“Some subroutines are so useful that they are always loaded into the computer. This set of subroutines is called the operating system. Useful operating-system subroutines include those that write or read characters typed on the keyboard, or that draw lines on the screen, or otherwise interact with the user. The computer’s operating system determines most of the look and feel of the interface to the user. It also governs the interface between the computer and whatever program is being run, since the operating system’s subroutines provide the program with a set of operations that are richer and more complex than the machine-language instructions.”