Lecture 4 -Computer Memory and Memory Types

8/3/2019 Lecture 4 -Computer Memory and Memory Types

1/55

Computer Memory and MemoryTypes

Lecture 4


2/55


3/55


4/55


5/55

Figure 12-4 presents a situation where data

is being read from memory device 1 whilememory device 2 remains "disconnected"from the bus. Disconnected is in quotesbecause the physical connection is still

'connection across which data can pass.


6/55


7/55

Notice that Figure 12-4 shows that theonly lines disconnected from the bus arethe data lines. This is because buscontention only occurs when multiple

ev ces are ry ng o ou pu o e samelines at the same time. Since only themicroprocessor outputs to the address

and control lines, they can remainconnected.


8/55

In order for this scheme to work, anadditional control signal must be sent toeach of the memory devices telling themwhen to be connected to the bus andwhen to be disconnected. This controlsignal is called a chip select.


9/55

A chip select is an active low signalconnected to the enable input of thememory device. If the chip select is high,the memory device remains idle and itsdata lines are disconnected from the bus.

communicate with the memory device, itpulls that device's chip select low thereby

enabling it and connecting it to the bus.


10/55

Each memory device has its own chipselect. For example, Table 12-1 showsthe only possible values of the chipselects for a system with four memory

.


11/55


12/55

The disconnection of the data lines isperformed using tristate outputs forthe data lines of the memory chips. A

third state added to it.


13/55

This output can be a logic 1, a logic0, or a third state that acts as a highimpedance or open circuit.It is like someone opened a switch

and nothing is connected.


14/55

This third state is controlled by the chipselect. When the active low chip selectequals 1, data lines are set to highimpedance, sometimes called the Zstate. A chip select equal to 0 causes

input or output


15/55


16/55

Memory Terminology

There are many different purposes formemory in the operation of a computer.Some memory is meant to store dataand programs only while the computer

s urne on w e o er memory smeant to be permanent.


17/55

Some memory contains application codewhile other memory is meant to store the

low-level driver code to control devicessuch as an IDE interface or a video card.Some memory may have a larger

faster


18/55

In order to understand what memorytechnologies to apply to which processoroperation, we need to understand a littlebit more about the technologiesthemselves. This section discusses

describe memory.


19/55

Random Access Memory (RAM):The primary storage is referred to as randomaccess memory (RAM) because it is possibleto randomly select and use any location of thememory directly store and retrieve data. Ittakes same time to any address of thememory as the first address. It is also calledread/write memory.


20/55

The storage of data and instructions inside

the primary storage is temporary. Itdisappears from RAM as soon as the power tothe computer is switched off. The memories,which loose their content on failure of power

supply, are known as volatile memories .Sonow we can say that RAM is volatile memory.


21/55

Types of RAM

The following are some common types of RAM:SRAM: Static random access memory usesmultiple transistors, typically four to six, foreach memory cell but doesn't have a capacitorin each cell. It is used primarily for cache.

DRAM: Dynamic random access memoryhas memory cells with a paired transistor andcapacitor requiring constant refreshing


22/55

FPM DRAM: Fast page mode dynamicrandom access memory was the originalform of DRAM. It waits through the entireprocess of locating a bit of data by column and

the next bit. Maximum transfer rate to L2cache is approximately 176 MBps.


23/55

SDRAM: Synchronous dynamic random

access memory takes advantage of the burstmode concept to greatly improve performance.It does this by staying on the row containing therequested bit and moving rapidly through the

, .is that most of the time the data needed by theCPU will be in sequence. SDRAM is the mostcommon form in desktops today. Maximum

transfer rate to L2 cache is approximately 528MBps.


24/55

DDR SDRAM: Double data ratesynchronous dynamic RAM is just likeSDRAM except that is has higher bandwidth,meaning greater speed. Maximum transfer

,MBps (for DDR SDRAM 133 MHZ).


