1.1 Introduction to Computer Systems
Computer systems consist of hardware, software two parts.
The hardware is visible touchable entity. Such as hard disk, video card, motherboard.
Software programs typically resided in various media, such as RAM, ROM, magnetic tape, optical disk, etc.
RAM:
Random Access Memory (English: Random Access Memory, abbreviation: RAM), also known as main memory, a CPU and an internal memory direct exchange of data. Temporary data storage medium which can be read at any time (except when a refresh), and fast, usually as the operating system or other programs that are running in the. Can be written at any time from any of a specified address when the RAM work (stored) or read (extracted) information. It ROM with the biggest difference is volatile data, data that is stored once the power outage will be lost. RAM for temporarily storing programs, data and intermediate results, and a digital computer system.
A ROM
a ROM read-only memory (Read-Only Memory) for short, is a solid state semiconductor read only memory data stored beforehand. Its characteristics is that once again the stored data can not be changed or deleted. Usually require frequent changes in the electronic data or computer systems, and data will not disappear because the power is turned off.
Computer software can be divided into two categories: system software and application software
program application it is for a specific task.
Hierarchy of a computer system
Can be obtained by abstract model, the bottom for the upper layer provides the interface to access help upper realize their functions.
From the perspective of the physical structure of the computer system may be divided into nine, computer composition principle this course, the main concern implementing logic layer and structural layer of a micro system.
From the software point of view of view of the hierarchy of a computer system
一些概念：
源程序：高级语言编写的程序。
目标程序：通过翻译程序将源程序翻译成的机器能识别的机器语言程序。
汇编语言：符号式的程序设计语言。用ADD,SUB,MUL,DIV等分别表示加减乘除等操作，并用符号表示指令或数据所在存储单元的位置。汇编语言通过汇编程序翻译为机器语言在实际机器上执行。
微程序：实际机器中的每一条机器指令翻译成一组微指令，微程序机器是对实际机器的分解。
- 五级计算机系统的层次结构
硬件研究的主要对象为实际机器和微程序机器，而软件的研究对象主要是操作系统以上的各级虚拟机。
区别计算机体系结构和计算机组成
计算机体系结构（简称结构）是指计算机概念性的结构与功能，是使用机器语言的程序员所见到的计算机系统的属性，比如：指令集、数据结构、存储器寻址技术、I/O机理等抽象属性，这些对于高级语言程序员是透明的。而正是这些属性定义了计算机软硬件的交界面，是上层软件编写时与硬件交互的接口。
计算机组成（简称组成）是指如何实现上面那些计算机体系结构体现的属性，这里面包含了很多对于程序员来说是透明的硬件细节。
以指令系统为例，指令系统体现了机器的属性，这是结构的问题；而如何取指令，分析指令，取操作数，运算，送结果…这些关于实现一条指令的具体细节的问题是组成方面的问题。两台计算机的指令系统相同，则结构相同，但是这些指令的实现方式完全可能不同，即它们的组成方式不同。
再具体一些，一台机器有没有乘法指令的功能是一个结构问题，而这个乘法指令是用乘法电路实现还是连续相加的加法电路实现就是一个组成问题了。
无论是过去还是现在，区分计算机组成和计算机结构这两个概念都是至关重要的。比如一堆体系结构相同的机器，它们的组成却有很大的差异，有些成本高性能高，有些成本低性能相对不好，这就能满足不同客户需求。同时一种机器的体系结构可能多年不变，但组成随着计算机技术的发展不断改变，在这样的情况下，并不影响原来开发的软件的正常使用。
PS:本笔记是高教出版社的计组的学习笔记，是博主自己对课文的整理和理解，当然里面也会包括一些课本原话啦
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