Upload
suman-halder
View
218
Download
0
Embed Size (px)
Citation preview
7/31/2019 Memory Intro
1/9
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.
7/31/2019 Memory Intro
2/9
Computer Organization
Suman Halder, Assistant Professor of CSE Department, CIEM Page 89
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.
7/31/2019 Memory Intro
3/9
Computer Organization
Suman Halder, Assistant Professor of CSE Department, CIEM Page 90
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
7/31/2019 Memory Intro
4/9
Computer Organization
Suman Halder, Assistant Professor of CSE Department, CIEM Page 91
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.
7/31/2019 Memory Intro
5/9
Computer Organization
Suman Halder, Assistant Professor of CSE Department, CIEM Page 92
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.
7/31/2019 Memory Intro
6/9
Computer Organization
Suman Halder, Assistant Professor of CSE Department, CIEM Page 93
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.
7/31/2019 Memory Intro
7/9
Computer Organization
Suman Halder, Assistant Professor of CSE Department, CIEM Page 94
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
7/31/2019 Memory Intro
8/9
Computer Organization
Suman Halder, Assistant Professor of CSE Department, CIEM Page 95
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.
7/31/2019 Memory Intro
9/9
Computer Organization
Suman Halder, Assistant Professor of CSE Department, CIEM Page 96
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.