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 FORTRAN Assembly Program was first introduced for the 709.
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.