The PostScript program is created on the computer either by someone using the language or by desktop publishing software or other applications software that translates, say, the movements of a mouse into a PostScript program. (Other page description languages are optimized for one of these purposes, not both.) That program is sent over a local-area network or through an RS-232 port to the laser printer. There it is converted into instructions for the printer by the PostScript interpreter, software resident in ROM. On the same circuit board as up to 2 megabytes of ROM is a Motorola 68000 series processor, which executes the instructions and causes the pages to be printed.
Things were more elementary with the first laser printers, which were in regular use at the Xerox Palo Alto Research Center (PARC) in the mid-1970s. They were controlled by a printing protocol called Press, which was not a programming language but a set of instructions that sent image data to a printer in a steady stream. It handled letters and simple images well, but for anything more detailed, got the printer to return the message: “Page Too Complex.” Thereupon the typical PARC engineer would simplify the image.
But when Warnock, a computer scientist with a Ph.D. from the University of Utah, joined the center in 1978, he immediately began work on a new printer protocol. Six years of experience at Evans & Sutherland in Mountain View, Calif., had taught him where to start.