1、Lesson 2 Computer Memory教學課件 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 Unit

2、 （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 a

3、s a 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 re

4、fer to RAM， 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. back教學課件 Any of the four major categories of memory in the diagram above can feed information direct

5、ly to the CPU. Each form of memory feeds the CPU at differing speeds and efficiency due to their different technological make up.3back教學課件 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， preser

7、ve 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 （

8、such as that 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. 5back教學課件 Registers T

9、he bottleneck 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） porti

10、on of the CPU 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 instantl

11、y available. If 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）.back教學課件 Level 1 cache is built directly on the CPU， just like the registers. It is small in si

12、ze， ranging anywhere 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 ont

13、o the CPU as like the level 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

14、 Memory， although a computer uses 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.back教學課件 Physical

15、 memory uses Dynamic Random 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. Op

16、erating systems memory management 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

17、 form of memory management that caters 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

18、hard disk drive （HDD） that acts as an 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

19、 the main memory for faster access speeds.back教學課件Key Words & Termsanywhere between 數目在之間architecture 體系結構bottle-neck瓶頸cache緩沖存儲器compiler編譯器considerably 相當地disc storage磁盤存儲器encode編碼feed to供應給hierarchy層級incarnation 化身intermediary調節者，中介laborious費力的，艱苦的back教學課件Key Words & Termslevel 1 cache一級緩存level 2

20、cache二級緩存numeric數值的physical memory物理存儲器prioritize把區分優先順序proximity接近swap file交換文件trade-off平衡，協定transfer speed傳輸速度virtual memory虛擬存儲器volatile易失性的back教學課件AbbreviationsDRAM（Dynamic Random Access Memory） 動態隨機存儲器HDD（Hard Disk Drive） 硬磁盤驅動器I/O（Input/Output） 輸入/輸出（設備，數據）SRAM（Static Random Access Memory） 靜態隨

21、機存儲器back教學課件Notes 1 Your brain has this capability and memory inside computers is an electronic incarnation of this concept. 該句是由and連接的兩個并列句，其中的this concept指代上文大腦能存儲和回憶的現象。 譯文：你的大腦有這種能力，而計算機內部的存儲器正是對這一概念的電子化身。 2 All the information in memory is encoded in fixed size cells called bytes. 本句中的called by

22、tes修飾cells。 譯文：存儲器上的所有信息都以固定大小的單元為單位進行編碼，這樣的單元被稱為字節。 3 Each form of memory feeds the CPU at differing speeds and efficiency due to their different technological make up. 該句中的atspeed意為“以速度”；due to譯為“由于，因為”；make up本身是動詞詞組，在這里當名詞來用，意為“制作”。 譯文：每種存儲器向CPU提供信息的速度和效率都不一樣，這是由存儲器不同的制作技術決定的。back教學課件Notes 4 The

23、 size of memory forms towards the bottom of the pyramid is made larger to hold all the other data that other forms cannot handle. 該句中的to hold all the other data that other forms cannot handle作目的狀語，其中，that other forms cannot handle又是一個定語從句，修飾data。 譯文：越往金字塔底部去，存儲器尺寸做得越大，目的是保存其他存儲形式不能處理的數據。 5 Informati

24、on is prioritized in terms of importance and stability to determine which form of memory would hold the data. 該句中的prioritized意為“把區分優先順序”；in terms of意為“按照，根據”；data意為“數據”，是復數名詞形式，其單數為datum。 譯文：信息按照重要性和穩定性被劃分為不同的優先級，這樣做的目的是決定用哪種存儲器保存這些數據。back教學課件Notes 6 Level 2 cache is usually situated in close proxim

25、ity to， but off， the CPU chip. 該句中的proximity意為“接近”；句中but off是對proximity to的補充，翻譯為“但有一定距離”。 譯文：二級緩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. 該句中的swap fil

26、e意為“交換文件”；cater for意為“迎合”；句中that caters for memory needs beyond a computer systems physical memory through the use of a swap file是一個定語從句，作a form of memory management的定語；through the use of a swap file在從句中作狀語，修飾caters for。 譯文：大部分操作系統都有一種內存管理的形式，即通過交換文件的方式滿足對計算機系統物理存儲器之外的存儲器的訪問需求。backExercisesTranslate

27、 the following phrases into English.1靜態隨機存儲器2動態隨機存儲器3虛擬存儲器4物理存儲器5一級緩存6二級緩存7硬盤驅動器訪問速度backExercisesTranslate 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， applica