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- A personal computer (PC) is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end user, with no intervening computer operator.

- Come in various types.

WORKSTATION

POCKET PC

TABLET PC

LAPTOP COMPUTER

DESKTOP COMPUTER

- A workstation is a high-end personal computer designed for technical or scientific applications. Intended primarily to be used by one person at a time, they are commonly connected to a local area network and run multi-user operating systems.

- A miniaturized personal computer

- Usually related to PDA (personal digital assistant) and smartphones

- made to be handheld (fits the palm)

- runs on an operating system

- also called palmtop

- A tablet PC refers to a laptop or slate-shaped mobile computer, equipped with a touchscreen or graphics tablet/screen hybrid to operate the computer with a stylus or digital pen, or a fingertip, instead of a keyboard or mouse.

- is a personal computer designed for mobile use small enough to sit on one's lap.

- includes most of the typical components of a desktop computer

- powered by a rechargeable battery charged from an AC/DC adapter

- also called notebook

- is a personal computer (PC) in a form intended for regular use at a single location, as opposed to a mobile laptop or portable computer.

- comes in different varieties of form factors

- includes the very essential components that lets you run a personal computer system

- consist or analog or digital modules

MOTHERBOARD

- the central printed circuit board of some complex electronic systems such as the personal computer

- also called the mainboard, system board or logic board

- all the parts of your computer can receive power and communicate with one another

NORTHBRIDGE

- is one of the two chips in the core logic chipset on a PC motherboard

- typically handles communications among the CPU, RAM, AGP or PCI Express, and the southbridge

- due to the difference in signalling between different RAMS, a northbridge will typically work with only one or two classes of CPUs and generally only one type of RAM

- known as a memory controller hub (MCH) or an integrated memory controller (IMC)

SOUTHBRIDGE

- the other core logic chipset in the motherboard

- is a chip that implements the "slower" capabilities of the motherboard in a northbridge/southbridge chipset computer architecture

FRONT-SIDE BUS

- is the bus that carries data between the CPU and the northbridge

BACK-SIDE BUS

- a computer bus used to connect the CPU to CPU cache memory(a smaller, faster memory which stores copies of the data from the most frequently used main memory locations.)

FORM FACTOR

- pertains to the physical dimensions of major system components

- usually patterned from the size specification of the motherboard

COMMON FORM FACTORS:

1.) Baby AT (BAT)

2.) ATX

3.) Micro ATX

4.) LPX

5.) NLX

6.) Flex ATX

1.) Baby AT (BAT)

- The Baby AT (BAT) format reduced the dimensions of the motherboard to a typical 9in wide by 10in long, and BAT motherboards are generally characterized by their shape, an AT-style keyboard connector soldered to the board and serial and parallel port connectors which are attached using cables between the physical ports mounted on the system case and corresponding connectors located on the motherboard.

Baby AT (BAT)

2.) ATX - The Intel Advanced/ML motherboard, launched in 1996, was

designed to solve these issues and marked the beginning of a new era in motherboard design. Its size and layout are completely different to the BAT format, following a new scheme known as ATX. The dimensions of a standard ATX board are 12in wide by 9.6in long; the mini ATX variant is typically of the order 11.2in by 8.2in.

- The ATX design gets round the problem by moving the CPU socket and the voltage regulator to the right-hand side of the expansion bus. Room is made for the CPU by making the card slightly wider, and shrinking or integrating components such as the Flash BIOS, I/O logic and keyboard controller. This means the board need only be half as deep as a full size Baby AT, and there's no obstruction whatsoever to the six expansion slots (two ISA, one ISA/PCI, three PCI).

ATX

3.) Micro ATX

- Introduced in the late 1990s, the MicroATX is basically a smaller version of Intel's ATX specification, intended for compact, low-cost consumer systems with limited expansion potential.

Micro ATX

4.) LPX

- The LPX format is a specialized variant of the Baby-AT used in low profile desktop systems and is a loose specification with a variety of proprietary implementations.

- Expansion slots are located on a central riser card, allowing cards to be mounted horizontally. However, this arrangement can make it difficult to remove the motherboard, and the more complex engineering required adds to system costs. As the riser card prevents good airflow within the system case, additional chassis fans are almost always needed.

LPX

5.) NLX

- Intel's NLX design, introduced in 1997, is an improvement on the LPX design for low-profile systems, with an emphasis on ease of maintenance. The NLX format is smaller, typically 8.8in wide by 13in long, so well suited for low-profile desktop cases.

