The UNIVAC 1101, or ERA 1101, was a computer system designed by Engineering Research Associates (ERA) and built by the Remington Rand corporation in the 1950s. It was the first stored program computer in the US.
Originally designed for the US Navy's Bureau of Ships (a cover for the NSA) and called Atlas (after a character in the popular comic strip Barnaby), the commercial version was renamed the 1101 because it was designed under "Task 13" (1101 is 13 in binary).
This computer was 38 feet long (11.5 m), 20 feet wide (6 m), and used 2700 vacuum tubes for its logic circuits. Its drum memory was 8.5 inches in diameter (21.6 cm), rotated at 3500 rpm, had 200 read-write heads, and held 16,384 24-bit words (a memory size equivalent to 48 KB) with access time between 32 microseconds and 17 milliseconds.
Instructions were 24 bits long, with 6 bits for the opcode, 4 bits for the "skip" value (telling how many memory locations to skip to get to the next instruction in program sequence), and 14 bits for the memory address. Numbers were binary with negative values in one's complement. The addition time was 96 microseconds and the multiplication time was 352 microseconds.
The single 48-bit accumulator was fundamentally subtractive, addition being carried out by subtracting the one's complement of the number to be added. This may appear rather strange, but the subtractive adder reduces the chance of getting negative zero in normal operations.
The machine had 38 instructions.
Engineering Research Associates built two Atlas systems for the Navy's Bureau of Ships, installing them in December 1950 and March 1953. There was talk of naming the commercial version MABEL, but Jack Hill suggested 1101 instead. The ERA 1101 was publicly announced in December 1951.
Engineering Research Associates built a third machine for their own offices, with the intention of creating a service for other companies needing computing resources. However this failed and in November 1954 Remington Rand donated the machine to Georgia Tech for a claimed value of $500,000. The two machines at NSA had been upgraded with core memory sometime prior to the middle of 1956. In November 1958 Georgia Tech upgraded this machine with 4096 words of core memory for a cost of $39,400. This 1101 was still running student jobs in 1961.
- List of UNIVAC products
- History of computing hardware
- Engineering Research Associates and the Atlas Computer (UNIVAC 1101) by George Gray,
from the Unisys History Newsletter, Volume 3, Number 3, June, 1999
- Introducing the ERA 1101: An operationally proven high-speed, electronic, general purpose digital computer, ERA, no-date. (8 pp) 
- ERA 1101 Documents list of 44 scanned course notes on 1101 by H. C. Snyder USN
- Summary of Characteristics Magnetic Drum Binary Computer, Engineering Research Associates Pub No. 25, 30 Nov 1948