Memory Intro

7/31/2019 Memory Intro

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Computer Organization

Suman Halder, Assistant Professor of CSE Department, CIEM Page 88

Module3

Memory unit design with special emphasis on implementation of CPU-memory interfacing. [2L]

Memory organization, static and dynamic memory, memory hierarchy, associative memory. [3L]

Cache memory, Virtual memory. Data path design for read/write access. [5L]

Primary Memory (Main Memory)

Primary memory is used for input/output operations, manipulation of text and calculation

operations, logic/ comparison operation, storage and retrieval operations.

Primary memory is made of semiconductor device. Primary memory is faster than secondary memory. Storage capacity of primary memory is smaller. Primary memory is expensive than secondary memory. Primary memory consumes less power.

Primary memory stores programs along with data, which are to be executed.

Secondary Memory (Auxiliary Memory)

Secondary memory used for storage and back-up operation.

The access time is considerably greater for secondary memory than for primary memory. Secondary memory is made of magnetic or optical device. Secondary memory is slower than primary memory. Storage capacity of secondary memory is larger. Secondary memory is cheaper than secondary memory. Secondary memory stores operating system, data files, compilers, assemblers, application

programs etc.

RAM (Random Access Memory)

RAM is a semiconductor memory that stores binary information temporarily during the operation

of the computer. It is called Random Access Memory because any of the locations on a chip can

be randomly selected and used to directly store and retrieve data and instructions. RAM is avolatile memory, i.e. it depends on a steady power supply and all the contents are destroyed if the

power is turned off. CPU can write into or read from this memory. There are two types of RAM

Static RAM and Dynamic RAM.

ROM (Read Only Memory)

ROM is semiconductor memory that stores binary information permanently. The information can

be read from this memory but it does not accept any input data or instructions from the users; i.e.it cannot be altered. ROM is a nonvolatile memory; it retains stored information even if the

power is turned off. The contents of ROM are permanent or semi-permanent. In embedded

computers, which frequently do not have disk drives, all of the required software may be storedin ROM. Software stored in ROM is often called firmware, because it is notionally more like

hardware than software. There are three types of ROM: PROM, EPROM and EEPROM.


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Table: Differences between ROM and RAM

Properties ROM RAM

Volatile Non-volatile Memory Volatile Memory

Read/write operation Only read operation Read and write bothoperation

Data erase Storing data cannot be

erased

Storing data can be erased

Memory location

access

Any Memory location

access in random manner

Any Memory location also

access in random manner

SRAM (Static RAM)

It is a type of computer memory that retains its contents as long as power is supplied. It does not

need constant refreshment like Dynamic RAM chips. A Static RAM chip can only store about

one fourth of the information that a Dynamic RAM of the same complexity can hold. StaticRAM, with access times of 15 to 30 nanoseconds, is much faster than Dynamic RAM, at 60nanoseconds or more. It is often used in caches; however Static RAM is 4-5 times as expensive

as Dynamic RAM.

Diagram of a Static Memory Cell based on a flip-flop

Each cell of a Static RAM is implemented using a flip-flop which is implemented using several

MOSFET transistors. External power is required to operate the transistors. As long as theexternal power is applied the static memory cell retains the data. The circuit of a single flip-flop

based cell which can store a binary 0 or 1 is shown in above figure.

DRAM (Dynamic RAM)

A common type of computer memory that needs to be refreshed every millisecond, during that

time they cannot be read by the processor. DRAM chips are small, simple, cheap, and easy to

make and hold four time as much information as a Static RAM chip of similar complexity.However, they are slower than Static RAM.


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Differences between SRAM and DRAM:

1. Standard DRAM does not have a fast-enough access time (the fastest is currently about 70 ns), While the SRAM is faster so the DRAM is used as the main memory in the computers and

the SRAM is used in the cache memory of the computers.

2.Because the DRAM capacitor loses its charge over time, and needs to have its chargerefreshed at regular intervals. Thus, dynamic memories are accompanied by controller circuits

to rewrite the bit and refresh the stored charge on a regular basis. But the SRAM does not

need such a controller.3. The SRAM requires four to six transistors while the DRAM is much simpler, it can be

implemented with three or one transistor. So more memory cells can be put in a single chip.

And the total size of the DRAM is larger than the size of SRAM.

4. Neither SRAMs nor DRAMs retain information when power is removed (volatile); but withbattery back-up, SRAMs can store important configuration information when main power is

removed because they don't require refreshing.

