AN/USQ-20

The AN/USQ-20 or Navy Tactical Data System (NTDS), was designed as a more reliable replacement for the 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.

The machine was the size and shape of an old fashioned double-door refrigerator, about six feet tall.

Instructions were represented as 30 bit words, in the following format:

   f  6 bits   function code 
   j  3 bits   jump condition designator 
   k  3 bits   partial word designator 
   b  3 bits   which index register to use 
   y  15 bits  operand address in memory

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 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.