问题 单项选择题

下面关于8237可编程DMA控制器的叙述中,错误的是

A.8237必须初始化,否则不能进行DMA操作

B.对8237编程时,IOR和IOW是8237的输入信号:当8237控制总线时,它们是 8237的输出信号

C.8237只有一条DMA请求线

D.8237用DACK信号作为对DREQ的响应,因此在某通道的DACK信号有效之前,同一通道的DREQ信号必须维持有效

答案

参考答案:C

解析: 本题考查8237可编程DMA控制器的相关概念。DMA(Direct Memory Access,直接存储访问)是一种完全由DMA控制器执行I/O交换的工作方式。8237具有4各用于连接I/O设备进行数据传送的通道,当I/O设备向某通道发出DMA请求时,该通道即可通过8237进行DMA数据传送,选项C说法错误。正确答案为选项C。

阅读理解

阅读理解

       Hundreds of years ago, life was much harder than it is today. There was no modern medicine. 

       Life today has brought new problems. One of the biggest is pollution. Water pollution has made our

rivers and lakes dirty. It kills fish and pollutes our drinking water. Noise pollution makes us talk louder and become angry more easily. Air pollution is the most serious kind of pollution: It's bad to all living things in

the world.

       Cars, planes and factories all pollute our air every day. Sometimes the polluted air is so thick that it is

like a quilt over a city. This kind of quilt is called smog(烟雾).

       Many countries are making rules to fight pollution. Factories must now clean their water before it is

poured into rivers; they mustn't blow dirty smoke into the air.

       We need to do many other things. We can put waste things in the dustbin instead of throwing it on the ground. We can go to work by bus or with our friends in the same car. If there are fewer people driving,

there will be less pollution. Rules are not enough. Every person must help to fight pollution.

1. Hundreds of years ago, life was much harder than it is today because____.     

A. there were not any modern machines     

B. there was no pollution     

C. both A and B     

D. there were not many people   

2. What is the biggest problem in today's life?     

A. Water pollution.    

B. Air pollution.     

C. Noise.            

D. Pollution.  

3. The most serious kind of pollution is___       .   

A. noise pollution    

B. air pollution     

C. water pollution    

D. A, and C

4. Factories must clean their water____           

A. before they are poured into rivers     

B. when they are poured into rivers     

C. after it is poured into rivers     

D. before it is poured into rivers   

5. From the passage we know that________    

A. a few years ago, there was no smog at all     

B. today people don't have to talk to each other in a loud voice     

C. We can drink water from the polluted rivers and lakes     

D. people are making rules in order to fight pollution

单项选择题