Operating Systems: Internals and Design Principles, Seventh Edition, by William Stallings. Published by Prentice Hall. Copyright © 2012 by Pearson Education, Inc.

Four main structural elements:

• Processor: Controls the operation of the computer and performs its data processing
functions. When there is only one processor, it is often referred to as
the central processing unit (CPU).

• Main memory: Stores data and programs. This memory is typically volatile; that
is, when the computer is shut down, the contents of the memory are lost. In
contrast, the contents of disk memory are retained even when the computer
system is shut down. Main memory is also referred to as real memory or primary

• I/O modules: Move data between the computer and its external environment.
The external environment consists of a variety of devices, including secondary
memory devices (e.g., disks), communications equipment, and terminals.

• System bus: Provides for communication among processors, main memory,
and I/O modules.
Processor-memory: Data may be transferred from processor to memory or
from memory to processor.

• Processor-I/O: Data may be transferred to or from a peripheral device by
transferring between the processor and an I/O module.

• Data processing: The processor may perform some arithmetic or logic operation
on data.

• Control: An instruction may specify that the sequence of execution be altered.
For example, the processor may fetch an instruction from location 149, which specifies that the next instruction will be from location 182.
Interrupts are provided primarily as a way to improve processor utilization.
For example, most I/O devices are much slower than the processor.
Classes of Interrupts
Program Generated by some condition that occurs as a result of an instruction execution, such as
arithmetic overflow, division by zero, attempt to execute an illegal machine instruction,
and reference outside a user’s allowed memory...

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