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Before the invention of index registers, and without Indirect Addressing , array operations had to be performed either by linearly repeating program code for each array element (i.e. over all address locations), or by using rather "dirty" Self-modifying Code techniques – both alternatives leading to quite significant disadvantages in program flexibility and maintenance, as well as being wasteful of computer memory; the latter a very scarce resource in computer installations of the early era (as well as in early microcomputers several decades later). In general, index registers became a standard part of computers during the technology's second generation (roughly 1955–1964). See, for example, the IBM 700/7000 Mainframe s. Early "small machines" with index registers include the AN/USQ-17 , around 1960 , and the real-time computers from Scientific Data Systems . The first Microprocessor with an index register appears to have been the Motorola 6800 , whose upgraded clone MOS Technology 6502 made good use of two such registers. |
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