PDP-7 Definition

The PDP-7 was a minicomputer which was shipped by Digital Equipment Corporation (DEC) in 1965. Its greatest claim to fame by far is that it is the computer for which the first version of UNIX was created.

Minicomputers were third generation computers that made efficient use of discrete transistors and magnetic core memories (i.e., arrays of tiny rings made from a magnetic ceramic material) in place of vacuum tubes to reduce their size, purchase price and operating cost to only small fractions of those for the mainframe computers which still dominated high performance computing.

Minicomputers also bridged the huge performance gap between the high capacity mainframes and the low powered microcomputers. The latter were relatively simple, single-user machines that ran simple operating systems such as CP/M or MS-DOS. Minicomputers, in contrast, ran full multi-user, multitasking operating systems such as VMS (developed by DEC) and UNIX.

A multitasking operating system is one in which multiple processes can execute (i.e., run) on a single computer seemingly simultaneously and without interfering with each other. A process, also referred to as a task, is a running instance of a program.

Established in 1957 by three graduates of the Massachusetts Institute of Technology (MIT) and initially operated out of an old wool mill in Maynard, Massachusetts, DEC was a pioneer in the U.S. computer industry. At its peak in 1990 it employed more than 120,000 people worldwide and earned more than $14 billion in revenue. The company was acquired in 1998 by Compaq Computer Corporation, which subsequently merged with Hewlett-Packard in 2001.

The PDP Series

During the 1960s DEC introduced its influential PDP series of minicomputers that featured magnetic core memories and a variety of other advanced technologies. A key to the success of this series was its suitability for that large market segment that could not afford mainframes.

PDP was an abbreviation for Programmed Data Processor. The company did not want its machines to be called computers because a study had predicted that the world market for computers would be very small, perhaps less than a hundred. It was the conventional wisdom of the time (even among the government and DEC's stockholders) that computers were big and expensive and required a dedicated computer center and a large supporting staff. DEC chose to avoid dealing with these stereotypes by entirely avoiding the term computer.

DEC's first computer, the PDP-1, entered production in 1960. It was priced at only $120,000 for a basic system at a time when other computers typically sold for well in excess of a million dollars. It also featured low operating costs and ease of use, including the ability to be operated by a single person and the building in of a CRT (cathode ray tube) display on which images could be drawn using an accompanying light pen.

The PDP-1 had a time-sharing operating system and a magnetic core memory that held 4096 words of 18 bits each. Memory capacity could be expanded in increments of 4096 words to a maximum of 65,536 words, and it could be supplemented by up to 24 magnetic tape drive storage units. Driving currents were automatically adjusted to compensate for temperature variations between 50 and 110 degrees Fahrenheit.

The PDP-1 is perhaps best remembered today for being the computer most important in the creation of the early hacker (i.e., computer expert) culture at MIT and elsewhere. It was also significant in that in 1962 it became the first computer used for playing a computer game, Steve Russell's Spacewar.

The PDP-1 was followed by a succession of models with a wide range of prices and performance levels. The most powerful of these were fully worthy of the large computer centers with big support staffs that were required by mainframes. Some early models, such as the PDP-3, were not actually built by DEC itself but rather by customers using DEC parts and facilities.

The PDP-4, DEC's second 18-bit model, was introduced in 1963 as a cheaper, but slower, alternative to the PDP-1. It was not commercially successful, with only about 54 units being sold. However, all of the company's subsequent 18-bit PDP models were based on its simplified instructions set (i.e., the set of commands that the computer's processor can understand and execute).

Among the more notable of the PDP-1's successors was the PDP-6, which was shipped in 1964. It was a large, high capacity model and sported a 36-bit word size. The price was the same as for the PDP-1, and approximately 23 units were built.

The PDP-7

The PDP-7, which was introduced in 1965, was developed as a less expensive alternative to the PDP-4, with a price of only U.S.$72,000 for a minimal system. It also had an 18-bit word length, and its standard main memory was 4K words (equivalent to nine kilobytes) but upgradeable to 64K words (144 KB). Minuscule by today's standards, this amount of RAM (random access memory) core memory was considered substantial at the time, especially given the low price of the system.

