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IBM 709

The IBM 709 was an early computer system introduced by IBM in August, 1958. It was an improved version of the IBM 704 and the second member of the IBM 700/7000 series. The IBM 709 added overlapped input/output, indirect addressing, and decimal instructions.

Instruction Format

The basic instruction format was a 3-bit prefix, 15-bit decrement, 3-bit tag, and 15-bit address. The prefix field specified the class of instruction. The decrement field often contained an immediate operand to modify the results of the operation, or was used to further define the instruction type. The three bits of the tag specified three index registers, the contents of which were subtracted from the address to produce an effective address. The address field either contained an address or an immediate operand.