1、Lesson 2 Computer Memory教学课件教学课件 Memory is something that stores，preserves and recalls data when needed.Your brain has this capability and memory inside computers is an electronic incarnation of this concept.1 Computer memory is used to store data that needs to be access by the Central Processing Un
2、it（CPU）.It is the CPU that performs the laborious tasks，the memory acts as storage for uncompleted tasks and other relevant information needed to accomplish those tasks.All the information in memory is encoded in fixed size cells called bytes.2 A byte can hold a small amount of information，such as a
3、 single character or a numeric value between 0 and 255.The CPU will perform its operations on groups of one，two，four，or eight bytes，depending on the interpretation being placed on the data，and the operations required.Forms of Memory Although the term“computer memory”is commonly used to refer to RAM，
4、there are various other forms of memory inside a computer such as the hard disk drive.The illustration below outlines the common memory architecture adopted within most modern computers.backback教学课件教学课件 Any of the four major categories of memory in the diagram above can feed information directly to
5、the CPU.Each form of memory feeds the CPU at differing speeds and efficiency due to their different technological make up.3backback教学课件教学课件 Registers and cache will transfer data to the CPU at greater speeds than RAM and hard disk drives.As the forms of memory in the top of the pyramid are costly to
6、 make，their size are limited to make computers affordable.The size of memory forms towards the bottom of the pyramid is made larger to hold all the other data that other forms cannot handle.4 It should also be noted here that physical RAM and cache are volatile in nature meaning they store，preserve
7、and recall data so long as there is electrical power flowing through the system.Once a computer system is shut off，the physical RAM and cache are cleared.Disc storage by way of floppy disk，hard disk，CD-ROM and DVD-ROM drives holds information those are required to be non-volatile in nature（such as t
8、hat project you have been working on!）.There is a trade-off between speed and cost，resulting in the development of such a pyramid-like architecture.Information is prioritized in terms of importance and stability to determine which form of memory would hold the data.5backback教学课件教学课件 Registers The bo
9、ttleneck in a memory and CPU architecture is the slow transfer speeds between the two.The fastest，and sadly the most expensive，form of memory resolves this problem by having the memory within the CPU itself.Data within registers are instantly fed to the Arithmetic and Logic Unit（ALU）portion of the C
10、PU making the relevant data instantly available.Registers are typically small in size and is controlled by the CPUs compiler.Cache This form of memory can be considered as an intermediary between the main physical RAM and the CPU.The cache makes any data frequently used by CPU instantly available.If
11、 the required information is not located in the cache，a fetch is made from the main memory.There are two levels of cache：level 1 cache（primary cache）and level 2 cache（secondary cache）.backback教学课件教学课件 Level 1 cache is built directly on the CPU，just like the registers.It is small in size，ranging anyw
12、here between 2 kilobytes（KB）and 128KB.As this cache is closer to the CPU than level 2 cache，its transfer speeds are faster as a result.Level 2 cache is usually situated in close proximity to，but off，the CPU chip.6 However，there are certain systems where the cache is built onto the CPU as like the le
13、vel 1 cache.The size of level 2 cache ranges from 256KB to 2 megabytes（MB）.Both levels of cache use Static Random Access Memory（SRAM）to hold the data.Main Memory This is where most of the information that a CPU requires resides.“Main Memory”commonly refers to Physical Memory，although a computer uses
14、 an operating systemimposed Virtual Memory in addition to physical memory.The amount of main memory on a computer is crucial because it determines how many programs can be executed at one time and how much data can be readily available to a program.backback教学课件教学课件 Physical memory uses Dynamic Rando
15、m Access Memory（DRAM）to store the data，and is considerably slower than the SRAM used by the cache.Information is exchanged between the main memory and the cache to ensure that the more commonly accessed information is placed in the cache to allow faster access speed.Operating systems memory manageme
16、nt will automatically remove data held on both the physical and virtual memory.The physical memory acts as an Input/Output（I/O）channel for data exchanged between the computer memory and other forms of electronic storages.Virtual Memory Most operating systems have a form of memory management that cat
17、ers for memory needs beyond a computer systems physical memory through the use of a swap file.7 There is a need for such memory management as operating systems themselves occupy a significant portion of physical memory.A swap file is a file located on a computers hard disk drive（HDD）that acts as an
18、extension to physical memory.However，the HDD has much slower access times than any of the forms of memory discussed above.Hence，information is swapped between the main memory and the swap file to ensure that the more frequently used information is located in the main memory for faster access speeds.
