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Show CHAPTER3 THE BBN BUTTERFLY PARALLEL PROCESSOR In general terms, a parallel processor is a computer containing some number of processing elements (PEs) which can operate concurrently. The BBN Butterfly is an MIMD, shared-memocy, homogeneous multiprocessor. An MIMD machine processes Multiple Instruction streams and Multiple Data streams concurrently. Each PE can be operating on different data with different instructions at the same time. The term MIMD is generally used to distinguish a machine from an SIMD machine, in which the same machine instructions are applied to different data. Vector- and array-processing machines are examples of SIMD machines. MIMD and SIMD machines are the most common types of parallel processors generally available today. The shared-memory designation indicates that multiple PEs can directly access the same memocy. Any Butterfly PE can share memory with any other PE. This scheme contrasts with machines such as the Cosmic Cube [41] in which each PE is associated with a memocy which is its own, and which can be accessed only by sending messages among the different elements. The terms "tightly-coupled" and "loosely-coupled" are also sometimes used to indicate shared vs. non-shared memory. The Butterfly is not a "pure" shared-memory machine, however (like the Sequent [42], for example), since its physical memory is partitioned among the various PEs in what is sometimes called a boudoir configuration. This topic will be explored more fully later in this chapter. A homogeneous parallel processor is one in which all PE's are identical. The advantages of homogeneity are lower cost (many identical parts mean low cost per part), scalability (the machine can be designed to operate with a variable number of |
