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Description of the UNIVAC I

UNIVAC I had a character set designed around the typewriter keyboard with a lower case alphabet, a shift character, the decimal digits, and the usual typewriter symbols. Capital letters and some of the symbols were printed by inserting shift characters into the text. In the computer each character was represented by six bits plus parity, which gave a possible 64 code combinations. Characters were examined when passed through the serial decimal adder and alphabetic characters were bypassed around the adder so they could not be altered. Bypassed alphabetics interrupted the carry chain and were inserted in the summand where they fell. If both operands were bypassed simultaneously, an error was detected and UNIVAC stopped. There were no bit manipulation instructions in UNIVAC I, and only by using tricks was it possible to change an individual bit. The values for operators chosen for the UNIVAC I versions for SHORT CODE were selected to correspond to the top row of keys on the console typewriter.

UNIVAC I machine instructions were composed of an operation code, usually a letter, and an address which was meant to be a three decimal digit quantity from 000 to 999. There was a peculiarity in the address decoding which was often used to advantage, namely that the letters S through Z could be substituted for the digits 2 to 9. Pairing these eight letters with the ten digits gave

80 combinations (SO-Z9) and I chose these to be the allowable variable identifiers; and as was the case with the BINAC, they were directly convertible into addresses. The remaining mathematical symbols and functions were given two decimal digit codes with 00 reserved for a skip.