A New Architecture for Minicomputers-The DEC PDP-11
C. GORDON BELL, ROGER CADY, HAROLD McFARLAND, BRUCE A. DELAGI, JAMES F. O'LOUGHLIN, RONALD NOONAN, and WILLIAM A. WULF
The minicomputer* has a wide variety of uses: communications controller, instrument controller, large-system preprocessor, real-time data acquisition systems, . . . desk calculator. Historically, Digital Equipment Corporation's (DEC) PDP-8 family, with 6000 installations has been the archetype of these minicomputers.
In some applications current minicomputers have limitations. These limitations show up when the scope of their initial task is increased (e.g., using a higher level language, or processing more variables). Increasing the scope of the task generally requires the use of more comprehensive executives and system control programs, hence larger memories and more processing. This larger system tends to be at the limit of current minicomputer capability, thus the user receives diminishing returns with respect to memory, speed efficiency, and program development time. This limitation is not surprising since the basic architectural concepts for current minicomputers were formed in the early 1960s. First, the design was constrained by cost, resulting in rather simple processor logic and
*The PDP-11 design is predicated on being a member of one (or more) of the micro, midi. mini. ... maxi (computer name) markets. We will define these names as belonging to computers of the third generation (integrated circuit to medium-scale integrated circuit technology), having a core memory with cycle time of 0.5~2 us. a clock rate of 5~10 MHz single processor with interrupts and usually applied to doing a particular task (e.g.. controlling a memory or communications lines, preprocessing for a larger system, process control). The specialized names are defined as follows.