IBM 701

Overview [1] #

IBM 701, known as the Defense Calculator while in development, was announced to the public on April 29, 1952, and was IBM’s first commercial scientific computer.

Its business computer siblings were the IBM 702 and IBM 650. It was based on the IAS machine.[2]

The system used vacuum tube logic circuitry and electrostatic storage, consisting of 72 Williams tubes with a capacity of 1024 bits each, giving a total memory of 2048 words of 36 bits each. Each of the 72 Williams tubes was 3 inches in diameter. Memory could be expanded to a maximum of 4096 words of 36 bits by the addition of a second set of 72 Williams tubes or (later) by replacing the entire memory with magnetic core memory. The Williams tube memory and later core memory each had a memory cycle time of 12 microseconds. The Williams tube memory required periodic refreshing, mandating the insertion of refresh cycles into the IBM 701's timing. An addition operation required five 12-microsecond cycles, two of which were refresh cycles, while a multiplication or division operation required 38 cycles (456 microseconds).

Instructions were 18 bits long, single address.

  • Sign (1 bit) - Whole word (-) or Half word (+) operand address
  • Opcode (5 bits) - 32 instructions
  • Address (12 bits) - 4096 Half word addresses
Numbers were either 36 bits or 18 bits long, signed magnitude, fixed point.

IBM 701 had only two programmer accessible registers:

  • The accumulator was 38 bits long (adding two overflow bits).
  • The multiplier/quotient was 36 bits long.

IBM 701 Emulator [2]#

The world's best—probably only—IBM 701 emulator!

To make programming more convenient than it was in 1953, they added a couple of things that aren't part of the 701 itself: a log that shows the instructions and registers in real-time, and an assembler that automatically punches a card deck and loads it into the card reader. (The assembler doesn't run on the 701.)



More Information#

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