Properly scaling calculations was the responsibility of the programmer. Instructions used the right half word plus the left sign bit to form addresses, yielding a 17-bit address space. The remainder of the left half word specified the operation. The first three bits after the sign specified an index register. The following bits specified an instruction class, class variation and instruction-dependent auxiliary information. Addresses were written in octal notation, with the two sign bits forming a prefix, so 2.07777 would be the highest word in memory 2. Arithmetic registers were provided for both halves of the data word and included an accumulator, an A register that held the data value retrieved from memory, and a B register that held the least significant bits of a multiplication, the magnitude of a division, as well as shifted bits. There was also a program counter, four index registers, and a 16-bit real-time clock register which was incremented 32 times a second. Trigonometric sine and cosine functions used 1.4 degree precision (256 values) via look-up tables. See also List of vacuum-tube computers References 1955 in computing 1955 in military history 1958 in military history 1983 disestablishments 1983 in military history Cold War military computer systems of the United States IBM vacuum tube computers Equipment of the United States Air Force Missile guidance AN/FSQ-7