Computer System Architecture The computer system may be organized in a number of different ways, which we can categorize roughly according to the number of general-purpose processors used. Single-Processor Systems Most systems use a single processor. The variety of single-processor systems may be surprising, however, since these systems range from PDAs to mainframes. On a single-processor system, there is one main CPU capable of executing a general-purpose instruction set, including instructions from user processes. Almost all systems have other special-purpose processors as well. They may come in the form of device-specific processors, such as a disk, keyboard, and graphics controllers; or, on mainframes, they may come in the form of more general-purpose processors, such as I/O processors that move data rapidly among the components of the system. All of these special-purpose processors run a limited instruction set and do not run user processes. Sometimes they are managed by