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Learning objective: Identify classifications of operating systems
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Operating system are usually broken down into four basic classifications. The four main types of operating system are listed below.
Single-user, single-task operating systems
A single-user, single-task operating system, like MS-DOS, with basic kernel functions that are non-reentrant: only one program at a time can use them. There is an exception with Terminate and Stay Resident (TSR) programs, and some TSRs can allow multitasking. However, there is still a problem with the non-reentrant kernel: once a process calls a service inside of operating system kernel (system call), it must not be interrupted with another process calling system call, until the first call is finished. [Wikipedia]
Single-user, multi-task operating systems
On basis of number of tasks the computer can handle at a time the operating systems can be classified into single-task or multi-tasking (also referred to as multi-programming) operating system. A single-task the name implies, this operating system is designed to manage the computer so that one user can effectively do one thing at a time. Multi-tasking operating systems are most commonly used by people on their desktop and laptop computers today. Microsoft's Windows and Apple's Mac OS platforms are both examples of operating systems that will let a single user have several programs in operation at the same time. For example, it's entirely possible for a Windows user to be writing a note in a word processor while downloading a file from the Internet while printing the text of an e-mail message. This is made possible either by using multiple CPUs or timesharing or a mix of both. [Wikipedia]
Multi-user systems
Multi-user is a term that defines an operating system or application software that allows concurrent access by multiple users of a computer. Time-sharing systems are multi-user systems. Most batch processing systems for mainframe computers may also be considered "multi-user", to avoid leaving the CPU idle while it waits for I/O operations to complete. However, the term "multitasking" is more common in this context. An example is a Unix server where multiple remote users have access (such as via Secure Shell) to the Unix shell prompt at the same time. Another example uses multiple X Window sessions spread across multiple terminals powered by a single machine - this is an example of the use of thin client. [Wikipedia]
Real-time operating systems
A real-time operating system (RTOS) is an operating system (OS) intended for real-time applications. Such operating systems serve application requests nearly real-time. A real-time operating system offers programmers more control over process priorities. An application's process priority level may exceed that of a system process. Real-time operating systems minimize critical sections of system code, so that the application's interruption is nearly critical. A real-time OS has an advanced algorithm for scheduling. Scheduler flexibility enables a wider, computer-system orchestration of process priorities, but a real-time OS is more frequently dedicated to a narrow set of applications. Key factors in a real-time OS are minimal interrupt latency and minimal thread switching latency, but a real-time OS is valued more for how quickly or how predictably it can respond than for the amount of work it can perform in a given period of time. [Wikipedia]
Thinking: What type of OS do you want for your PC? or your car's breaks?
Key terms: Multi-user systems, Real-time operating systems, Single-user, multi-task operating systems, Single-user, single-task operating systems
Resources:
To maximize your learning, please visit these Web sites and review their content
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Quick links:
DOS @ Wikipedia
Classification of operating systems @ ICTExams
Multi-user @ Wikipedia
Real-time_operating_system @ Wikipedia
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