5. The power consumed per bit of DRAM is less than power consumed per bit for SRAM. Thepower dissipated per bit of DRAM is less than 0.05mW while the SRAM consumes 0.2mW.6. The DRAM is less expensive per bit than SRAM.

7. The SRAM has the property of being "bi-stable" in other words, as long as a current is appliedto it, it can retain its on/off state.

8. DRAM does not have a fast-enough access time (the fastest is currently about 70 ns) while theSRAM is faster so it is used as cache memory to reduce the accessing to the DRAM (main

memory).

Table: Differences between SRAM and DRAM

Properties SRAM DRAM

Access time Faster than DRAM, 15 to 30

nanoseconds.

Slower than SRAM, 60 nanoseconds

or moreCost SRAM 4 to 5 times more expensivethan DRAM

Cheap than SRAM

Refreshment Refreshment not required. Refreshment required in everymillisecond.

Storing

capacity

SRAM can store of information

that a DRAM of the same

complexity can hold.

DRAM holds four time as much

information as a SRAM chip of

similar complexity.

SDRAM (Synchronous Dynamic RAM)

This is a type of enhanced memory that allows data to be transferred at any point in the systemsclock cycle rather than just at certain point. This makes for dramatically faster overall memory

performance.

NVRAM (Non-Volatile RAM)

A high speed static RAM and EEPROM are packed into a single IC to form a nonvolatile RAM.When power is gone the entire contents of the memory are quickly transferred automatically in

parallel in the EE-PROM and stored data from EE-PROM are transferred back into RAM when

the power is turned on and can be read from RAM. The advantage of NVRAM is that a battery


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backup is not needed to save the working data in the event of power failure. Example of

NVRAM is Intel 2004.

PROM (Programmable Read Only Memory)

This memory has nichrome or polysilicon wires arranged in a matrix; these wires can be

functionally viewed as diodes or fuses. This memory can be programmed by the user forconverting critical and lengthy operations into micro programs that are fused into a chip. Once

they are in a hardware form, they can be executed at a very high speed. Once operations have

been written into a PROM chip, they cannot be altered. Permanent program, data or any otherkinds of information can be store in a PROM. An example of PROM is 74S287.

EPROM (Erasable Programmable Read Only Memory)

EPROM is a memory chip that maintains its contents without electrical power. It can be erased

and reprogrammed by removing a protective cover and exposing the chip to high intensity short

wave ultraviolet light for some time before it can accept any new contents. The technique oferasing contents is not easy; EPROM IC has to be removed from computer for exposure to the

ultraviolet light. EPROMs are used to store programs which are permanent but need updating.

EPROMs are cheap and reliable.

EPROM uses MOS technology. They store 1 as charge and 0 as no charge on the insulated

floating-gate of the transistor. The ultraviolet light causes to leak off the charge. Thus the data

are erased and allows user to reprogram the EPROM. For programming it requires high voltage30V or higher but for read operation it needed only 5V.

EEPROM (Electrically Erasable Programmable Read Only Memory)

A memory chip that maintains its contents without electrical power and whose contents can be

erased and reprogrammed with special electrical pulses either within the computer or from

external source. It is also known as EAPROM (Electrically Alterable PROM). The chip can beerasing in shorter time compared to EPROM. The technique of erasing contents is easy; EPROM

IC need not be removed from computer for erasing. Example of EEPROM is Intel 2816A

Flash Memory

A special form of nonvolatile EEPROM that can erase at signal levels normally found inside the

PC, so that you can reprogram the contents with whatever you like without pulling the chips outof your computer. Also once flash memory has been programmed, you can remove the expansion

board it is mounted on and plug it to another computer if you wish.

Hard Disk

Seagate introduced the first hard disk for PCs in 1979, which held 5 MB of data. The Hard Disk

holds the operating system, which boots the system, stores the different multi-megabyte

application and files that we rely on, and even provides virtual memory for systems lean onRAM. Hard disk performance also has a profound effect on overall system performance. Hard

Disk also called as Hard Drive or Hard Disk drive or Fixed Disk drive.


