Chapter 21 bus connection
Computer processor is undoubtedly the most important part, but not the only part. A computer also require a random access memory (RAM) to store the machine code instructions for the processor to perform. The computer must have some way to make these instructions into the RAM (input devices), and some of the ways to see the results of program execution (the output device). Recall, RAM is volatile, when power is removed, its contents will be lost. Another useful so the computer means is a permanent storage device, when the computer is powered off, they can store the code and data.
Composition of a complete computer all integrated circuits must be mounted on the circuit board. In some smaller machines, all of the integrated circuits may be mounted on a board, but more commonly, a different member mounted separately in two or more plate, communicate with each other through the bus plates. Briefly, a bus is a set of digital signals supplied to each of the circuit boards of the computer , the signals can be divided into four categories:
• address signal . These signals are provided by the microprocessor, used to address RAM cell, it can also be used to address counter connected to
other components on the computer.
• data output signal . Also provided by the microprocessor, or other device used to write data to the RAM. Careful scrutiny to lose
the meaning of the (input) and output (output) of. The data output signal is an output from the microprocessor, and RAM into which
the data input signal his device.
• data input signal . Is provided by the rest of the computer, the signal is read by the microprocessor. The data input signal via
normally output from the RAM, i.e., indicates that the microprocessor reads the memory contents. However, other components may also provide the data
input signal to the processor.
• control signal . Composition by a variety of signals, the control signals are generally consistent with the specific processor computer. A control
signal from the microprocessor, or may be transferred from the microprocessor to other components. For example, the microprocessor can be used to control a channel
number to indicate that some data to write to a memory address.
The need to emphasize the three-state output concept, since it crucial to the operation of the bus. All chips connected to the bus
can use the data input signals on the bus to the processor data transfer. At any time, many of the circuit board is connected to the bus
, only one input is used to determine what the data signals on the bus Yes. Other circuit board is not selected, the output of the third state.
SRAM与DRAM的区别：R A M芯片如2 1 0 2就会保持已存储的内容；如果断电，则内容会丢失。在这方面， D R A M也是如此。但D R A M还需要周期性地对存储器进行访问，即使这些内容是不需要的。这称之为刷新， 1秒钟内含有好几百次刷新（当然现在比这更快），就好像隔一段时间就推一下某个人使他不要入睡一样
本章最后介绍的外围设备是长期存储设备。前面讲过，随机访问存储器—不论是用继电
器、电子管还是晶体管构成的—在当电源关闭时，内容会丢失。所以，一个完整的计算机
也需要长期存储器。长久以来使用的方法是在纸上或卡片上打孔，就像I B M的穿孔卡片。在
早期小型计算机中，是通过在滚动的纸带上打孔来保存程序和数据的这便于以后重新装入到
内存中。但是，穿孔卡片和纸带存在一个问题，即介质不能重复使用，当打上一个孔后就不
容易再恢复。另一个问题是效率低，就当时来说，如果想要真切的看到某一比特，可能要花
费太多空间。所以，现在最流行的长期存储器类型是磁介质存储器，磁带开始比较流行，但是，因为不能很快地移动到磁带上的任一点进行
访问，频繁的前进和倒回要花费很多时间，所以其并不是理想的存储介质，磁盘至此随机访问，是比较良好的介质。
第22章 操作系统 第23章 定点数和浮点数 第24章高级语言和低级语言 第25章图形化革命
22章上学期刚学过，略过未看
23，24章很简单
25章当小说读读，感觉没什么感想..