Science Fair Project Encyclopedia
The LINC (Laboratory Instrument Computer) was a 12-bit, 2048-word computer. The LINC and the PDP-8 can be considered the first mini computers and perhaps the first personal computers as well. Although its instruction set was small, it was larger than the ingenious and tiny PDP-8 instruction set.
Unlike today's personal computers, the LINC sold for about $50,000. It interfaced well with laboratory experiments. Analog inputs and outputs were part of the basic design. It was designed in 1962 by Charles Molnar and Wesley Clark at Lincoln Laboratory, Massachusetts (Part of MIT), for NIH researchers. The LINC's design was literally in the public domain, perhaps making it unique in the history of computers. 24 LINC computers were assembled in a summer workshop at MIT. Digital Equipment Corporation (starting in 1964) and Spear Inc. of Waltham, MA manufactured them commercially.
The Control Panel
The LINC control panel was capable of much more than single-stepping programs. It was, to all intents and purposes, the debugger. It allowed execution to be stopped when the program counter matched a set of switches. More important, another function allowed execution to be stopped when a particular address was accessed. The single-step and the resume functions could be automatically repeated. The repetition rate could be varied over four orders of magnitude by means of an analog knob and a four-position decade switch, from about one step per second to about half of the full speed. Running a program at one step per second and gradually accelerating it to full speed provided an extremely dramatic way to experience and appreciate the speed of the computer.
A very noteworthy feature of the LINC was the LINCtape, seen in the photographs below. It was a fundamental part of the machine design, not an optional peripheral, and the machine's OS relied on it. In retrospect, the easiest way to understand the LINCtape's role is to think of it as a linear diskette with a slow seek time. The magnetic tape drives on large machines of the day stored large quantities of data, took many minutes to spool from end to end, but could not reliably update blocks of data in place. In contrast, the LINCtape was a small, nimble device which stored about 400K, had a fixed formatting track allowing data to be repeatedly read and re-written to the same locations, and took less than a minute to spool from one end to the other. The tape was formatted in fixed-sized blocks, and was used to hold a directory and file system. Filenames were six characters long. The file system allowed for two files--a source file and an executable binary file to be stored under the same name. Since the basic LINC had only 1024 12-bit words of core memory (RAM)--and the big, expanded LINC had only 2048--normal operations depended heavily on swapping to and from LINCtape. (Digital later patented and marketed an extremely similar design under the name DECtape; Digital's patents on DECtape were eventually tested in court and found invalid).
LINCtape is also fondly remembered for its reliability, which was distinctly higher than that of the diskettes that supplanted it. LINCtape incorporated a very simple form of redundancy--all data was duplicated laterally in two locations across the tape. LINC users demonstrated this by punching holes in a tape with an ordinary office paper punch. Tape damaged in this way was perfectly readable. The formatting track made operation almost independent of tape speed, which was, in fact, quite variable. As can be seen in the pictures below, there was no capstan; the motion of the tape during reading and writing was directly controlled by the reel motors. There was no fast forward or rewind—or, you might say, reading and writing was performed at fast forward and rewind speeds. In some modes of operation, the data transfers were audible over the built-in loudspeaker and produced a very characteristic series of squawks with varying pitch.
The LINC keyboard, manufactured by Soroban (IBM? Hiroshima?), had a set of keys with locking solenoids for each key. When the user pressed a key, the LINC would lock all the keys in their current up or down positions, read the key into a hardware register, then, when the running program read the register, the hardware would release all locking solenoids. This had the effect of slowing down typing and preventing even 2-key rollover. This keyboard was abandoned in the LINC-8 and PDP-12 follow-on computers (see below).
The LINC hardware allowed a 12-bit word to be rapidly and automatically displayed on the screen as a 2-wide by 6-high matrix of pixels, making it possible to display full screens of flicker-free text. The standard display routines generated 4 by 6 character cells, giving the LINC one of the coarsest, ugliest-looking character sets ever designed.
The LINC-8 And PDP-12 Computers
Digital Equipment Corporation manufactured two next-generation LINC-compatble computers, the LINC-8 and PDP-12. The first follow-on, the LINC-8, booted (slowly) to a PDP-8 program called PROGOFOP (PROGram OF OPeration) which interfaced to the separate LINC hardware. The PDP-12 was the last and most popular follow-on to the LINC. It was a capable and improved machine, and was more stable than the LINC-8, but architecturally was still a shotgun marriage of a LINC and a PDP-8, full of many small technical glitches. (For example, the LINC had an overflow bit which was a small but important part of the LINC's machine state; the PDP-12 had no provision for saving and restoring the state of this bit across PDP-8 interrupts.)
The MINC-11 Computer
Digital produced a version of the PDP-11/03 called the MINC-11, housed in a portable cart, and equippable with Digital-designed laboratory I/O modules supporting capabilities such as analog input and output. A programming language, MINC BASIC, included integrated support for the laboratory I/O modules. MINC stood for "Multi-Instrument Computer." The name undoubtedly was intended to evoke memories of the LINC, but the 16-bit machine had no architectural resemblance to, or compatibility with, the LINC.
|The LINC Computer||The PDP-12 Computer|
|The Last LINC Computer (1992, MIT)|
See also: PDP
LINC is also the name of a fourth-generation programming language.
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