title: AN/USQ-20
text: # AN/USQ-20
  
  The AN/USQ-20, or CP-642 or Naval Tactical Data System (NTDS), was designed as a more reliable replacement for the Seymour Cray-designed AN/USQ-17 with the same instruction set. The first batch of 17 computers were delivered to the Navy starting in early 1961.
  
  A version of the AN/USQ-20 for use by the other military services and NASA was designated the UNIVAC 1206. Another version, designated the G-40, replaced the vacuum tube UNIVAC 1104 in the BOMARC Missile Program.
  
  ## Technical
  
  The machine was the size and shape of an old-fashioned double-door refrigerator, about six feet tall (roughly 1.80 meters).
  
  Instructions were represented as 30-bit words in the following format:
  
  Numbers were represented as 30-bit words. This allowed for five 6-bit alphanumeric characters per word.
  
  The main memory was 32,768 words of core memory.
  
  The available processor registers were:
  
  * One 30-bit arithmetic (A) register.
  * A contiguous 30-bit Q register (total of 60 bits for the result of multiplication or the dividend in division).
  * Seven 15-bit index (B) registers (note: register B0 is always zero).
  