NLX

6.) Flex ATX

- The FlexATX is a natural evolution of the Intel's microATX form factor which was first unveiled in late 1999. The FlexATX addendum to the microATX specification addresses the requirements of only the motherboard and not the overall system solution. As such, it does not detail the interfaces, memory or graphics technologies required to develop a successful product design. These are left to the implementer and system designer. The choice of processor is, however, limited to socket-only designs.

- The principal difference between FlexATX and microATX is that the new form factor reduces the size of the motherboard - to 9in x 7.5in. Not only does this result in lower overall system costs, it also facilitates smaller system designs. The FlexATX form factor is backwards compatible with both the ATX and micro-ATX specifications - use of the same motherboard mounting holes as both of its predecessors avoids the need to retool existing chassis.

Flex ATX

Form Factors & Sizes Name PCB Size (mm)

AT 350305

ATX 305244

Baby-AT 330216

BTX 325266

COM Express 12595

DTX 244203

EBX 203146

EPIC (Express) 165115

ESMexpress 12595

ETX / XTX 11495

FlexATX 229191

LPX 330229

microATX 244244

microATX 171171

Mini-DTX 203170

Mini-ITX 170170

mobile-ITX 7545

Nano-ITX 120120

NLX 254228

PC/104 (-Plus) 9690

Pico-ITX 10072

WTX 356425

EXPANSIONS SLOTS:

1.) Industry Standard Architecture (ISA)

2.) Peripheral Component Interconnect (PCI)

3.) Peripheral Component Interconnect Extended (PCIX)

4.) Peripheral Component Interconnect Express (PCIE)

5.) Accelerated Graphics Port (AGP)

6.) Micro Channel Architecture (MCA)

7.) VESA Local Bus (VL Bus or VLB)

1.) Industry Standard Architecture (ISA)

- a computer bus standard introduced by IBM in 1981

- the bus width is 8 or 16 bits

- data transfer was implemented parallel

- operated at 8MHz

Industry Standard Architecture (ISA)

Extended Industry Standard Architecture

3.) Peripheral Component Interconnect (PCI)

- a computer bus for attaching hardware devices in a computer introduced by Intel in 1993

- the bus width is 32 or 64 bits

- data transfer was implemented parallel

- operated at 133 MB/s

Peripheral Component Interconnect (PCI)

4.) Peripheral Component Interconnect Extended (PCIX)

- is a computer bus and expansion card standard that enhances the 32-bit PCI Local Bus

- a double-wide version of PCI, runs at four times faster clock speed but similar implementation and protocol

- developed jointly by IBM, HP, and Compaq in 1998

- the bus width was 64 bits, data transfer was implemented parallel, and operated at 1064 MB/s

Peripheral Component Interconnect Extended (PCIX)

5.) Peripheral Component Interconnect Express (PCIE)

- jointly developed by Intel, Dell, IBM, HP in 2004

- replaced the PCI and PCIX

- the bus width is 1-32 (called lanes), data transfer was implemented serially and operated at the following:

v1.x: 250 MB/s

v2.0: 500 MB/s

v3.0: 1 GB/s

- a "narrow but fast" serial connection instead of a "wide but slow" parallel connection

Peripheral Component Interconnect Express (PCIE)

Peripheral Component Interconnect Express (PCIE)

6.) Accelerated Graphics Port (AGP)

- a high speed channel used for attaching video cards to PC motherboards

- superseded by the PCIE

Accelerated Graphics Port (AGP)

7.) Micro Channel Architecture (MCA)

- a 16 or 32 bit parallel computer bus developed by IBM

- was developed in order to address problems in the Industry Standard Architecture (ISA)

Micro Channel Architecture (MCA)

8.) VESA Local Bus (VL Bus or VLB)

- Video Electronics Standards Association Local Bus

- worked alongside the ISA bus

- In the early 1990s the I/O bandwidth of the ISA bus was becoming a critical bottleneck to PC graphics performance. The need for faster graphics was being driven by increasing adoption of Graphical User Interfaces in PC operating systems.

- acted as a high-speed conduit for memory-mapped I/O and DMA, while the ISA bus handled interrupts and port-mapped I/O

VESA Local Bus (VL Bus or VLB)

BIOS (Basic Input/Output System)

- a software placed in a ROM chip on the motherboard (ROM BIOS)

- contains codes that can control the keyboard, display screen, disk drives, serial communications, and a number of miscellaneous functions

- inspects the computer to determine whether everything is functioning normally POST (power-on self test)

- assigns resources to peripherals and devices

BIOS (Basic Input/Output System)

- determines which drive to boot

- some BIOS are copied from ROM to RAM when the PC is booted shadowing

BIOS (Basic Input/Output System)

BIOS (Basic Input/Output System)

PROCESSOR

- is considered the brain of the computer and is where most of the calculations take place

- also called CPU (Central Processing Unit), central processor , processor or microprocessor

- contains the ALU (Arithmetic Logic Unit) and the CU (Control Unit)

ALU (Arithmetic Logic Unit)

- the part of a computer that performs all arithmetic computations, such as addition and multiplication, and all comparison operations.

