IBM 709

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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 of scientific computers. The IBM 709 added overlapped input/output, indirect addressing, and three "convert" instructions (which provided support for decimal arithmetic, leading zero suppression, and several other operations).

An optional hardware emulator executed legacy IBM 704 programs on the IBM 709. This was the first commercially available emulator prior to 1960. Registers and frequently used 704 instructions were emulated in 709 hardware. Complex 704 instructions were emulated in 709 software.

The 709 was built using vacuum tubes. IBM introduced a transistorized version of the 709, called the IBM 7090, in November 1959.

The FORTRAN Assembly Program was first introduced for the 709.

[edit] Instruction and data formats

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.

Fixed point numbers were stored in binary sign/magnitude format.
Single precision floating point numbers had a magnitude sign, an 8-bit excess-128 exponent and a 29 bit magnitude
Alphanumeric characters were 6-bit BCD, packed six to a word.