AN/FSQ-7

The AN/FSQ-7 (originally named Whirlwind II during the design stage) was an early, very large (architecturally, by the standards of its day, as well as physically) vacuum tube real-time computer system; they were the heart of the 'direction centers', the main nodes in the SAGE system.

It was designed by IBM, in cooperation with Lincoln Laboratory, starting in October, 1952, and built by IBM (and was a significant factor in their growth in expertise in computers). The first prototype went into operation in 1955; the first operational system was shipped in July, 1958.

The main memory, and the system generally, used parity for error detection. The machine used core memory (which was invented for it); typically a 64KW bank, and a 4KW bank, in most installations. Its word length was 32 bits, the two halves often being handled independently (as much of the data handled was coordinates). Instructions were also 32 bits; the left half-word contained a number of fields, including the opcode, and the right half-word (plus one bit from the left) was an address.

The CPU could perform roughly 55,000 to 83,000 instructions per second (initially there were about 48 different instructions in total; later there were 59), and used one's complement arithmetic. Minimal support for subroutines was provided; the return instruction had to be modified before it could be executed, using an address stored in a register by the call. It had 8 registers that were visible to the programmer:

• a program counter
• an accumulator (the half-words were sometimes usable independently)
• 4 (initially 2) index registers (the right half-word of the accumulator could also be used for indexing)
• 'I/O address counter'
• 'I/O word counter'

For I/O, in addition to interfaces to air defence elements (radars, etc), the AN/FSQ-7 was connected to a number of display consoles, and used several drums for mass storage; it also had punched card readers.

Each SAGE site included two complete AN/FSQ-7 systems, with one on 'hot standby' at all times, using redundancy to provide an extremely reliable system; marginal checking was used to discover failing tubes before they failed permanently. Twenty-four of the original dual AN/FSQ-7's were eventually built; each contained 49,000 tubes, weighed 275 tons, and used up to three megawatts of power.

As SAGE became operational, it became evident that its 'combat centers' (each of which oversaw several direction centers) could usefully be computerized as well. A variant of the AN/FSQ-7 named the AN/FSQ-8 was produced for this; three were eventually built. It did not connect directly to radar units, as the AN/FSQ-7 did; and it also had a specialized display system.

Further reading

• Bernd Ulmann, AN/FSQ-7: The Computer That Shaped the Cold War, De Gruyter Oldenbourg, Munich, 2014
• Morton M. Astrahan, John F. Jacobs, History of the Design of the SAGE Computer - The AN/FSQ-7, Annals of the History of Computing, Vol. 5, No. 4, October 1983, pp. 340-349

External links

• sage - documentation at Bitsavers
  • AN/FSQ-7 (SAGE) Manual List
  • Introduction to AN/FSQ-7 (C686-416L-ST)
  • AN/FSQ-7 Central Computer System Theory of Operation (Preliminary)
  • AN/FSQ-7 Central Computer System Theory of Operation (Volume I)
  • AN/FSQ-7 Central Computer System Theory of Operation (Volume II)
  • The Computer Museum Member's First Field Trip To Northbay AN/FSQ-7 SAGE Site
• Introduction to AN/FSQ-7 Combat Direction Central and AN/FSQ-8 Combat Control Central
• Theory of Programming for AN/FSQ-7 (31P2-2FSQ7-112)
• The 24 AN/FSQ-7 Computers IBM Built for SAGE are Physically the Largest Computers Ever Built