Introduction To Computers

With multiprocessors multiple CPUs share memory. A multicomputer or cluster computer has multiple CPUs, each of which has its own memory. Multicomputers were developed because large multiprocessors are difficult to engineer and prohibitively expensive;^[17] they are universal in cloud computing because of the size of the machine needed.^[18] The different CPUs often need to send and receive messages to each other;^[19] to ensure good performance, the operating systems for these machines need to minimize this copying of packets.^[20] Newer systems are often multiqueue—separating groups of users into separate queues—to reduce the need for packet copying and support more concurrent users.^[21] Another technique is remote direct memory access, which enables each CPU to access memory belonging to other CPUs.^[19] Multicomputer operating systems often support remote procedure calls where a CPU can call a procedure on another CPU,^[22] or distributed shared memory, in which the operating system uses virtualization to generate shared memory that does not physically exist.^[23]

A distributed system is a group of distinct, networked computers—each of which might have their own operating system and file system. Unlike multicomputers, they may be dispersed anywhere in the world.^[24] Middleware, an additional software layer between the operating system and applications, is often used to improve consistency. Although it functions similarly to an operating system, it is not a true operating system.^[25]

Embedded operating systems are designed to be used in embedded computer systems, whether they are internet of things objects or not connected to a network. Embedded systems include many household appliances. The distinguishing factor is that they do not load user-installed software. Consequently, they do not need protection between different applications, enabling simpler designs. Very small operating systems might run in less than 10 kilobytes,^[26] and the smallest are for smart cards.^[27] Examples include Embedded Linux, QNX, VxWorks, and the extra-small systems RIOT and TinyOS.^[28]

A real-time operating system is an operating system that guarantees to process events or data by or at a specific moment in time. Hard real-time systems require exact timing and are common in manufacturing, avionics, military, and other similar uses.^[28] With soft real-time systems, the occasional missed event is acceptable; this category often includes audio or multimedia systems, as well as smartphones.^[28] In order for hard real-time systems be sufficiently exact in their timing, often they are just a library with no protection between applications, such as eCos.^[28]

A hypervisor is an operating system that runs a virtual machine. The virtual machine is an application that emulates hardware; in other words, it operates as much as possible like the actual hardware the operating system was designed to run on.^[14][29] Virtual machines can be paused, saved, and resumed, making them useful for operating systems research, development,^[30] and debugging.^[31] They also enhance portability by enabling applications to be run on a computer even if they are not compatible with the base operating system.^[14]