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The C.mmp, with three racks visible, including the front panel of the crossbar switch.

The C.mmp was an early MIMD multiprocessor system developed at Carnegie Mellon University by William Wulf (1971).

Sixteen PDP-11 minicomputers were used as the processing elements (named Compute Modules in the system). Each CM had a local memory of 8K and a local disk subsystem.

Each of the Compute Modules shared these communication pathways:

  • An Interprocessor bus - used for distribution of system-wide clock, interrupt and process control messaging among the CMs
  • A 16x16 crossbar switch - used to connect the 16 CMs on one side and 16 banks of shared memory on the other.

Since the PDP-11 only had an address space of 16-bits, an additional address translation unit was added to expand the address space to 25 bits for the shared memory space.

The operating system was called HYDRA. It was an capability-based object-oriented operating system, and was operated from a PDP-10 multi-user system. System resources were represented as objects and protected through capabilities.

Among the programming languages available on this system was a subset of ALGOL 68. This language was in fact more a superset than a subset, as the features supporting parallelism were vastly improved, to make good use of the C.mmp.

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