Whither Fortran?
Michael Metcalf, John K. Reid · ACM SIGPLAN Fortran Forum · 1991
Programming in the early days of computing was tedious in the extreme. Programmers required a detailed knowledge of the instructions, registers, and other aspects of the central processing unit (CPU) of the computer for which they were writing code. The source code itself was written in a numerical notation, so called octal code. In the course of time mnemonic codes were introduced, a form of coding known as machine or assembly code. These codes were translated into the instruction words by programs known as assemblers. In the 1950s it became increasingly apparent that this form of programming was highly inconvenient, if only because of the length of time required to write and test a program, although it did enable the CPU to be used in a very efficient way.