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Hard disk is a storage device that uses a set of thin, flat,

rotating, magnetically coated circular objects made of

metal or glass called platters to store data or programs. Acentral spindle supports the platters. A small gap

separates each platter from its neighbors, which allows a

read-write head mounted on an actuator arm to fitbetween the platters. A typical hard disk platter rotates at

3600 rpm, and the read/write heads (the electromagnetic

device used to read and write to and from magnetic

media) float on a cushion of air from 10 to 25 millionthsof inch thick so that the heads never come into contact

with the recording surface. The whole unit is hermetically sealed to prevent airborne

contaminants from entering and interfering with these close tolerances. The speed of storinginformation of hard Disk is called average access time. Average access time measurement in

millisecond.

Hard disk drives usually have more than one platter and there are two heads for each platter ofthe hard drive (one on the top and one on the bottom). The heads read and write the data on the

platter.

A tiny dust particle could cause a head crash. Hard disk is very vulnerable to vibrations: a minor

bump can make the head crash into disks surface. The damage usually cannot be repaired,causing data loss and hard disk damage.

When the data is read from or written on a portion of a track of disk, the read/write head moves

to the desired track from its current position. After reaching the desired track, there is a rotational

delay in bringing the specified portion of the track beneath the head. The rotational delay iscalled latency. The time required in the movement of the head, is known as seek time. The

access time for a data is the sum of seek time and latency. Usually manufacturers specify the

average access time; since the actual seek time varies based upon where the heads are and wherethe next bit of data is located.

A hard disk is composed of certain basic components:

Rigid disks with magnetic media for data storage. Read/write heads, usually one per surface Head movement mechanics. A computer interface/controller. Electronic interface between the heads and the computer interface One or more cables between the drive and computer.CHS: Cylinder-head-sector, also known as CHS, is a method of giving addresses to data on ahard drive.


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Heads: Every hard drive consists of platters and read-write heads. If a drive has double sided

four platters, then it has eight read-write heads, one on the top and bottom of each platter. The

head value is the number of read-write heads in the drive.

Tracks: Each platter is divided into thousands of

tightly packed concentric circles, known as tracks, that

can be read from or written to by that surfaces head.These tracks resemble the structure of annual rings of a

tree. All the information stored on the hard disk is

recorded in tracks. Starting from zero at the outer sideof the platter, the number of tracks goes on increasing

to the inner side. Each track can hold a large amount of

data counting to thousands of bytes.

Cylinders: The cylinder value is the number of tracks

on one side of each platter. There are the same numbers

of cylinders on each side of each platter. A cylinder is avertical set of all of the tracks with the same number on each surface of a diskette or hard disk.

Thus cylinder 0 is the set of tracks numbered 0, cylinder 1 is the set of tracks numbered 1, and so

forth. For a diskette, cylinder 0 consists of track 0 on side 1and track 0 on side 2; cylinder 1

consists of track 1 on side 1 and track 1 on side 2; and so forth. When writing a file, the systemfills all the tracks on a cylinder and then advanced the read-write heads to the next cylinder.

Sectors: Each track is further divided down into smaller units called sectors. As sector is thebasic unit of data storage on a hard disk. A single track typically can have thousands of sectors

and each sector can hold more than 512 bytes of data. A few additional bytes are required for

control structures and error detection and correction. Old drives used the same number of sectors

on every track. But modern drives the outer tracks are storing more sectors than the inner tracksof the plotter. Sectors are often grouped together to form Clusters.

Cluster: A cluster is a group of sectors. A cluster size is always a power of 2, such as 1, 2, 4 or 8sectors. How many sectors together form a cluster depends upon the capacity of the disk. A hard

disk typically has four sectors per cluster. On a disk device that uses one sector per cluster, there

are sector and cluster the equal in size. As the capacity goes on increasing, so also increasing themaximum cluster number. According, we have 12 bit, 16 bit and 32 bit FAT. In a 12 bit FAT

each entry is of 12 bits. Since each entry in FAT represents a cluster number, the maximum

cluster number possible in a 12 bit FAT is 212

(4096). Similarly, in case of a 16 bit FAT the

maximum cluster number is 216

(65536). Also, for a 32 bit FAT the maximum cluster number is228 (268435456. Only 28 of the 32 bits are used in this FAT).

A file begins on a cluster boundary and requires a minimum of one cluster even if the file

occupies only one sector.

Thus one can figure the number of bytes of data that will fit on disk by calculating:

(# Cylinders) (# heads) (Number of sectors/track) (Number of bytes/sect)

For example, a hard disk contains 3148 cylinders, 16 heads, 63 sectors/track and 512 bytes /sect.


