operating system

1. FUNDAMENTALS

1.1 Definition of OS

What is an Operating System?
An operating system is a program that manages the computer hardware. It acts as an intermediate between the user of the computer and the computer hardware.

According to their task completion, they are varied different kinds of Operating system are used for different computer systems. For example,

1. Mainframe Operating system are designed to optimize utilization of hardware.
2. Personal Computer (PC) operating system support complex games, business applications.
3. Handheld operating system used to interact with user easily.
    

Goal of Operating System :

1. It should be convenient to the user.
2. It should be efficient to the user.


Abstract view of the Components of a Computer System :

The operating system controls every task your computer carries out and manages system resources.

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Operating System Functions

At the simplest level, an operating system does two things:

1. It manages the hardware and software resources of the system. In a desktop computer, these resources include such things as the processor, memory, disk space and more (On a cell phone, they include the keypad, the screen, the address book, the phone dialer, the battery and the network connection).
2. It provides a stable, consistent way for applications to deal with the hardware without having to know all the details of the hardware.

The first task, managing the hardware and software resources, is very important, as various programs and input methods compete for the attention of the central processing unit (CPU) and demand memory, storage and input/output (I/O) bandwidth for their own purposes. In this capacity, the operating system plays the role of the good parent, making sure that each application gets the necessary resources while playing nicely with all the other applications, as well as husbanding the limited capacity of the system to the greatest good of all the users and applications.
­ The second task, providing a consistent application interface, is especially important if there is to be more than one of a particular type of computer using the operating system, or if the hardware making up the computer is ever open to change. A consistent application program interface (API) allows a software developer to write an application on one computer and have a high level of confidence that it will run on another computer of the same type, even if the amount of memory or the quantity of storage is different on the two machines.
Even if a particular computer is unique, an operating system can ensure that applications continue to run when hardware upgrades and updates occur. This is because the operating system -- not the application -- is charged with managing the hardware and the distribution of its resources. One of the challenges facing developers is keeping their operating systems flexible enough to run hardware from the thousands of vendors manufacturing computer equipment. Today's systems can accommodate thousands of different printers, disk drives and special peripherals in any possible combination
Memory hierarchy:
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Diagram of the computer memory hierarchy

The term memory hierarchy is used in computer architecture when discussing performance issues in computer architectural design, algorithm predictions, and the lower level programming constructs such as involving locality of reference. A "memory hierarchy" in computer storage distinguishes each level in the "hierarchy" by response time. Since response time, complexity, and capacity are related,^[1] the levels may also be distinguished by the controlling technology.
The many trade-offs in designing for high performance will include the structure of the memory hierarchy, i.e. the size and technology of each component. So the various components can be viewed as forming a hierarchy of memories (m₁,m₂,...,m_n) in which each member m_i is in a sense subordinate to the next highest member m_i-1 of the hierarchy. To limit waiting by higher levels, a lower level will respond by filling a buffer and then signaling to activate the transfer.
There are four major storage levels.^[1]

1. Internal – Processor registers and cache.
2. Main – the system RAM and controller cards.
3. On-line mass storage – Secondary storage.
4. Off-line bulk storage – Tertiary and Off-line storage.

This is a most general memory hierarchy structuring. Many other structures are useful. For example, a paging algorithm may be considered as a level for virtual memory when designing a computer architecture.