Computers – A Gateway to Information

Memory

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Memory Memory consists of electronic components

that store instructions waiting to be executed by the processor, data needed by those instructions, and the results of processing the data

Stores three basic categories of items:

The operating system and other system software

Application programs

Memory addresses a collection of cells, each

with a unique physical/memory address

Each cell can hold one byte or 8 bits

101 102 103

201

301

202 203

302 303

With one by we can represent one character in ASCII CodeExample: “A” is 65 in ASCII code and 01000001 in binary representation

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Memory Each location in memory has an address Memory size is measured in kilobytes (KB or

K), megabytes (MB), gigabytes (GB), or terabytes (TB)

Page 223 Figure 4-17

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Memory Access time is the amount of time it takes the

processor to read from memory◦ Measured in nanoseconds

Page 229 Figures 4-24 – 4-25

MEMORY There are two kinds of computer memory: primary and

secondary.

Primary memory is accessible directly by the processing unit.

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Primary Memory RAM chips usually reside on a memory

module and are inserted into memory slots

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Memory The amount of RAM necessary in a computer

often depends on the types of software you plan to use

Page 226 Figure 4-21

Main Memory Holds instructions and data needed for

programs that are currently running

RAM is usually a volatile type of memory

Contents of RAM are lost when power is turned off

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Memory types- RAM RAM: random access memory

◦ SRAM: static RAM No need to be refreshed

◦ DRAM: dynamic RAM Need to be refreshed periodically Main memory

◦ SDRAM (Synchronous DRAM)◦ EDRAM (Enhanced DRAM)◦ EDO (Extended Data Out) ◦ FLASH RAM◦ Ferroelectric RAM

Primary Memory is referred to as random access memory (RAM) because it is possible to randomly select and use any location of the memory directly store and retrieve data. It takes same time to any address of the memory as the first address.

It is also called read/write memory

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The storage of data and instructions inside the primary storage is temporary.

It disappears from RAM as soon as the power to the

computer is switched off.

The memories, which loose their content on failure of power supply, are known as volatile memories

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Read Only Memory (ROM) There is another memory in computer, which is called

Read Only Memory (ROM). The storage of program and data in the ROM is permanent. The ROM stores some standard processing programs

supplied by the manufacturers to operate the computer. The ROM can only be read by the CPU but it cannot be

changed.

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The basic input/output program is stored in the ROM that examines and initializes various equipment attached to the PC when the switch is made ON.

The memories, which do not loose their content on failure of power supply, are known as non-volatile memories.

ROM is non-volatile memory.

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PROM is another type of primary memory in computer, which is called Programmable Read Only Memory (PROM).

You know that it is not possible to modify or erase programs stored in ROM, but it is possible for you to store your program in PROM chip. Once the programmes are written it cannot be changed and remain intact even if power is switched off.

Programs or instructions written in PROM or ROM cannot be erased or changed.

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EPROM: Erasable Programmable Read Only Memory, which over come the problem of PROM & ROM.

EPROM chip can be programmed time and again by erasing the information stored earlier in it.

When the EPROM is in use information can only be read.

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Cache Memory The speed of CPU is extremely high compared to the access

time of main memory. Therefore the performance of CPU decreases due to the slow

speed of main memory. Cache Memory is a small memory chip is attached between

CPU and Main memory whose access time is very close to the processing speed of CPU.

CACHE memories are accessed much faster than conventional RAM. It is used to store programs or data currently being executed or temporary data

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Memory Memory cache speeds the processes of the

computer because it stores frequently used instructions and data

auxiliary memory or secondary storage

1-Hard Disk, in computer science, one or more inflexible platters coated with material that allows the magnetic recording of computer data.

Hard disks provide faster access to data than floppy disks and are capable of storing much more information. Because platters are rigid, they can be stacked so that one hard-disk drive can access more than one platter.

Most hard disks have from two to eight platters.

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Magnetic Tape: Magnetic tapes are used for large computers like mainframe computers where large volume of data is stored for a longer time

Floppy Disk: It is similar to magnetic disk discussed above. They are 5.25 inch or 3.5 inch in diameter. They come in single or double density and recorded on one or both surface of the diskette. The capacity of a 5.25-inch floppy is 1.2 mega bytes whereas for 3.5 inch floppy it is 1.44 mega bytes.

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floppy disk * small magnetic data storage disk: a small flexible

magnetically coated disk in a rigid plastic case on which data can be stored or retrieved by a computer

* a regular floppy disk holds approximately 1.44 megabytes.

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CD- ROM CD-ROM, short for compact disc read-only memory,

a rigid plastic disk that stores a large amount of data through the use of laser optics technology.

Because they store data optically, CD-ROMs have a much higher memory capacity than computer disks that store data magnetically.

However, CD-ROM drives, the devices used to access information on CD-ROMs, can only read information from the disc, not write to it.

CD-ROMs can store large amounts of data and so are popular for storing databases and multimedia material. The most common format of CD-ROM holds approximately 630 megabytes .

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Secondary Storage A nonvolatile storage medium Contents retained while power is off Hard disk drives are most common

◦ Records data magnetically on a circular disk◦ Provides fast access to large amounts of data

Optical devices store data on CD’s as pits USB flash memory devices

◦ High capacity device plugs into USB port◦ Portable, reliable, and fits easily in a pocket

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Primary memory is more expensive than secondary memory.

The size of primary memory is less than that of secondary memory.

Primary memory is faster than secondary memory.

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Memory hierarchy

Speed SpaceCost

Secondary Memory

Computer Architecture

• Coordination of many levels of abstraction

• Under a rapidly changing set of forces• Design, Measurement, and Evaluation

I/O systemInstr. Set Proc.

Compiler

OperatingSystem

Application

Digital DesignCircuit Design

Instruction Set Architecture

Firmware

Datapath & Control

Layout