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3.5.6 SIMD Machines

Several commercial SIMD machines were introduced in the 1970s, but they were not very widely used. Interest in this class of machines was renewed in the late 1980s with the introduction of the Connection Machine (CM-1) from Thinking Machines, Inc., and the MasPar MP-1. Part of the renewed interest is certainly the result of VLSI technology, which had advanced by that time to the point where several small processors could be put on a single chip. By themselves these processors were too simple to compete with general purpose single-chip processors such as the Motorola 68020 or Intel 80386, but literally thousands of them could be packaged in a small space and built into a cost-effective system. For example, 32 MP-1 processors fit on a single chip, and 32 chips were placed on a single board, for a total of 1024 processors (and their associated memory) in approximately 4 square feet.

The CM-1 was based on 1-bit processors. Every operation in the machine processed 1-bit operands and produced 1-bit results. Operations on larger data elements, for example 32-bit integers, required one cycle per bit. Attached to each processor was a local memory with a capacity of 4K bits. Memory references, like processor operations, were 1-bit operations, i.e. a fetch copied 1 bit from memory into a 1-bit processor register. 16 processors were implemented on a single chip. Within a chip, processors were connected with a grid, and up to 4096 chips were connected via a 12-dimensional hypercube. All processors obeyed instructions issued by a central control processor, which in turn was connected to a front-end workstation.

Figure 20: Switching in an X-net.