The PDP-7's CPU (central processing unit) was implemented using a large number of small circuit cards. As was the case with DEC's earlier models, all logic was formed from discrete components (i.e, individual transistors, diodes and resistors); that is, no integrated circuits (ICs) were used.

The PDP-7 also featured DEC's first mass storage-based operating system. This was made possible by the company's new DECtape random access, block addressable, small format magnetic tape system. For the first time, tape was divided into sectors so that it could be used as an input/output storage system that was both interactive and inexpensive. The tapes were used in a way similar to how floppy disks were later used.

Like its predecessors, input and output was conducted via a teletypewriter, also referred to as a teletype machine, a kind of electromechanical typewriter that was commonly used for communication, and a punched paper tape drive unit for low cost storage of programs and data was included. There was also a high quality DEC 340 CRT display unit with a round, ten-inch screen which could draw simple vector graphics, and an accompanying light pen could be used to draw on the display screen.

The system also included an advanced Fortran II compiler, a symbolic assembler, a text editor, a debugging system, maintenance routines and a library of arithmetic, utility and programming aids that had been developed on the PDP-4. Fortran is a programming language that was developed in the 1950s and which is still widely used for scientific and numerical applications. A compiler is a specialized program for converting source code into machine code that a CPU can directly understand and execute. An assembler is a computer program for translating an assembly language into machine code. Source code is the original form in which software is written in a programming language prior to being compiled, and assembly languages are a type of low level (i.e., very close to machine code but easier for humans to read and write) programming language.

The PDP-7 was well received in data acquisition and laboratory applications, and it was considered sufficiently reliable (at least when properly programmed) to be suitable even for use in the control of nuclear reactors. Ultimately, 120 of the systems were produced and sold.

The PDP-7 and the Birth of UNIX

In 1969 Ken Thompson wrote the first version of UNIX in assembly language using an otherwise little-used PDP-7 at Bell Labs, the research arm of AT&T, the former U.S. telecommunications monopoly. One of the factors that made this possible was the proficiency that he had gained with that system while writing an early computer game called Space Travel. This game, incidentally, became one of the first programs to run on UNIX.

However, the PDP-7 was already obsolete when it was used for creating the first version of UNIX, and thus in 1970 the UNIX group proposed purchasing a PDP-11 for $65,000. The PDP-11, which had just been launched that year, incorporated some important advances (including greater ease of programming), and it became a highly successful and influential model. It was DEC's first and only 16-bit system.

In 1971, the group used their new PDP-11 to rewrite UNIX in a high-level language, instead of its original assembly language, so that it could more easily be ported to (i.e., transferred to) other types of computers. They briefly tried using Fortran before creating their own language based on BCPL (Basic Combined Programming Language), which they called B. They then extended B to produce the C language, which is still in widespread use today, after which they rewrote the UNIX source code in C. This made it easier to port UNIX to run on new hardware, as all that was needed was a C compiler to convert its C source code into the machine code for the specific type of computer.

The severe limitations of the PDP series and other computers of the day forced Thompson and Ritchie to be ruthlessly efficient in their designs for UNIX and C, as was the case with the other operating systems and languages of that era (although those other systems and languages have long since faded from use). Despite the fact that memory sizes, processor speeds, data access times, storage capacities and display capabilities have grown vastly greater than could have even been imagined at that time, this extreme efficiency has continued to serve Unix-like operating systems (i.e., the descendants and clones of the original UNIX) well and is widely acknowledged to be an important factor in the enduring and growing success of such systems.

The PDP-15, which was shipped in 1970, was DEC's final 18-bit computer, and it was the only one that was implemented with integrated circuits rather than discrete components. It was the largest selling of all the PDP models, with more than 400 units ordered in just the first eight months of production.

There are still a few PDP-7s in operable condition, including one that is currently being restored in Oslo, Norway.

Created May 30, 2005. Updated September 27, 2007.
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