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Computer Memory Computer Memory System OverviewSystem Overview

ObjectivesObjectives

Discuss the overview of the memory Discuss the overview of the memory elements of a computerelements of a computer

Describe the characteristics of the Describe the characteristics of the computer memory systemcomputer memory system

Understand the heirarchy of the computer Understand the heirarchy of the computer memorymemory

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CharacteristicsCharacteristics LocationLocation CapacityCapacity Unit of transferUnit of transfer Access methodAccess method PerformancePerformance Physical typePhysical type Physical characteristicsPhysical characteristics OrganisationOrganisation

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LocationLocation Internal memory is often equated with main Internal memory is often equated with main

memory.memory. The processor requires its own local memory The processor requires its own local memory

in the form of registers e.g. PC,IR,ID…..in the form of registers e.g. PC,IR,ID….. The control unit portion of the processor may The control unit portion of the processor may

also require its own internal memory.also require its own internal memory. Cache is another form of internal memoryCache is another form of internal memory External memory consists of peripheral External memory consists of peripheral

storage devices e.g. disk, tape, that are storage devices e.g. disk, tape, that are accessible to the processor via I/O cntrller.accessible to the processor via I/O cntrller.

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CapacityCapacity

Word sizeWord size The natural unit of organisationThe natural unit of organisation Common word lengths are 8, 16 and 32 bits.Common word lengths are 8, 16 and 32 bits. External memory capacity is typically External memory capacity is typically

expressed in terms of bytes (1GB, 20MB)expressed in terms of bytes (1GB, 20MB)

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Unit of TransferUnit of Transfer Internal memoryInternal memory

The unit of transfer is equal to the number of data The unit of transfer is equal to the number of data lines of the memory module.lines of the memory module.

Equal to the word length but is often larger, such as Equal to the word length but is often larger, such as 64,128 or 256 bits.64,128 or 256 bits.

Related concepts : word, addressable unit, unit of Related concepts : word, addressable unit, unit of transfertransfer

External memoryExternal memory Usually a block which is much larger than a word and Usually a block which is much larger than a word and

these are referred as blocks.these are referred as blocks. Addressable unitAddressable unit

Smallest location which can be uniquely addressedSmallest location which can be uniquely addressed Word internallyWord internally

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Access Methods (1)Access Methods (1) SequentialSequential

Start at the beginning and read through in orderStart at the beginning and read through in order Access time depends on location of data and previous Access time depends on location of data and previous

locationlocation e.g. tapee.g. tape

DirectDirect Individual blocks have unique addressIndividual blocks have unique address Access is by jumping to vicinity plus sequential search Access is by jumping to vicinity plus sequential search

(pointer)(pointer) Access time depends on location and previous locationAccess time depends on location and previous location e.g. diske.g. disk

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Access Methods (2)Access Methods (2) RandomRandom

Individual addresses identify locations exactlyIndividual addresses identify locations exactly Access time is independent of location or previous Access time is independent of location or previous

accessaccess e.g. RAM, cachee.g. RAM, cache

AssociativeAssociative Data is located by a comparison with contents of a Data is located by a comparison with contents of a

portion of the storeportion of the store Access time is independent of location or previous Access time is independent of location or previous

accessaccess e.g. cachee.g. cache

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PerformancePerformance Access timeAccess time – for random-access, this is the time it takes – for random-access, this is the time it takes

to perform a read or write operation, that is, the time from to perform a read or write operation, that is, the time from the instant that an address is presented to the memory to the instant that an address is presented to the memory to the instant that data have been stored or made available the instant that data have been stored or made available for use. For non-random-access is the time for the for use. For non-random-access is the time for the read/write mechanism to position at the desired locationread/write mechanism to position at the desired location

Memory cycle timeMemory cycle time – applied to random-access, consists – applied to random-access, consists of the access time plus any additional time required of the access time plus any additional time required before a second access can commence. This additional before a second access can commence. This additional time may be required for transients to die out on signal time may be required for transients to die out on signal lines or to regenerate data if they are read destructively.lines or to regenerate data if they are read destructively.

Transfer rateTransfer rate – this is the rate that data can be – this is the rate that data can be transferred into or out of a memory unit. transferred into or out of a memory unit.

Transfer rate calculationTransfer rate calculation

For random-access :For random-access : Transfer is based in 1 cycle time (clock period)Transfer is based in 1 cycle time (clock period)

For non-random-access :For non-random-access : TTnn = T = TAA + N/R + N/R

Where :Where : Tn = average time to read/write N bits;Tn = average time to read/write N bits; TA = average access time;TA = average access time; N = Number of bits;N = Number of bits; R = Transfer rate (bps)R = Transfer rate (bps) 1010

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Physical Types of memoryPhysical Types of memory

SemiconductorSemiconductor RAMRAM

MagneticMagnetic Disk & TapeDisk & Tape

OpticalOptical CD & DVDCD & DVD

OthersOthers BubbleBubble HologramHologram

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Physical CharacteristicsPhysical Characteristics VolatileVolatile – information decays naturally or is lost when – information decays naturally or is lost when

electrical power is switched off. electrical power is switched off. NonvolatileNonvolatile – information once recorded remains – information once recorded remains

without deterioration until deliberately changed and no without deterioration until deliberately changed and no electrical electric power is needed to retain information. electrical electric power is needed to retain information. e.g magnetic-surface memory. e.g magnetic-surface memory.

NonerasableNonerasable – cannot be altered, except by – cannot be altered, except by destroying the storage unit. E.g. ROM. destroying the storage unit. E.g. ROM.

Power consumptionPower consumption

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Cost, capacity and access timeCost, capacity and access time

There is a trade-off among the three key There is a trade-off among the three key characteristics of memory (cost, capacity characteristics of memory (cost, capacity and access time)and access time) Faster access time, greater cost per bitFaster access time, greater cost per bit Greater capacity, smaller cost per bitGreater capacity, smaller cost per bit Greater capacity, slower access timeGreater capacity, slower access time

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Memory Hierarchy - DiagramMemory Hierarchy - Diagram

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Memory HierarchyMemory Hierarchy

RegistersRegisters In CPUIn CPU

Internal or Main memoryInternal or Main memory May include one or more levels of cacheMay include one or more levels of cache ““RAM”RAM”

External memoryExternal memory Backing storeBacking store

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Cont..Cont..

Based on figure above, as one goes down Based on figure above, as one goes down the hierarchy the following occurthe hierarchy the following occur

Decreasing cost per bitDecreasing cost per bit Increasing capacityIncreasing capacity Increasing Access timeIncreasing Access time Decreasing the frequency of access of the Decreasing the frequency of access of the memory by the processormemory by the processor

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Memory OrganisationMemory Organisation

Smaller, more expensive, faster memories Smaller, more expensive, faster memories are supplemented by larger, cheaper, slower are supplemented by larger, cheaper, slower memories. The key to the success of this memories. The key to the success of this organisation is organisation is decreasing frequency of decreasing frequency of access.access. This concept is explain detail in This concept is explain detail in discussing on discussing on cache memory ( Part 2 )cache memory ( Part 2 ) virtual memoryvirtual memory

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Hierarchy ListHierarchy List

RegistersRegisters L1 CacheL1 Cache L2 CacheL2 Cache Main memoryMain memory Disk cacheDisk cache DiskDisk OpticalOptical TapeTape