COMPUTERS AND MORE

One of the earliest examples of a computer which used parallel functional units was the CDC 6600, which first appeared in the early 1960s. It was superseded in the late 1960s by the CDC 7600 and later by the Cray-1, which can be seen as a logical extension of the 6600/7600 systems from scalar to vector operation.

The 6600 remains an important system for students of computer architecture, however, since it has been particularly well documented. An understanding of its organisation and operation provides a proper background not only for an appreciation of the design of the Cray-1, but almost all modern RISC processors. The common feature in all these systems is the scoreboard mechanism used to control the issuing of instructions to functional units, and the return of results to the common registers.

Vector Processing

The need to provide facilities for processing sequences of vector elements was recognised in the very early days of digital computer design. The von Neumann concept, for example, included the notion of allowing instructions to be treated as data, which meant that the address part of an instruction accessing a vector element could be incremented during the execution of a program loop and thus produce the effect of processing a vector. In practice, however, this technique allows so much scope for program error that even the very first stored program computer (the Manchester Mark 1) used B-lines instead, and this latter technique has been used almost universally ever since. Thus virtually any digital computer can be used to process vectors. The differences between machines lie in the addressing facilities which they provide to support accesses to data structures, and whether or not they include instructions which implicitly process a sequence of vector elements. Computers with this latter facility have been described as Single Instruction Multiple Data (SIMD) arrangements, in contrast with the Single Instruction Single Data (SISD) arrangement of conventional computers.