Heterogeneous Element Processor

Article summary

The Heterogeneous Element Processor (HEP) was introduced by Denelcor, Inc. in 1982. The HEP's architect was Burton Smith. The machine was designed to solve fluid dynamics problems for the Ballistic Research Laboratory. A HEP system, as the name implies, was pieced together from many heterogeneous components -- processors, data memory modules, and I/O modules. The components were connected via a switched network. A single processor, called a PEM (Process Execution Module), in a HEP system (up to sixteen PEMs could be connected) was rather unconventional; via a "program status word (PSW) queue" up to fifty processes could be maintained in hardware at once. The largest system ever delivered had 4 PEMs. The eight-stage instruction pipeline allowed instructions from eight different processes to proceed at once. In fact, only one instruction from a given process was allowed to be present in the pipeline at any point in time. Therefore, the full processor throughput of 10 MIPS could only be achieved when eight or more processes were active; no single process could achieve throughput greater than 1.25 MIPS. This type of multithreading processing classifies today the HEP as a barrel processor, while it was described as an MIMD pipelined processor by its designers. The hardware implementation of the HEP PEM was emitter-coupled logic.…

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