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Real-time Operating Systems (RTOS)

Real-time operating systems are no-frills operating systems designed for one thing—performance. They are typically used in scientific research, industrial robots and devices like mobile phones. They offer little, if any, user interaction.

Real-time computing means that the system must conform to very strict time-constraints or deadlines. For example, if the time taken for a robot to lift an object and move it from point A to point B is 10 seconds, then it must be 10 seconds always—nothing less, nothing more. Imagine what would happen if a car’s fuel injection system injected petrol into the engine before it was ready for it—at best, the engine might sputter, but at worst, it could suffer serious damage!

Real-time OSes are the toughest to write—programmers need to use specialised algorithms to manage resources and ensure that the system’s deadlines are always met.

Single-Task Operating Systems

As the name suggests, these operating systems are capable of running only one task at a time. All system resources are devoted to this single task, and are returned to the OS once the task is complete. Most mobile phone OSes are single-task systems.

Multi-Tasking Operating Systems

All modern OSes—Windows, Linux, Unix, Mac OS X and others—are capable of multi-tasking: running many programs at the same time. Resources are shared by all the programs turn by turn on a ‘time-sharing’ basis. The operating system decides which program is allowed to use the computer’s resources and when.

Multi-tasking operating systems are loved by one and all—users can run many different applications at the same time, programmers can write programs that perform multiple tasks simultaneously rather than waiting for one task to finish before beginning the other, and finally, chip designers are happy because multitasking makes better use of the capabilities of their chips. We will talk more about multi-tasking later.

Multi-User Operating Systems

Multi-User Operating Systems are designed so that many users can use the system’s resources at the same time. Such OSes are called “Server” or “Mainframe” operating systems. In addition to managing different tasks, multi-user OSes need to ensure that each user is getting the most out of the machine, and that problems with one user do not affect other users.

This was usually the scenario in older corporate environments where all resources would be present on one central computer, and all users would connect to it from their terminals—basically a keyboard and a monitor—to access their programs.

Multi-Processing Operating Systems

To do their job better, many OSes support the use of more than one processor. This way, tasks can be carried out in parallel, speeding up the system. This was especially useful for servers, which need to perform a large number of operations as quickly as possible. There are two ways for an operating system to work in a multiprocessor setup. The first, called Asymmetric Multi-processing, gives one processor to the OS to use exclusively for its own tasks. Tasks for other applications are then distributed among the remaining processors. In the second approach, Symmetric Multi-processing, all tasks, including those of the OS, are distributed evenly among all the processors.

Hardware Abstration Layer (HAL)

Applications need to talk to the system’s hardware all the time. Application programmers should not be bothered with having to write new code for new hardware, so operating systems offer them the Hardware Abstraction Layer, or HAL. The HAL is a collection of Device Driverswhich talk to the different kinds of hardware. They give applications a set of instructions which can be used to access this hardware. For example, there are different drivers for different sound-cards, but all of them give applications the same set of instructions to use. Applications use these instructions to create sound, and it is up to the driver to make the sound come out of the speakers. This way, all the application has to know is what functions it can use.