LINC-8
| Industry | Computer hardware |
|---|---|
| Founded | 1957 |
| Headquarters | Maynard, Massachusetts , United States |
LINC-8 is a minicomputer developed by Digital Equipment Corporation and manufactured between 1966 and 1969. It combines a LINC system with a PDP-8 within a single chassis, allowing users to run software designed for either platform.
Architecture
The LINC-8 contains a PDP-8 processor alongside a partially emulated LINC CPU, operating exclusively in either 'LINC mode' or 'PDP-8 mode,' with specific instructions enabling mode switching. Interrupt handling is performed solely by the PDP-8 CPU, which restricts compatibility for programs dependent on the LINC’s native interrupt mechanisms.
Arithmetic operations differ between the two: the LINC employs 12-bit ones' complement arithmetic, whereas the PDP-8 uses 12-bit two's complement. Their memory addressing schemes also vary; the LINC divides memory into 1024-word segments and selects two segments simultaneously—one for instructions and one for data—with indirect addressing applied to the data segment. While capable of addressing up to 32K words, most implementations were limited to 8K due to hardware limitations and the complexity of expanding memory, which required CPU clock reductions for each additional 4K segment.
Segment zero is allocated to the PDP-8 program PROGOFOP, responsible for managing interrupts and traps. PROGOFOP facilitates partial operation of the LINC CPU by handling unsupported instructions through the PDP-8, which is crucial for executing complete LINC applications.
Multiple operating systems were created for the LINC-8, some adapted from original LINC OS versions. Typical boot sequences load PROGOFOP first, followed by tape-based commands that initiate the LINC-oriented OS. Other OS variants emphasize PDP-8 compatibility, occasionally sacrificing portability but enabling more flexible memory allocation by reserving smaller memory regions compared to full LINC mode.
The PDP-8 organizes its memory into 128-word pages using direct or indirect addressing extended via Data and Instruction Fields, allowing access up to 32K words, though hardware constraints generally kept memory below 8K words.
LINC peripherals managed through dedicated instructions include analog inputs such as knobs and jacks, relays for controlling external devices, LINCtape drives (precursors to DECtape), a CRT display derived from a modified Tektronix oscilloscope, and a Teletype Model 33 ASR. The CRT lacked traditional sweep circuits and relied entirely on digital-to-analog converters and intensifiers for operation; its knob-based control scheme anticipated modern mouse interfaces.
Certain peripherals are actually PDP-8 devices; when unimplemented LINC instructions occur, they generate PDP-8 interrupts that simulate functions like tape drive control. Console key presses also produce PDP-8 interrupts handled by PROGOFOP to mimic original LINC console behavior.
A distinctive feature is the FETCH/EXEC stop mechanism, which monitors instruction execution until a condition triggers a PDP-8 interrupt that halts the LINC program. Emulated console operations allow users to examine memory contents and execute individual instructions through switch registers. Booting some operating systems involves manually entering direct tape read commands via switches followed by simulated start signals. In summary, the LINC-8 replicates the original LINC console panel’s full functionality by means of PDP-8-based emulation.
Purpose
The LINC-8 was created as a compact computer specifically for laboratory use, providing moderate processing capabilities at an affordable price. Its hardware was specifically engineered to assist in experiment monitoring and control. The system featured a LINCtape magnetic tape drive, invented by Wesley A. Clark, known for its robustness and suitability in laboratory settings, ensuring data integrity even under harsh treatment.
Current status
By 1969, DEC introduced the PDP-12, a superior combination computer designed for laboratory applications, which led to the discontinuation of the LINC-8. Only a few hundred units of the LINC-8 were ever produced, making it a rare machine today. As of 2008, there have been ongoing initiatives within the Update computer society at Uppsala University to develop an emulation of the LINC-8 for contemporary computer platforms.