cdrom, floppy and hard disk structure

CDROM, Floppy and Hard Disk CDROM, Floppy and Hard Disk StructureStructure

Plus some basic conceptsPlus some basic concepts

Table of Contents CD

• History• Structure• Data Recording• How The CD Drive Works• CD File Systems• Multiple Sessions• CD-ReWritable (CD-RW)• DVD

Floppy Disk• History• Structure• Data Recording/Retrieval• Formatting• 3½ Inch (2HD) Disks• Hard Disk• Some Basic Concepts

• Boot Sector• Cluster• FAT• NTFS

PART 1

CD-ROM

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History

• Compact Disc - Digital Audio (CD-DA), the original CD specification developed by Philips and Sony in 1980

• Specifications were published in Red Book, continued to be updated (lastest version in 1999)

• In 1985 a standard for the storage of computer data by Sony and Philips, CD-ROM (Compact Disc Read Only Memory)

• Developments in the technology have been ongoing and rapid– Compact disc Interactive (CD-I)– Compact Disc Television (CD-TV)– Compact Disc Recording (CD-R)– Digital Video Disc (DVD)

Structure

• A CDROM Drive uses a small plastic-encapsulated disk that can store data

• This information is retrieved using a Laser Beam

• A CD can store vast amounts of information because it uses light to record data in a tightly packed form

Structure (cont’d)

• On surface of CDROM, it was be "punched" to according the spiral called the pits. These positions do not have "punch" as land.– The 0.12 micron deep pit, approximately 0.6 microns

wide.– The pit and land length from 0.9 to 3.3 microns.– The distance between the spiral is 1.6 micron.– Track density on a CDROM is about 16,000 tracks per

inch.

CD Layers

• The thickness of a CD can vary between 1.1 and 1.5mm• A CD consists of four layers

• The biggest part is clear polycarbonate (nominally 1.2mm)

• There is a very thin layer of reflective metal (usually aluminum) on top of the polycarbonate

• Then a thin layer of some protective material covering the reflective metal

• A label or some screened lettering on top of protective material

CD Layers (cont’d)

CD Safety• The label side of a CD is the most

vulnerable part of the disk• The other side is protected by the

thick (1.2mm) and hard polycarbonate

• It is possible to carefully clean and even to polish this surface to remove fingerprints and even scratches

• Many flaws on the polycarbonate surface will simply go unread.

CD vs. Magnetic Media

• In Magnetic Media (like floppy/hard disk) the surface is arranged into concentric circles called “tracks”

• Number of sectors per track is constant for all tracks

• The CD has one single track, starts at the center of the disk and spirals out to the circumference of the disk

• This track is divided into sectors of equal size

CD Data Recording

• In the recording, Lazer gun was used to write data to disk– Signal corresponding to 0 => laser

off.– Signal corresponding to 1 => laser

on => burned disk surface into a point of losing the ability to reflect

• Information is recorded on a CD using a series of bumps

Laser Laser gungun

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Data Recording (cont’d)• The unmarked areas between pits are called "lands”• Lands are flat surface areas

• The information is stored permanently as pits and lands on the CD-ROM. It cannot be changed once the CD-ROM is mastered, this is why its called CD-ROM

Data Reading• Laser reflection on

rotating disk surface, the pit will be lost reflected rays => that is “0” signal, the land they received reflected rays => that is “1” signal

Laser Laser gungun

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Prism

Sensitive Sensitive diodediode

How The CD Drive Works• A motor rotates the CD• The rotational speed varies so

as to maintain a constant linear velocity (the disk is rotated faster when its inner "SPIRALS" are being read)

How The CD Drive Works (cont’d)

• A laser beam is shone onto the surface of the disk• The light is scattered by the pits and reflected by the lands,

these two variations encode the binary 0's and 1's• A light sensitive diode picks up the reflected laser light and

converts the light to digital data

How The CD Drive Works (cont’d)

CD-ROM Drive Speed

• The CD-ROM drives are classified by their rotational speed• Based on the original speed of a CD-Audio (e.g. A "2X"

CD-ROM drive will run at twice the speed of a CD- Audio)

Speed Information transfer rate1X 150 Kbytes/s2X 300 Kbytes/s… …

CD Physical Specifications

Diameter 120mm ±0.3mmTransparent Layer Thickness 1.2mm ±0.1mmTotal Thickness 1.1mm - 1.5mmTransparent Layer Index of Refraction

1.55 ±0.10

Reflectance of Metal Layer through Transparent Layer

70% minimum

Laser Wavelength 780nm ±10nmTrack Pitch 1.6 micron ±0.1 micronScanning Linear Velocity 1.20m/s - 1.40m/s (±0.01m/s)

