IBM 4020 Military Computer

The IBM 4020 Military Computer was a Cold War era military computer. Two of them were used in the two IBM AN/FSQ-31 SAC Data Processing Systems.

Details:
 * Instruction rate : Up to 400,000 instructions per second
 * Cycle time : 2.5 microseconds
 * Word size : 48 bits + 2 parity bits
 * Core storage : up to 131,072 words
 * operates in parallel mode
 * 69 instructions
 * single address instructions
 * Add or subtract : 2.5 microseconds (fixed point)
 * Add or subtract : 7.7 to 26 μs (floating point)
 * Multiply : average 24 μs (24 bit precision fixed point)
 * Multiply : 5 to 74 μs (floating point)
 * Multiprocessing with Automatic Priority

The instruction format includes:
 * 7 bit op code
 * 2 bit real data indicator
 * 3 bit byte displacement
 * 3 bit mode selector
 * 1 sign bit
 * 8 bit 'byte activity'
 * 1 bit Double index flag
 * 1 bit Indirect address flag
 * 4 bit index register selection
 * 18 bit address.

Man-machine communications included a light-gun to indicate an area of interest on a visual display unit.

Circuit logic packaging was based on 7 types of Q-pacs each holding 1 to 4 circuits. Transistors and semiconductor diodes were soldered to the outside of the Q-pac encapsulation for ease of cooling and replacement.