Computer Memory

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Computer Memory:

Computer Memory Somnath Chaudhuri 1 9 th April’2011

Introduction:

Introduction Hardware refers to the physical equipment used for the input, processing, output and storage activities of a computer system. Central processing unit (CPU) manipulates the data and controls the tasks performed by the other components. Primary storage internal to the CPU; temporarily stores data and program instructions during processing. 2

How the CPU Works:

How the CPU Works 3

How the CPU Works (Continued):

How the CPU Works (Continued) Binary form: The form in which data and instructions can be read by the CPU – only 0s and 1s. Machine instruction cycle: The cycle of computer processing, whose speed is measured in terms of the number of instructions a chip processes per second. Clock speed: The preset speed of the computer clock that times all chip activities, measured in megahertz and gigahertz. Word length: The number of bits (0s and 1s) that can be processed by the CPU at any one time. Bus width: The size of the physical paths down which the data and instructions travel as electrical impulses on a computer chip. 4

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What is Memory Main memory consists of a number of storage locations, each of which is identified by a unique address The ability of the CPU to identify each location is known as its addressability Each location stores a word i.e. the number of bits that can be processed by the CPU in a single operation. Word length may be typically 16, 24, 32 or as many as 64 bits. A large word length improves system performance , though may be less efficient on occasions when the full word length is not used 5

Computer Memory :

Computer Memory Two basic categories of computer memory: Primary storage and secondary storage . Primary stores small amounts of data and information that will be immediately used by the CPU. Secondary stores much larger amounts of data and information (an entire software program, for example) for extended periods of time. 6

Primary Storage:

Primary Storage Primary storage or main memory stores three types of information for very brief periods of time: Data to be processed by the CPU; Instructions for the CPU as to how to process the data; Operating system programs that manage various aspects of the computer’s operation. Primary storage takes place in chips mounted on the computer’s main circuit board, called the motherboard . Four main types of primary storage: register, random access memory (RAM), cache memory and read-only memory (ROM). 7

Types of Primary Storage:

Types of Primary Storage Registers: registers are part of the CPU with the least capacity, storing extremely limited amounts of instructions and data only immediately before and after processing. Random access memory (RAM): The part of primary storage that holds a software program and small amounts of data when they are brought from secondary storage. Cache memory: A type of primary storage where the computer can temporarily store blocks of data used more often. 8

Types Primary Storage (Continued):

Types Primary Storage (Continued) Read-only memory (ROM): Type of primary storage where certain critical instructions are safeguarded; the storage is nonvolatile and retains the instructions when the power to the computer is turned off. Flash memory: A form of rewritable read-only memory that is compact, portable, and requires little energy. 9

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Random Access Memory (RAM) holds its data as long as the computer is switched on All data in RAM is lost when the computer is switched off Described as being volatile It is direct access as it can be both written to or read from in any order Its purpose is to temporarily hold programs and data for processing. In modern computers it also holds the operating system 10

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Types of RAM 1. Dynamic Random Access Memory (DRAM) Contents are constantly refreshed 1000 times per second Access time 60 – 70 nanoseconds 2. Synchronous Dynamic Random Access Memory (SDRAM) Quicker than DRAM Access time less than 60 nanoseconds 3. Direct Rambus Dynamic Random Access Memory (DRDRAM) New type of RAM architecture Access time 20 times faster than DRAM More expensive 11

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4. Static Random Access Memory (SRAM) Doesn’t need refreshing Retains contents as long as power applied to the chip Access time around 10 nanoseconds Used for cache memory Also for date and time settings as powered by small battery Types of RAM 5. Video Random Access memory Holds data to be displayed on computer screen Has two data paths allowing READ and WRITE to occur at the same time A system’s amount of VRAM relates to the number of colours and resolution A graphics card may have its own VRAM chip on board 12

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Read only memory (ROM) ROM holds programs and data permanently even when computer is switched off Data can be read by the CPU in any order so ROM is also direct access The contents of ROM are fixed at the time of manufacture Stores a program called the bootstrap loader that helps start up the computer Access time of between 10 and 50 nanoseconds 13