25/55

Memory Modules

dual in-line memory module (DIMM)

,memory module.

small outline dual in-line memory moduleSODIMM


26/55

From the top: SIMM, DIMM and

SODIMM memory modules


27/55

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

called Read Only Memory (ROM). Again it isthe ICs inside the PC that form the ROM. Thestorage of program and data in the ROM ispermanent. The ROM stores some standard

process ng programs supp e y t emanufacturers to operate the personalcomputer. The ROM can only be read by theCPU but it cannot be changed.


28/55

The basic input/output program is stored in

the ROM that examines and initializes variousequipment attached to the PC when the switchis made ON. The memories, which do notloose their content on failure of power supply,are known as non-volatile memories. ROM isnon-volatile memory.


29/55

PROMThere is another type of primary memory in

computer, which is called Programmable ReadOnly Memory (PROM). You know that it is notpossible to modify or erase programs stored inROM, but it is possible for you to store your

program n c p. nce t e programsare written it cannot be changed and remainintact even if power is switched off. Thereforeprograms or instructions written in PROM orROM cannot be erased or changed.


30/55

EPROM:This stands for Erasable Programmable Read

Only Memory, which over come the problem ofPROM & ROM. EPROM chip can beprogrammed time and again by erasing theinformation stored earlier in it. Information

time ultraviolet light and it erases chip isreprogrammed using a special programmingfacility. When the EPROM is in use information

can only be read.


31/55

Cache Memory:

The speed of CPU is extremely high comparedto the access time of main memory. Thereforethe performance of CPU decreases due to theslow speed of main memory. To decrease the

m sma c n opera ng spee , a sma memorychip is attached between CPU and Mainmemory whose access time is very close to theprocessing speed of CPU. It is called CACHE

memory.


32/55

CACHE memories are accessed much fasterthan conventional RAM. It is used to store

programs or data currently being executed ortemporary data frequently used by the CPU.So each memory makes main memory to befaster and larger than it really is. It is also very

expens ve o ave gger s ze o cac ememory and its size is normally kept small.


33/55

Registers:The CPU processes data and instructions with

high speed, there is also movement of databetween various units of computer. It isnecessary to transfer the processed data withhigh speed. So the computer uses a number of

spec a memory un ts ca e reg sters. eyare not part of the main memory but they storedata or information temporarily and pass it onas directed by the control unit.


34/55

SECONDARY STORAGEYou are now clear that the operating speedof primary memory or main memory shouldbe as fast as possible to cope up with the

. -devices are very expensive and hence thecost per bit of storage is also very high.


35/55

Again the storage capacity of the main

memory is also very limited. Often it isnecessary to store hundreds of millions ofbytes of data for the CPU to process.Therefore additional memory is required in allthe computer systems. This memory is calledauxiliary memoryor secondary storage.


36/55

In this type of memory the cost per bit ofstorage is low. However, the operating speedis slower than that of the primary storage.Huge volume of data are stored here on

primary storage as and when required. Mostwidely used secondary storage devices aremagnetic tapesand magnetic disk.


37/55

Magnetic Tape: Magnetic tapes are used forlarge computers like mainframe computerswhere large volume of data is stored for alonger time. In PC also you can use tapes in

.in tapes is inexpensive.


38/55

Tapes consist of magnetic materials that storedata permanently. It can be 12.5 mm to 25 mmwide plastic film-type and 500 meter to 1200meter long which is coated with magnetic

ma er a . e ec s connec e o e cen raprocessor and information is fed into or readfrom the tape through the processor. It similarto cassette tape recorder.


39/55

Magnetic Tape


40/55

Advantages of Magnetic Tape:

Compact: A 10-inch diameter reel of tape is2400 feet long and is able to hold 800, 1600 or6250 characters in each inch of its length. The

maximum capacity of such tape is 180 millioncharacters. Thus data are stored much morecompactly on tape.


41/55

Fast: Copying of data is easier and fast.

Economical: The cost of storingcharacters is very less as compared to otherstorage devices.

Magnetic tapes can be used for long termstorage and a tape can be used repeatedlywith out loss of data.


