Talk:IBM 709

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The IBM 709 did not include decimal instructions, per se. Only binary arithmetic was supported, primarily in 36-bit signed integer and 36-bit floating-point formats. For larger integers, the Accumulator and M-Q registers could be combined programatically to represent a 70-bit integer plus a 1-bit sign (represented twice.) The index-register functions additially provided limited 15-bit unsigned integer arithmetic (addition and subtraction only.) Decimal data required a programmed radix conversion to a binary representation before being processed.

The primary improvements of the 709 over the previous 704 involved more magnetic core memory and apparantly the first use of independent I/O channels. Whereas I/O on the 704 was a programmed function of the central processor - data words were transferred to or from the I/O register, one at at time, using a "copy" instruction - the 709 came with the IBM-766 Data Synchronizer, which provided two independently "programmed" I/O channels. Up to three Data Synchronizers could be attached to a 709, each able to control up to 20 tape drives and a card-reader/punch/printer set. This allowed six times as many I/O devices on the 709, and allowed I/O to proceed on multiple devices while program execution continued in parallel.

The IBM-738 Magnetic Core Storage used on the 709 was also a milestone of hybrid technology. Although the core array drivers were all vacuum tube, the read sense amplifiers were a very early use of transistors in computing.