| OPERATING
SYSTEM _ INTRODUCTION |
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| Definitions: |
An
Operating System is a system software which may be
viewed as an organized collection of software consisting
of procedures for operating a computer and providing
an environment for execution of programs.
It is the most fundamental of all the system programs,
which controls all the computer’s resources
and provides the base upon which the application programs
can be written.
It is a layer of software on the top of the bare hardware,
which will shield programmers from the complexity
of the hardware.
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| Computer
System Operation: |
A modern general-purpose
computer system consists of one or more CPUs and a
number of device controllers connected through a common
bus that provides access to shared memory. Each device
controller is in charge of a specific type of device.
The CPU and the device controllers can execute concurrently,
competing for memory cycles. A memory controller is
provided to synchronize access to the shared memory.
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When a computer is powered on or rebooted, an initial
program called bootstrap program runs. Typically it
is stored in read-only memory (ROM) or electrically
erasable programmable read-only memory (EEPROM), which
is also known as firmware. It initializes all aspects
of the system, from CPU registers to device controllers
to memory contents. The bootstrap program must know
how to load the operating system and to start executing
that system. To accomplish this goal, the bootstrap
program must locate and load the operating system
kernel into the memory. After the “init”
process the operating system waits for some event
to occur.
The occurrence of an event is usually signaled by
an interrupt from either the hardware or the software.
Hardware may trigger an interrupt at any time by sending
a signal to the CPU and software may trigger an interrupt
by executing a special operation called a system call.
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When
the CPU is interrupted, it stops what it is doing
and immediately transfers execution to the service
routine for the interrupt. On completion of the service
routine for the interrupt, the CPU resumes the interrupted
computation.
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| Storage
Structure: |
Computer
programs must be loaded into the main memory (RAM)
for execution. Main memory is the only large storage
area that the processor can access directly. It commonly
implemented in a semiconductor technology, which forms
an array of memory words. Each word has its own address.
Interaction is achieved through a sequence of load
and store instructions to specific memory addresses.
The load instruction moves a word from main memory
to an internal register within the CPU, whereas the
store instruction moves the content of a register
to main memory. Apart from the explicit loads and
stores, the CPU automatically loads instructions from
main memory for execution.
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| For execution, the programs
and data to reside in the main memory permanently. This
may not be possible because: |
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Main memory may be
too small to store all needed programs and data. |
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Main memory is a volatile storage
device. |
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Most
computer systems provide secondary storage as an extension
of main memory. The main requirement for secondary
storage is that it be able to hold large quantities
of data permanently. |
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A
wide variety of storage systems in a computer system
can be organized in a hierarchy according to speed
and cost. The higher levels are expensive, but they
are fast. As we move down the hierarchy, the cost
per bit generally decreases, whereas access time generally
increases. In addition to differing in speed and cost,
the various storage systems are either volatile or
nonvolatile.
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