PARALLEL ELEMENT PROCESSING ENSEMBLE As studies of ballistic missile defense systems progressed, the postulated threats expanded greatly in terms of the number of objects arriving simultaneously and the sophistication of the penetration aids. This increase in threat influenced ABM design and especially increased the estimate of throughput needed for ABM data processors. In response, a new concept of architecture for the ABM data processor was suggested.14 Because a large part of the processing associated with radar tracking and discrimination required that the same set of algorithms be repeatedly applied to each object, parallel elements might carry on the processing. In 1964, research began on a content-addressable memory invented by Lee and Pauli of Bell Laboratories. This memory offered an approach to the needed parallel processing, and a follow-on development program supported by the Advanced Ballistic Missile Defense Agency (ABMDA) led to the Parallel Element Processing Ensemble (PEPE) concept. PEPE was a programmable, special-purpose computing machine that augmented conventional sequential computing in ABM data processing. Processing capacity was largely independent of traffic because an independent parallel element was assigned to each object in track. Each parallel element was, in fact, a small digital computer, with an arithmetic unit and memory. In addition, each contained a special-purpose input unit called a "correlator," which associated radar replies with the appropriate track by simultaneously comparing each radar reply with predicted track positions. Most of the control circuitry was in an ensemble control unit, which was connected in turn to a more conventional "host" sequential digital computer. The host computer stored instructions for the parallel ensemble, sequenced through them, and passed them to the ensemble control unit. The host computer also did the processing that could be most effeciently handled by a sequential computer.15 By the mid-1960s, a study was under way to adapt PEPE to ABM. The intent was to realistically assess feasibility and cost factors. Several studies were launched, primarily in the areas of software development and testing. Hardware Feasibility Using readily available components, a processor with 16 elements was built with integrated circuits and tested with an IBM 360/65 as a host computer. This "IC Model" of PEPE was used in the demonstration tests discussed below. A study which showed the feasibility of using more advanced large-scale integrated circuits in PEPE was completed toward the end of the development project.16 Software Development Since the job to be done by parallel processing elements would be done the same way by a sequential computer, similar programming methods could be used. A parallel version of FORTRAN, P-FOR, became PEPE's basic programming language. P-FOR was supported by a compiler and an assembler to convert programs into machine code. Also, programs could be written for input to the assembler using PAL, the Parallel Assembly Language. The language and software system were available well before any hardware so that programs could be tested by simulation. The P44 precompiler converted each operation on parallel data in a P-FOR program into a DO loop on an array in a standard FORTRAN program. The FORTRAN program could be readily tested on any machine with FORTRAN capability. In addition to P44, which tested P-FOR programs at the source level, the Parallel ABM System Simulation (PASS) simulated operation at the machine level. PASS Tests I to IV, each testing a broader system, were planned. PASS I and II demonstrated PEPE's capability for basic ABM processing and were completed. PASS III and IV were replaced by tests defined by ABMDA, as noted below. Application Studies To identify problems and evaluate the advantages of PEPE, several specific applications were studied:17 • SAFEGUARD. Routines planned for SAFEGUARD as developed in NIKE-X simulations were converted to parallel form in PASS I and 11. 18 • VIRADE. As discussed previously under Defense of Strategic Forces, the VIRADE concept added the problems of changing sites to the basic ABM problems. 19 • ABMDA defined tests. For a final evaluation of PEPE as part of the Bell Laboratories development program, ABMDA defined two systems: Zero Order Software (ZOS) and Preliminary Hardsite Defense (PHSD). These replaced PASS III and IV. The final PHSD demonstration was against a threat defined by General Research Corporation and transmitted to Bell Laboratories by data link from Santa Barbara, California in interrupted real time. The PEPE system used in this test was the 16-element IC model supplemented by sequential simulation, and it achieved essentially all the test objectives. 20 Lessons Learned • The ability of PEPE to carry a large, constantly growing portion of SAFEGUARD data processing was established. The threat level defined for the current SAFEGUARD System did not make PEPE cost effective. Its cost effectiveness would have to be established for a given threat, for a given ABM system, and with the current state of the processor art considered. • The feasibility of increasing system capability by removing processing from a sequential computer and assigning it to a parallel processor was established. • The high level language, P-FOR, was found to be a powerful tool in rapidly programming a complex system. 14. Ballistic Missile Defense, Advanced Development Program, Advanced Data Processing, Vol 1, Bell Laboratories, September 30, 1969. The first of three annual reports; introduces the Parallel Element Processing Ensemble (PEPE) concept and details its architecture and software and initial simulations. 15. IEEE COMP CON '72 Digest: PEPE Computer Architecture, B. A. Crane, M. J. Gilmartin, J. H. Huttenhoff, P. T. Rux and R. R. Shively

The PEPE Support Software System, D. E. Wilson

Application of PEPE to Radar Data Processing, G. D. Bergland and C. F. Hunnicutt

Parallel Processing of Ballistic Missile Defense Radar Data with PEPE, J. A. Cornell. These papers present a brief overview of the characteristics, PHSD test implementation, and performance of PEPE in a general distribution publication. It was presented as an example of "Innovative Architecture, " the theme of the Conference. 16. Ballistic Missile Defense, Advanced Development Program, Advanced Data Processing. (U), Vol 2, Bell Laboratories, September 30, 1971. (SECRET) Presents an overview of the PEPE IC Model, details and results of PEPE application studies to Ballistic Missile Defense, ZOS, PHSD and Off-loading, and results of hardware implementation studies. 17. Ballistic Missile Defense, Advance Development Program, Advanced Data Processing (U), Vol 2, Bell Laboratories, September 30, 1969. (SECRET) Presents the results of studies of the Application of PEPE to SAFEGUARD, VIRADE, and to Coherent Waveform Processing. 18. Ballistic Missile Defense, Advanced Development Program, Advanced Data Processing, Vol 1, Parts 1 and 2, Bell Laboratories, September 30, 1970. This second annual report presents a detailed description of the Integrated Circuit (IC) PEPE model brassboard hardware and its support software and the PASS II evaluation studies. 19. Ballistic Missile Defense, Advanced Development Program, Advanced Data Processing (U), Vol 2, Bell Laboratories, September 30, 1970. (SECRET) Presents results of GPSS simulation studies of Ballistic Missile Data Processing and design alternatives and a study of the application of PEPE to SPRINT missile guidance. 20. Ballistic Missile Defense, Advanced Development Program, Vol 1, Advanced Data Processing, Bell Laboratories, September 30, 1971. The third annual and final report of PEPE studies at Bell Laboratories. Presents an overview of the PEPE system, hardware and software, and the principal final year studies and demonstrations, ZOS, PHSD, and offloading. Includes an introduction to proposed LSI implementation.