Introduction

the hardware, the operating system, the application programs,

and the users（四部分：硬件、操作系统、应用程序和用户）

 

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Chapter 1 Introduction

All forms of memory provide an array of bytes. Each byte has its

own address. Interaction is achieved through a sequence of load or store

instructions to specific memory addresses. The load instruction moves a byte

or word from main memory to an internal register within the CPU , whereas the

store instruction moves the content of a register to main memory. Aside from

explicit loads and stores, the CPU automatically loads instructions from main

memory for execution.

A typical instruction–execution cycle, as executed on a system with a von

Neumann architecture, first fetches an instruction from memory and stores

that instruction in the instruction register. The instruction is then decoded

and may cause operands to be fetched from memory and stored in some

internal register. After the instruction on the operands has been executed, the

result may be stored back in memory. Notice that the memory unit sees only

a stream of memory addresses. It does not know how they are generated (by

the instruction counter, indexing, indirection, literal addresses, or some other

means) or what they are for (instructions or data). Accordingly, we can ignore

how a memory address is generated by a program. We are interested only in

the sequence of memory addresses generated by the running program.

Ideally, we want the programs and data to reside in main memory

permanently. This arrangement usually is not possible for the following two

reasons:

1. Main memory is usually too small to store all needed programs and data

permanently.

2. Main memory is a volatile storage device that loses its contents when

power is turned off or otherwise lost.

Thus, 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.