Data handling by control word techniques
G. A. Blaauw · 1958
Early computing machines executed programs which could be specified by pluggable wiring, paper tape, or cards. These programs remained unchanged during their execution. The invention of the stored program made it possible to treat a program as data. This invention was a basic step forward in the development of computing machines. It then was possible for any instruction of a program to be modified by the program itself. In practice this general facility was used mainly for the modification of addresses. Subsequently, it became apparent that programmed address computation, though sufficient in theory, was cumbersome in practice. Too much computing time and program space were required to perform these auxiliary operations. As a remedy, an address register, also called index register or B-line, was provided, whose contents could be added to the operand address. In recent machines, several index registers, up to one hundred, have been made available.