Heterogeneous Element Processor

The Heterogeneous Element Processor (HEP) was introduced by Denelcor in 1982 as the world's first commercial MIMD computer. 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 in a HEP system (up to sixteen 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 eight-statge 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.

Processes were classified as either user-level or supervisor-level. User-level processes could create supervisor-level processes, which were used to manage user-level processes and perform I/O. Processes of the same class were required to be grouped into one of seven user tasks and seven supervisor tasks.

Each processor, in addition to the PSW queue and instruction pipeline, contained instruction memory, 2,048 64-bit general purpose registers and 4,096 constant registers. Constant registers were differentiated by the fact that only supervisor processes could modify their contents. Interestingly, the processors themselves contained no data memory; instead, data memory modules could be separately attached to the switched network.

The HEP implemented a type of mutual exclusion in which all registers and locations in data memory had associated "empty" and "full" states. Reading from a location set the state to "empty," while writing to it set the state to "full." A programmer could allow processes to halt after trying to read from an empty location or write to a full location, enforcing critical sections.

The switched network between elements resembled, in many ways, a modern computer network. On the network were sets of nodes, each of which had three links. When a packet arrived at a node, it consulted a routing table and attempted to forward the packet closer to its destination. If a node became congested, any incoming packets were passed on without routing. Packets treated in such a manner had their priority level increased; when several packets vied for a single node, a packet with a higher priority level would be routed before ones with lower priority levels.

Although it was known to have poor cost-performance, the HEP received attention due to what were, at the time, several revolutionary features. HEP systems were purchased by Los Alamos, the Argonne National Laboratory, the Ballistic Research Laboratory, and The HEP attracted widespread attention despite its terrible cost performance because of its many interesting hardware features that facilitated programming. The Denelcor HEP was acquired by several institutions, including Los Alamos, Argonne National Laboratory, Ballistic Research Laboratory, and Germany's Messerschmitt. Messerschmitt was the only client to put the HEP into use for "real" applications; the other clients used it for experimenting with parallel algorithms.