LINC-8

LINC-8 was the name of a minicomputer manufactured by Digital Equipment Corporation between 1966 and 1969. It combined a LINC computer with a PDP-8 in one cabinet, thus being able to run programs written for either of the two architectures.

The LINC-8 contained one PDP-8 CPU and one LINC CPU, partially emulated by the PDP-8. At any one time, the computer was in either 'LINC mode' or 'PDP-8 mode' - both processors could not run in parallel. Instructions were provided to switch between modes. In the LINC-8, all interrupts were handled by the PDP-8 CPU, and programs that relied on the interrupt architecture of the LINC could not be run.

The LINC was a 12 bit ones' complement accumulator machine, whereas the PDP-8, while also a 12 bit accumulator machine, operated in two's complement arithmetic.

Memory addressing on the two architectures was also different. On the LINC, the full address space was divided into 1024-word segments, two of which were selected for use at any one time: the instruction field and the data field. Direct access of data in the instruction field was possible using 10 bit addresses. The data field could only be indirectly addressed. The Instruction field and Data field are theoretically capable of being chosen from up to 32 areas of 1K 12-bit words each as the maximum architecture is 32K total words. As a practical matter, few LINC-8 systems ever were expanded to 8K total. Memory expansion is accomplished first by adding PDP-8 memory extension hardware and extended memory instructions and a few minor LINC processor modifications to address the memory beyond the basic 4K total. Once this is accomplished, 4K memory "wings" can be added in a daisy-chained buss arrangement, which in theory could be expanded out as many as 7 times to implement the entire 32K. As a practical matter, it is always difficult to implement on the "regular" PDP-8, and, in the case of the LINC-8, it became necessary to slow down the CPU slightly just to add on the first additional 4K.

Thus, as a practical matter, LINC-8 memory segments are limited to segment 0-3, or perhaps 0-7 on the few 8K implementations. However, basic 4K machines cannot address beyond 0-3 while extended memory models could attempt to address segments 0-37 octal even if non-existent memory.

By convention, the segment 0 area is not available for normal fully emulated LINC operations. This is because the PDP-8 program usually known as PROGOFOP is loaded there to handle all interrupts, traps, etc. It is possible to write a program for a "partial" LINC CPU, meaning using only the hardware that actually exists. Whenever an operation is performed that it cannot handle, the PDP-8 operation resumes. However, the LINC operation could have been terminated for a variety of reasons. As such, it is always recommended that PROGOFOP be loaded when attempting to use "complete" LINC programs on this system.

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