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Unit 3: Using Operating System

• The system call interface simplifies program construction by removing the details of Notes
implementing the request to the operating system.
3.1.2.2 The System Call Model: Layers of Interaction
There are layers of interaction in the system to ensure fair access to resources, protect privacy,
and provide convenience.

Figure 3.5

3.2 Types of Operating System

Within the broad family of operating systems, there are generally four types, categorized
based on the types of computers they control and the sort of applications they support. The
categories are real-time operating system, single user single task, single user multitasking
and multi-user.
3.2.1 Real-Time Operating System (RTOS)

Real-time operating systems are used to control machinery, scientific instruments and industrial
systems such as embedded systems (programmable thermostats, household appliance
controllers), industrial robots, spacecraft, industrial control (manufacturing, production, power
generation, fabrication, and refining), and scientific research equipment.
An RTOS typically has very little user-interface capability, and no end-user utilities, since the
system will be a “sealed box” when delivered for use. A very important part of an RTOS is
managing the resources of the computer so that a particular operation executes in precisely the
same amount of time, every time it occurs. In a complex machine, having a part move more
quickly just because system resources are available may be just as catastrophic as having it not
move at all because the system is busy.
An RTOS facilitates the creation of a real-time system, but does not guarantee the final result will
be real-time; this requires correct development of the software. An RTOS does not necessarily
have high throughput; rather, an RTOS provides facilities which, if used properly, guarantee
deadlines can be met generally (soft real-time) or deterministically (hard real-time). An RTOS
will typically use specialized scheduling algorithms in order to provide the real-time developer
with the tools necessary to produce deterministic behavior in the final system. An RTOS is
valued more for how quickly and/or predictably it can respond to a particular event than for
the given amount of work it can perform over time. Key factors in an RTOS are therefore a
minimal interrupt latency (the time between the generation of an interrupt by a device and the
servicing of the device which generated the interrupt) and a minimal thread switching latency
(the time needed by the operating system to switch the CPU to another thread).

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