018 in Durham, North Carolina.