42/55

Magnetic Disk: You might have seen thegramophone record, which is circular like a disk

and coated with magnetic material. Magneticdisks used in computer are made on the sameprinciple. It rotates with very high speed insidethe computer drive. Data is stored on both the

sur ace o e s . agne c s s are mospopular for direct accessstorage device. Eachdisk consists of a number of invisible concentriccirclescalled tracks.


43/55

Information is recorded on tracks of a disksurface in the form of tiny magnetic spots. The

presence of a magnetic spot represents one bitand its absence represents zero bit. Theinformation stored in a disk can be read manytimes without affecting the stored data. So the

rea ng operat on s non- estruct ve. ut youwant to write a new data, then the existing datais erased from the disk and new data isrecorded.


44/55

Floppy Disk: It is similar to magnetic diskdiscussed above. They are 5.25 inch or 3.5inch in diameter. They come in single ordouble density and recorded on one or bothsurface of the diskette. The capacity of a5.25-inch flo is 1.2 me a b tes whereasfor 3.5 inch floppy it is 1.44 mega bytes. It ischeaper than any other storage devices andis portable. The floppy is a low cost device

particularly suitable for personal computersystem.


45/55

Floppy Disk


46/55

Optical Disk:With every new application and software there

is greater demand for memory capacity. It isthe necessity to store large volume of data thathas led to the development of optical diskstorage medium. Optical disks can be divided

nto t e o ow ng categor es:Compact Disk/ Read Only Memory (CD-ROM):CD-ROM disks are made of reflective metals.CD-ROM is written during the process of

manufacturing by high power laser beam.


47/55

Here the storage density is very high, storage

cost is very low and access time is relativelyfast. Each disk is approximately 4 1/2 inchesin diameter and can hold over 600 MB ofdata. As the CD-ROM can be read only we

cannot write or make changes into the datacontained in it.


48/55

Write Once, Read Many (WORM): The

inconvenience that we can not write any thingin to a CD-ROM is avoided in WORM. AWORM allows the user to write datapermanently on to the disk. Once the data is

wr en can never e erase w ouphysically damaging the disk.


49/55

Here data can be recorded from keyboard,

video scanner, OCR equipment and otherdevices. The advantage of WORM is that itcan store vast amount of data amounting togigabytes (109 bytes). Any document in a

WORM can be accessed very fast, say lessthan 30 seconds.


50/55

Erasable Optical Disk: These are opticaldisks where data can be written, erased andre-written. This also applies a laser beam towrite and re-write the data. These disks may

.


51/55

Erasable optical disks are based on atechnology known as magnetic optical(MO).To write a data bit on to the erasable opticaldisk the MO drive's laser beam heats a tiny,precisely defined point on the disk's surfaceand magnetizes it.


52/55

Storage Speed CapacityRelativeCost ($)

Permanent?

Registers Fastest Lowest Highest No

RAM Very Fast Low/Moderate High No

Floppy

Disk

Hard Disk Moderate Very High Very Low Yes


53/55

The benefits of secondary storagecan be summarized as follows:

Capacity. Organizations may store the

equivalent of a roomful of data on sets of disksthat take up less space than a breadbox. Asimple diskette for a personal computer holdsthe e uivalent of 500 rinted a es or onebook. An optical disk can hold the equivalentof approximately 400 books


54/55

Reliability. Data in secondary storage isbasically safe, since secondary storage isphysically reliable. Also, it is more difficultfor unscrupulous people to tamper with

ata on s t an ata store on paper na file cabinet.Convenience. With the help of a

computer, authorized people can locateand access data quickly.


55/55

Cost. Together the three previous

benefits indicate significant savings instorage costs. It is less expensive to storedata on tape or disk (the principal meansof secondary storage) than to buy and

house filing cabinets. Data that is reliableand safe is less expensive to maintainthan data subject to errors. But the

greatest savings can be found in thespeed and convenience of filing andretrieving data.

Lecture 4 -Computer Memory and Memory Types

Documents