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Disk capacity = (3148 cyl) (16 heads) (63 sectors/track) (512 bytes/sect)

which is 1,624,670,208 bytes, or around 1.6 Gig.

Advantages of Hard Disk

Very high storage capability.

Hard disks are very reliable. Very fast access time compared to Floppy Disk or CDROM Read/write operation can be done easily. It has very long lifeDisadvantages of Hard Disk

Expensive compared to Floppy Disk or CDROM Hard Disk is not removable.CD-ROM (Compact Disk ReadOnly Memory)

The first optical data storage disk, developed by Philips, had 60 times the capacity of a 5.25-inchfloppy disk. CD-ROMs are made up plastic (polycarbonate). The plastic disk is than coated witha very thin layer of aluminum, to make the surface reflective. Commercially available recordable

CDs are coated with cyanine, phthalocyanine or azo dyes. The cyanine disks are green,

phthalocyanine disks are yellow and azo disks are blue.

Compact disks are 4.72 inches in diameter and 527 MB to 742 MB will be fit on it, depending on

the number of sectors on the disk and the format used on it. The typical disk used in computer-

based CD-ROM drive stores 630 MB using 333,000 sectors. A 60 minutes audio CD with270,000 sectors and up to 99 tracks.

There are different types of CDs. They are: Audio CD holds only music Video CD stores movie Photo CD Data CD containing different types of softwareCD-RW disc (Compact Disc: Re-Writable)

A recordable CD can be erased and rerecorded many times. CD-RW discs can only be written in

a CD-RW recorder, not in a normal CD-Recorder. We can reformat CD-RW and overwrite all

existing contents with new contents by this drive. In general a CD-RW disk can be re-written

about a thousand times.

Advantages of CD-RW

Use when you need to erase the data and re-write new information (for example, updatingfiles).

Use to make a practice CD or to test the contents of a CD before making a permanent one. More cost effective for near line data storage requirements than CD-R.CD-R


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Those CDs are recordable only one time is called CD-R. The writing on CD-R is done on a

special drive called CD-RW recorder or CD writer. This drive can also read a CD.

Advantages of CD-R

Good for permanent data storage.

Less expensive per disc than CD-RW disc. Readable on virtually all CD-ROM and CD-R drives. Use when you do not need to erase the data.Physical format of CD is two types; Mode 1 is standard for writing computer data and textproviding error detection code (EDC) and error correction code (ECC) Mode 2 provides for

video and audio using CRC for error correction. There are two form types within a Mode 2

sectors. Form 1 contains static information. Form 2 contains time-dependent information. Anormal CDROM track contains only Mode 1 data type sectors. With CDROM-XA track contains

only Mode 2 sectors. The XA stands for extended architecture; the XA standard makes possible

multimedia CDROM.

Single-speed (1X) CDROM drives transferred data into the computer at 150 KB/second. If the

drives spin twice as fast then data transfer will twice as fast as well. These double-speed (2X)

drives transferred data into the computer at 300 KB/second. Others drives are follows quad-speed (4X, 600KB/second), six-speed (6X, 900KB/second), eight-speed (8X, 1200 KB/second),

ten-speed (10X, 1500KB) so on. In modern technology 52X CDROM derives transferred 7800

KB/second.

A CD can contain [(75 sectors per second) X (60 seconds per minute) X (number of minutes on

CD)] sectors. The amount of data contained in the sector depends on what physical format it is

recorded in; for "regular" CD-ROM data, you can fit 2,048 bytes (2 KB) of data into a sector.

Advantages of CDROM

It is more reliable It has long life It can be removed from the drive Very high storage capability Very low costDisadvantage of CDROM

Longer access time compared over a hard disk. Most CDROM are nor erasable, except CD-RWDVD (Digital Versatile Disk)

DVD looks like CDROM, but DVD can store more than 26 times as much data as CDROM.DVD can store as much as 17 GB, depending on the disk you use. DVD-ROM is an optical disc

technology that can read both CD-ROM and DVD-ROM discs and may be able to read CD-R

and CD-RW discs. DVD-ROM drives are read-only, that is, you can use them to read CDs, butnot to make CDs.

There are two main differences between a hard disk and a diskette drive.


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For hard disk, the read-write head rides just above the disk surface without ever touching it,whereas for diskette, the read-write head actually touches the surface.

A hard disk device is constantly spinning, whereas a diskette device starts and stops for eachread/write operation.

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Memory Intro