History
Index registers, commonly known as a B-line in early British computers, were first used in the British Manchester Mark 1 computer, in 1949. In general, index registers became a standard part of computers during the technology's second generation, (roughly 1954–1966) Machines in the IBM 700/7000 mainframe series had them, starting with the IBM 704 in 1954, though they were optional on some smaller machines such as the IBM 650 and IBM 1401.
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 a true index register appears to have been the Motorola 6800, whose upgraded clone MOS Technology 6502 made good use of two such registers. (The earlier Intel 8080 did allow indirect addressing via a register.)
Modern computer designs generally do not include dedicated index registers; instead they allow any general purpose register to contain an address, and allow a constant value and, on some machines, the contents of another register to be added to it as an offset to form the effective address. Early computers designed this way include the PDP-6 and the IBM System/360.
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