IBM 4020 Military Computer

The IBM 4020 Military Computer{{cite book|url=http://bitsavers.org/pdf/ibm/4020/4020_Military_Computer_General_Info_Oct59.pdf|title=The IBM 4020 Military Computer - General Information|date=October 31, 1959|publisher=IBM Federal Systems Division}} 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 : {{nowrap|7.7 to 26 μs}} (floating point)
• Multiply : average 24 μs (24 bit precision fixed point)
• Multiply : {{nowrap|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.