19、backback教学课件教学课件Key Words&Termsanywhere between 数目在之间architecture 体系结构bottle-neck瓶颈cache缓冲存储器compiler编译器considerably 相当地disc storage磁盘存储器encode编码feed to供应给hierarchy层级incarnation 化身intermediary调节者，中介laborious费力的，艰苦的backback教学课件教学课件Key Words&Termslevel 1 cache一级缓存level 2 cache二级缓存numeric数值的physical me
20、mory物理存储器prioritize把区分优先顺序proximity接近swap file交换文件trade-off平衡，协定transfer speed传输速度virtual memory虚拟存储器volatile易失性的backback教学课件教学课件AbbreviationsDRAM（Dynamic Random Access Memory）动态随机存储器HDD（Hard Disk Drive）硬磁盘驱动器I/O（Input/Output）输入/输出（设备，数据）SRAM（Static Random Access Memory）静态随机存储器backback教学课件教学课件Notes
21、1 Your brain has this capability and memory inside computers is an electronic incarnation of this concept.该句是由and连接的两个并列句，其中的this concept指代上文大脑能存储和回忆的现象。译文：你的大脑有这种能力，而计算机内部的存储器正是对这一概念的电子化身。2 All the information in memory is encoded in fixed size cells called bytes.本句中的called bytes修饰cells。译文：存储器上的所有信
22、息都以固定大小的单元为单位进行编码，这样的单元被称为字节。3 Each form of memory feeds the CPU at differing speeds and efficiency due to their different technological make up.该句中的atspeed意为“以速度”；due to译为“由于，因为”；make up本身是动词词组，在这里当名词来用，意为“制作”。译文：每种存储器向CPU提供信息的速度和效率都不一样，这是由存储器不同的制作技术决定的。backback教学课件教学课件Notes 4 The size of memory fo
23、rms towards the bottom of the pyramid is made larger to hold all the other data that other forms cannot handle.该句中的to hold all the other data that other forms cannot handle作目的状语，其中，that other forms cannot handle又是一个定语从句，修饰data。译文：越往金字塔底部去，存储器尺寸做得越大，目的是保存其他存储形式不能处理的数据。5 Information is prioritized in
24、terms of importance and stability to determine which form of memory would hold the data.该句中的prioritized意为“把区分优先顺序”；in terms of意为“按照，根据”；data意为“数据”，是复数名词形式，其单数为datum。译文：信息按照重要性和稳定性被划分为不同的优先级，这样做的目的是决定用哪种存储器保存这些数据。backback教学课件教学课件Notes 6 Level 2 cache is usually situated in close proximity to，but off，
25、the CPU chip.该句中的proximity意为“接近”；句中but off是对proximity to的补充，翻译为“但有一定距离”。译文：二级缓存通常位于非常靠近CPU芯片的位置，但在CPU芯片的外部。7 Most operating systems have a form of memory management that caters for memory needs beyond a computer systems physical memory through the use of a swap file.该句中的swap file意为“交换文件”；cater for意为
26、“迎合”；句中that caters for memory needs beyond a computer systems physical memory through the use of a swap file是一个定语从句，作a form of memory management的定语；through the use of a swap file在从句中作状语，修饰caters for。译文：大部分操作系统都有一种内存管理的形式，即通过交换文件的方式满足对计算机系统物理存储器之外的存储器的访问需求。backbackExercisesTranslate the following phr
27、ases into English.1静态随机存储器2动态随机存储器3虚拟存储器4物理存储器5一级缓存6二级缓存7硬盘驱动器访问速度backbackExercisesTranslate the following sentences into Chinese.1DRAM gets the“dynamic”in its name because it is refreshed thousands of times per second.2RAM：A temporary storage memory area in computer where the operating system，appli
28、cation programs，and data in current use are kept.3ROM is sustained by a small longlife battery in your computer.4RAM cache：is a portion of memory made of high-speed static RAM.backbackExercisesIdentify the following to be true or false according to the text.1The speed of RAM is higher than the speed
29、 of cache.2Level 2 cache is built in the CPU.3The CPU can perform operations on group of ten bytes.4The capacity of Disc storage is usually larger than that of cache.Filling in each of the following blanks according to the text.1Disc storage holds information those are required to be in nature.2Regi
30、sters are typically small in size and is controlled by the CPUs .3RAM and cache are in naturemeaning they store，preserve and recall data so long as there is electrical power flowing through the system.4Physical memory uses to store the data，and is considerably slower than the SRAM used by the cache.backback
