A 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.

Most system use a single processor. 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 in the form of device-specific processors or 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 the operating system, in that the operating system sends them information about their next task and monitors their status. This arrangement relieves the main CPU of the overhead of disk scheduling. PCs contain a microprocessor in the keyboard to convert the keystrokes into codes to be sent to the CPU. These type of special-purpose processors are low-level components built into the hardware. The operating system cannot communicate with these processors and they do their jobs autonomously. This does not turn a single-processor system into a multiprocessor. If there is only one general-purpose CPU, then the system is a single-processor system.

Multiprocessor systems have two or more processors in close communication, sharing the computer bus and sometimes the clock, memory and peripheral devices.