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Memory The memory unit is an essential component in any

digital computer since it is needed for storing

programs and data CPU needs faster memory to process data.

Not all accumulated information is needed by theCPU at the same time

Therefore, it is more economical to use low-coststorage devices to serve as a backup for storing theinformation that is not currently used by CPU

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Computer Memory: types Primary Storage Secondary Storage

Evolution of Manand Storage

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Memory Types The memory unit that directly communicate with

CPU is called the main memory Devices that provide backup storage are called

auxiliary memory

The memory hierarchy system consists of allstorage devices employed in a computer system

from the slow by high-capacityauxiliarymemoryto a relatively faster main memory, to an evensmaller and faster cache memory

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Memory hierarchy

Register

Cache

Main Memory

Magnetic Disk

Magnetic Tape

Memory Hierarchy is to obtain the highest possible access speed whileminimizing the total cost of the memory system

Kilobyte

Megabyte

Gigabyte

Terabyte Petabyte

Exabyte

Zettabyte

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Primary Storage Primary storageor main memorystores three

types of information for very brief periods oftime:

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 computers operation. Primary storage takes place in chips mounted on

the computers main circuit board, called themotherboard.

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Main Types of Primary Storage Registers

Random access memory (RAM)

Cache memory Read-only memory (ROM)

Register RAMCache

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Cache memory If the active portions of the program and data

are placed in a fast small memory, the averagememory access time can be reduced,

Thus reducing the total execution time of theprogram

Such a fast small memory is referred to ascache memory

The cache is the fastest component in thememory hierarchy and approaches the speed ofCPU component

The typical access time ratio between cache and

main memory is about 1 to 7

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Cache memory Mapping The basic characteristic of cache memory is its fast access

time,

Therefore, very little or no time must be wasted when

searching the words in the cache The transformation of data from main memory to cache

memory is referred to as a mapping process, there arethree types of mapping: Associative mapping

Direct mapping

Set-associative mapping

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The operation of cache memory

1. Cache fetches data

from next to currentaddresses in mainmemory

2. CPU checks to seewhether the nextinstruction it requires is incache

3. If it is, then theinstruction is fetched from

the cache a very fastposition

4. If not, the CPU has to

fetch next instructionfrom main memory - amuch slower process

Main

Memory

(DRAM)

CPU

Cache

Memory

(SRAM)

= Bus connections

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HIT / MISS concept

The performance of cache memory is frequentlymeasured in terms of a quantity called hit ratio

When the CPU refers to memory and finds the word in

cache, it is said to produce a hit Otherwise, it is a miss

Hit ratio = hit / (hit+miss)

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Types of main memory

There are two types of main memory, Random Access Memory (RAM) and ReadOnly Memory (ROM)

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. Inmodern computers it also holds the operating system

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Random-Access Memory (RAM)

Static RAM (SRAM)

Each cell stores bit with a six-transistor circuit.

Retains value indefinitely, as long as it is kept powered.

Dynamic RAM (DRAM) Each cell stores bit with a capacitor and transistor.

Value must be refreshed every 10-100 ms.

Tran. Access

per bit time Persist? Sensitive? Cost Applications

SRAM 6 1X Yes No 100x cache memories

DRAM 1 10X No Yes 1X Main memories,frame buffers

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ROMROM 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

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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

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

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Secondary Storage

Memory capacity that can store very large amounts ofdata for extended periods of time.

Magnetic tape (sequential access)

Magnetic disks (direct access)

Secondary Storage:It is nonvolatile.

It takes much more time to retrieve data because of the electromechanical innature.It is cheaper than primary storage.Auxiliary memory access time is usually 1000 times that of main memory

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Magnetic Tape

Magnetic tape: A secondary storagemedium on a large open reel or in asmaller cartridge or cassette.

Sequential access: Data access in whichthe computer system must run throughdata in sequence in order to locate a

particular piece of data.

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Hard DriveHard drives: A form of secondarystorage that stores data on plattersdivided into concentric tracks andsectors, which can be read by aread/write head that pivots across therotating disks.

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Optical Storage Devices

Compact Disk, Read-Only Memory (CD-ROM)

Digital Video Disk (DVD)

Blu-ray disc

VS

The Winner

Direct access: Data access in which any piece of data be retrieved in a nonsequential manner by locating it using the datas address.

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Uses backing storage e.g. hard disk as a temporary location for programsand data where insufficient RAM available

Swaps programs and data between the hard-disk and RAM as the CPUrequires 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 inmilliseconds (Note: a millisecond is a thousandth of a second)

Virtual memory is much slower than RAM

Virtual memory

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TERMS : Memory

Main memory consists of a number ofstorage locations, each of which isidentified by a unique address

The ability of the CPU to identifyeach location is known as itsaddressability

Each location stores a word i.e. thenumber of bits that can be processedby the CPU in a single operation.Word length may be typically 16, 24,32 or as many as 64 bits.

The access time = seek time + transfer timeSeek time: required to position the read-write head to a location

Transfer time: required to transfer data to or from the device

The average time required toreach a storage location inmemory and obtain its contents

is called the access time

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