Major Components of Operating System

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Chapter 1
At a top level, a computer consists of processor, memory, and I/O components, with one or more modules of each type. These components are interconnected in some fashion to achieve the main function of the computer, which is to execute programs. Thus, there are 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
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On most processors, most of these are not visible to the user. Some of them may be accessible by machine instructions executed in what is referred to as a control or kernel mode. • Program counter (PC): Contains the address of the next instruction to be fetched • Instruction register (IR): Contains the instruction most recently fetched All processor designs also include a register or set of registers, often known as the program status word (PSW), that contains status information. The PSW typically contains condition codes plus other status information, such as an interrupt enable/disable bit and a kernel/user mode bit.
A program to be executed by a processor consists of a set of instructions stored in memory. In its simplest form, instruction processing consists of two steps: The processor reads (fetches) instructions from memory one at a time and executes each instruction. Program execution consists of repeating the process of instruction fetch and instruction execution. Instruction execution may involve several operations and depends on the nature of the instruction. The processing required for a single instruction is called an instruction cycle. Using a simplified two-step description, the instruction cycle is depicted in Figure 1.2. The two steps are referred to as the fetch stage and the execute stage. Program execution halts only if
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