CD File Systems

1. ISO-9660The base standard defines three levels of compliance

– Level 1 limits file names to 8+3 format. Many special characters (space, hyphen, equals, and plus) are forbidden

– Level 2 and 3 allow longer filenames (up to 31) and deeper directory structures (32 levels instead of 8)

– Level 2 and 3 are not usable on some systems, special MS-DOS

CD File Systems (cont’d)2. Rock Ridge• Extensions to ISO-9660 file system• Favored in the Unix world• Lifts file name restrictions, but also allows Unix-style

permissions and special files to be stored on the CD• Machines that don't support Rock Ridge can still read

the files because it's still an ISO-9660 file system (they won't see the long forms of the names)

• UNIX systems and the Mac support Rock Ridge• DOS and Windows currently don't support it

CD File Systems (cont’d)

3. Joliet• Favored in the MS Windows world• Allows Unicode characters to be used for all text fields

(including file names and the volume name)• Disk is readable as ISO-9660, but shows the long

filenames under MS Windows4. HFS (Hierarchical File System)

Used by the Macintosh in place of the ISO-9660, making the disk unusable on systems that don't support HFS

Multiple Sessions

• Allows CDs to be written more than once (not re-written)• Some CD writers support this feature• About 640MB of data can be written to the CD, as some

space is reserved for timing and other information• Each session written has an overhead of approximately

20MB per session

CD-ReWritable (CD-RW)

• It is essentially CD-R• Allows discs to be written and

re-written up to 1000 times• The storage capacity is the

same as that for CD-R• Based on phase-change

technology• The recording layer is a

mixture of silver, indium, antimony and tellurium

CD-RW Recording Process• The recording layer is polycrystalline• The laser heats selected areas of the recording track to the

recording layer's melting point of 500 to 700 degrees Celsius

CD-RW Recording (cont’d)

• The laser beam melts the crystals and makes them non-crystalline (amorphous phase)

• The medium quickly cools, locking in the properties of the heated areas

• The amorphous areas have a lower reflectivity than the crystalline areas

• This creates a pattern which can be read as pits and lands of the traditional CD

• To erase a CD-RW disc, the recording laser turns the amorphous areas back into crystalline areas

DVD

• same size (120mm) and thickness (1.2mm) as CD

• Improvements in the logarithms used for error correction

• Much greater data accuracy using smaller Error Correction Codes (ECC)

• More effective use of the track space

• Digital Versatile Disk (Formerly Digital Video Disk)

DVD vs. CD

• DVD uses a tighter spiral (track or helix) with only 0.74 microns between the tracks (1.6 microns on CDs)

• DVD recorders use a laser with a smaller wavelength, 635nm or 650 nm (visible red light) vs. 780nm (infrared) for CDs

• DVD has smaller "burns" (pits) in the translucent dye layer (0.4 microns minimum vs. 0.83 microns minimum on CDs)

• These technologies allow DVDs to store large amounts of data

DVD (cont’d)

• Standard single-sided DVDs store up to 4.7GB of data• Dual-sided discs hold about 8.5GB of data (9.4GB for back-

to-back layers dual-sided discs)• In back-to-back layers discs, it must be turned over to

access the data on the reverse side • DVD uses MPEG2 compression for high quality pictures• DVD drives have a much faster transfer rate than CD drives • DVD-ROM drives will read and play existing CD-ROM

and CD-A disks

DVD (cont’d)

Types Characteristics

DVD-5 4.7GB Single-Side, Single-Layer capacity 4,7 GB.

DVD-9 8.5GB Single-Side, Dual-LayerCapacity 8,5 GB.

DVD-10 9.4GB Double-Side, Single-LayerCapacity 9,4 GB.

DVD-18 17.1GB Double-Side, Dual-LayerCapacity 17,1 GB.

PART 2

Floppy Disks(floppies or diskettes)

History1. The 8-inch disk- First attempt by IBM in 1967, the result was a diskette

storing 80KBytes of data- 250KB, 800KB and 1MB floppies untill 1975- The problem was their poor media quality

2. The 5¼-inch minifloppy- First developed in 1976, storing 110KB- In 1978, double-sided drive doubled the

capacity, and a new "double density" format increased it to 360 KB

History (cont’d)

3. The 3-inch compact floppy disk

- No more capacity than the more popular (and cheap) 5¼" floppies

- More reliable thanks to its hard casing

- Their main problems were their high prices

History (cont’d)

4. The 3½-inch floppy disk

- originally offered in a 360 KB single-sided and 720 KB double-sided double-density format

- A newer "high-density" format, displayed as "HD" on the disks and storing 1440 KB of data, in the mid-80s

- Another advance in the oxide coatings allowed a new "extended-density" ("ED") format at 2880 KB in 1991