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Types of ROM 1. Programmable Read Only Memory (PROM) Empty of data when manufactured May be permanently programmed by the user 2. Erasable Programmable Read Only Memory (EPROM) Can be programmed, erased and reprogrammed The EPROM chip has a small window on top allowing it to be erased by shining ultra-violet light on it After reprogramming the window is covered to prevent new contents being erased Access time is around 45 – 90 nanoseconds 14

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Types of ROM 3. Electrically Erasable Programmable Read Only Memory (EEPROM) Reprogrammed electrically without using ultraviolet light Must be removed from the computer and placed in a special machine to do this Access times between 45 and 200 nanoseconds 4. Flash ROM Similar to EEPROM However, can be reprogrammed while still in the computer Easier to upgrade programs stored in Flash ROM Used to store programs in devices e.g. modems Access time is around 45 – 90 nanoseconds 5. ROM cartridges Commonly used in games machines Prevents software from being easily copied 15

Caches:

Caches The rate of data fetching by the CPU from the main memory is about 100 times faster than from secondary memory. But there is also a mismatch between main memory and CPU. CPU can process the data 10 times faster than the main memory. Which limits the performance of the CPU due to mismatch in CPU and main memory speed. So Cache memory act as a buffer b/w main memory and CPU. Cache: A smaller, high speed storage device used to increase the speed of processing by making current programs and data available to the CPU at a rapid rate. The basic characteristic of cache memory is its fast access time. 16

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Small amount of memory typically 256 or 512 kilobytes Temporary store for often used instructions Level 1 cache is built within the CPU (internal) Level 2 cache may be on chip or nearby (external) Faster for CPU to access than main memory Cache memory 17

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The operation of cache memory 1. Cache fetches data from next to current addresses in main memory 2. CPU checks to see whether the next instruction it requires is in cache 3. If it is, then the instruction is fetched from the cache – a very fast position 4. If not, the CPU has to fetch next instruction from main memory - a much slower process Main Memory (DRAM) CPU Cache Memory (SRAM) = Bus connections 18

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Virtual memory Uses backing storage e.g. hard disk as a temporary location for programs and data where insufficient RAM available Swaps programs and data between the hard-disk and RAM as the CPU requires them for processing A cheap method of running large or many programs on a computer system Cost is speed: the CPU can access RAM in nanoseconds but hard-disk in milliseconds Virtual memory is much slower than RAM 19

Secondary Storage:

Secondary Storage Memory capacity that can store very large amounts of data for extended periods of time. It is nonvolatile. It takes much more time to retrieve data because of the electromechanical nature. It is cheaper than primary storage. It can take place on a variety of media 20

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Secondary storage Sequential access Direct access Magnetic tape Magnetic disk Optical disk Floppy disk Hard disk CD-ROM WORM 21

Secondary Storage (Continued):

Secondary Storage (Continued) Hard disk: A form of secondary storage that stores data on platters divided into concentric tracks and sectors, which can be read by a read/write head that pivots across the rotating disks. Floppy disk: A form of easily portable secondary storage on flexible disks; also called floppy disks.

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Optical storage devices: A form of secondary storage in which a laser reads the surface of a reflective plastic platter. Compact disk, read-only memory (CD-ROM): A form of secondary storage that can be only read and not written on. Digital video disk (DVD): An optical storage device used to store digital video or computer data. Fluorescent multilayer disk (FMD-ROM): An optical storage device with much greater storage capacity than DVDs. Secondary Storage (Continued)

Memory Hierarchy:

Memory Hierarchy 24

An Example Memory Hierarchy:

An Example Memory Hierarchy registers on-chip L1 cache (SRAM) main memory (DRAM) local secondary storage (local disks) Larger, slower, and cheaper (per byte) storage devices remote secondary storage (distributed file systems, Web servers) Local disks hold files retrieved from disks on remote network servers. Main memory holds disk blocks retrieved from local disks. off-chip L2 cache (SRAM) L1 cache holds cache lines retrieved from the L2 cache memory. CPU registers hold words retrieved from L1 cache. L2 cache holds cache lines retrieved from main memory. L0: L1: L2: L3: L4: L5: Smaller, faster, and costlier (per byte) storage devices 25

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