Computer systems can be made faster and more reliable by using multiple CPUs. Four organizations for multi-CPU systems are multiprocessors, multicomputers, virtual machines, and distributed systems. Each of these has its own properties and issues.

A multiprocessor consists of two or more CPUs that share a common RAM. Often these CPUs themselves consists of multiple cores. The cores and CPUs can be interconnected by a bus, a crossbar switch, or a multistage switching network. Various operating system configurations are possible, including giving each CPU its own operating system, having one leader operating system with the rest being follower, or having a symmetric multiprocessor, in which there is one copy of the operating system that any CPU can run. In the latter case, locks are needed to provide synchronization. When a lock is not available, a CPU can spin or do a context switch. Various scheduling algorithms are possible, including time sharing, space sharing, and gang scheduling.

Multicomputers also have two or more CPUs, but these CPUs each have their own private memory. They do not share any common RAM, so all communication uses message passing. In some cases, the network interface board has its own CPU, in which case the communication between the main CPU and the interface-board CPU has to be carefully organized to avoid race conditions. User-level communication on multicomputers often uses remote procedure calls, but distributed shared memory can also be used. Load balancing of processes is an issue here, and the various algorithms used for it include sender-initiated algorithms, receiverinitiated algorithms, and bidding algorithms.

Distributed systems are loosely coupled systems each of whose nodes is a complete computer with a complete set of peripherals and its own operating system. Often these systems are spread over a large geographical area. Middleware is often put on top of the operating system to provide a uniform layer for applications to interact with. The various kinds include document-based, file-based, objectbased, and coordination-based middleware. Some examples are the World Wide Web, CORBA, and Linda.