Structure

• Made from circular sheets of plastic which are coated with a magnetic material

• A central hole for coupling to the disk drive• An envelope seals the disk to protect and "clean" the disk• An aperture in the envelope to

expose a section of the disk to allow magnetic heads to read and write

• A button on the corner to switch the disk to write-protected mode

Physical Structure

• A disk is divided into many concentric circles (lines of recorded data) called “tracks”

• The disk is also divided into wedge-shaped segments called “sectors”

• The number of sectors per track is the same in all tracks• So the outer sectors are larger than the inners, but has the

same capacity

Data Recording

• An electric current flows through a coil of wire

• A magnetic field is produced• This field is used to

magnetise the coating of iron oxide on a floppy disk

• Varying electrical current, the signal is passed through the coil and the variations are "recorded" on the disk

• Similar to the operation of a domestic tape recorder• An electric current flows

through a coil of wire• A magnetic field is produced• This field is used to

magnetise the coating of iron oxide on a floppy disk

• Varying electrical current, the signal is passed through the coil and the variations are "recorded" on the disk

• Similar to the operation of a domestic tape recorder• An electric current flows

through a coil of wire• A magnetic field is produced• This field is used to

magnetise the coating of iron oxide on a floppy disk

• Varying electrical current, the signal is passed through the coil and the variations are "recorded" on the disk

• Similar to the operation of a domestic tape recorder

Data Retrieval

• The disk is rotated at low speed (200 to 600 revolutions/min)

• The disk moves under the head

• A very small electric current is induced into the head and the stored data is retrieved

Double Sided Disks

• Are used in a disk drive with two read/write heads• A pressure pad is fitted for each head

Formatting (IBM)

• During the "formatting" process each sector has written in it a :- 51 bytes prologue field containing the track and sector number- 512 bytes data field- 28 bytes unusable field and delay gap between sectors- The track, sector, and the data field are followed by a CRC

(cyclic redundancy check) checksum- Whenever data are read from the disk a new checksum is

calculated and compared to the written one- An error message is generated if the two don't agree

Hard Disk

• Fixed and removable

• Fast (disk rotates at 60 to 200 times per second)

• Currently 20 – 180 GB (may be limited by the version of the operating system)

• Like floppies, uses the magnetic properties of the coating material, but the technology is different

Boot Sector (Boot Record)• A vital sector, disk will be unusable if this sector damages• MBR at CHS 0, 0, 1 in hard disks, contains Partition Table• Each partition has its own boot sector too• Each operating system has its own boot sector format• For Booting, Bootstrap Loader loads Boot Sector data it in a

particular address of memory (0000:7C00h) and sets the PC• In hard disks, the small program in MBR attempts to locate

an active (bootable) partition in partition table• If found, the boot record of that partition is read into memory

(location 0000:7C00) and runs

DOS/Win Formatted Disk

• A DOS/Win formatted floppy/hard disk’s Boot Sector contains– A jump and a NOP opcode– BPB (BIOS Parameter Block)

- Sectors per cluster- Number of Root directory entries- Sectors per FAT- Volume Label- …

– A program, to load OS if bootable/show error msg if not in floppies, to locate the active partition in hard disks

– Error messages

Cluster

• Data units of disk must be addressed, which units belong to which file / are free / are damaged (bad sectors) / …

• On disks having large capacity, purposing one sector as a unit makes addressing table so large Cluster is defined

• Represents the smallest amount of disk space that can be allocated

• The smaller the cluster size, the more efficiently disk space usage, the more number of bits to address one unit

• The number of sectors per cluster is stored in theBoot Record

FAT

• FAT-12/FAT-16/FAT-32 are Microsoft favorite File Allocation Tables (before NTFS)

• FAT-12 uses 12 bits for addressing, a max. of 4096 units, considering one sector as a cluster, 2MB can be addressed

• FAT-16 with max.(128) sectors/cluster (64KB cluster size wasting large amount of disk space) up to 4GB, this is why Win95 cannot support more than 4GB partiotions

• FAT-32, the same system,32 bit fields for addressing

NTFS• NT File System• Better performance• Less wasted space• More security• Supports all sizes of

clusters (512b - 64 KB)• The 4 KB cluster is somehow standard• Practically no partition size limitation• Very flexible, all the system files can be relocated, except

the first 16 MFT (Master File Table) elements

NTFS (cont’d)• NTFS disk is symbolically divided into two parts• The first 12% is assigned to MFT area• The rest 88% represents usual space for files storage• MFT area can simply reduce if needed, clearing the space for

recording files• At clearing the usual area, MFT can be extended again

Question

• Why are the bumps on the reflective layer of the CD called pits?

• Do you know what a Boot Loader program is? And how it works?