IBM 7090 - Instruction and Data Formats

Instruction and Data Formats

The basic instruction format was the same as the IBM 709, a three-bit prefix, 15-bit decrement, three-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 (seven in the 7094), the contents of which were subtracted from the address to produce an effective address. The address field contained either 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 eight-bit excess-128 exponent and a 27-bit magnitude
• Double precision floating point numbers, introduced on the 7094, had a magnitude sign, an eight-bit excess-128 exponent, and a 54-bit magnitude. The double precision number was stored in memory in an even-odd pair of consecutive words; the sign and exponent in the second word were ignored when the number was used as an operand.
• Alphanumeric characters were six-bit BCD, packed six to a word.

Octal notation was used in documentation and programming; console displays lights and switches were grouped into three-bit fields for easy conversion to and from octal.