CU (Control Unit)

- implements the microprocessor instruction set. It extracts instructions from memory and decodes and executes them, and sends the necessary signals to the ALU to perform the operation needed.

CHARACTERISTICS OF A MICROPROCESSOR INSTRUCTION SET - the set of instructions that a microprocessor can execute BANDWIDTH - the amount of data that can be processed at a single

instruction CLOCK SPEED - determines the amount of instructions that a processor

can execute at a given time (per second)

MICROPROCESSOR LAYOUT

TYPES OF MICROPROCESSOR ARCHITECTURE

1.) CISC (Complex Instruction Set Computer)

2.) RISC (Reduced Instruction Set Computer)

CISC (Complex Instruction Set Computer)

- an architecture in which the CPU can support as many as two hundred instructions or more.

- implemented in most personal computers

- can execute several low-level operations in a single instruction

RISC (Reduced Instruction Set Computer)

- a microprocessor architecture that recognizes only a limited number of instructions but executes them very fast since the instructions are very simple

- RISC chips are cheaper to design and produce since they only require fewer transistors

RISC

801

- a microprocessor prototype created by John Cocke in 1975

- was never marketed but plays a pivotal role in computer history, becoming the first RISC microprocessor

RISC RISC 1 and RISC 2 - The first "proper" RISC chips that were created at

Berkeley University in 1985 ARM - the technology was utilized by Intel in their processor

design - developed by Cambridge based Advanced Research

Machines (originally Acorn Research Machines) - powered the the old Acorn Archimedes and the Apple

Newton handwriting recognition systemsearch Machines)

OVERCLOCKING

- a technique for getting a performance boost from your system, without purchasing any additional hardware

- run a microprocessor faster than the speed for which it has been tested and approved

- also called speed margining

THINGS TO CONSIDER WHEN OVERCLOCKING:

- heat

- cooling system

- damage to hardware

- understanding of the different types of system memory

HEAT SINK

- a component designed to lower the temperature of an electronic device

- dissipates heat into the surrounding air - usually made up of aluminum alloy - take heat away from your components and put it somewhere

else - can be ACTIVE or PASSIVE

Active Heat Sink

- a heat sink that has a fan

Passive Heat Sink

- a heat sink that does not have a fan

Active Heat Sink Passive Heat Sink

IDEAL PC AIRFLOW

Common PC Cooling Techniques

Water Cooling

Liquid Nitrogen

Liquid Helium

Water Cooling

- coolant is used to transfer heat instead of air

- can be either in a separate unit outside of the PC case, or integrated within the PC case

- can be: Integrated, Internal or External

heat is transferred from the CPU into a "cooling block (a hollow heat sink with both an inlet and outlet for the liquid coolant)

heated coolant is then pumped into a reservoir

it travels into a radiator where it is cooled, typically by a fan

it is then pumped out again to the cooling block, and the cycle

Liquid Nitrogen

- a copper or aluminum pipe is mounted on top of the processor or graphics card

-the liquid nitrogen is poured into the pipe, resulting in temperatures well below -100C

- liquid nitrogen evaporates at -196 C, far below the freezing point of water, it is valuable as an extreme coolant

Liquid Helium

- colder than liquid nitrogen

- liquid helium evaporates at -269 C, and temperatures ranging from -230 to -240 C have been measured from the heat sink

1.) SERIAL PORT (COM PORT)

- one of the most basic external connections to a computer for more than two decades

- takes a byte of data and transmits the 8 bits in the byte one at a time

- it has the advantage of needing only one data line (wire) to transmit data.

- sends a start bit to signify the start of the data and a stop bit to signify the end of the data. It may also send a parity bit

1.) SERIAL PORT

- serial port communication is bi-directional (half duplex or full duplex)

- rely on a chip, UART (Universal Asynchronous Receiver Transmitter) transforms the parallel form of data into serial form of data

- most UART chips have built in buffer (16 to 64 KB)

- serial data transfer speed are from 115 Kbps to 460 Kbps

-

UART CHIP

Serial Port Pin Out

Serial Port Pin Out

TXD: Transmit Data; The data sent from the Data Terminal and received by the Data Set.

RXD: Receive Data; The data sent from the Data Set and received by the Data Terminal.

DTR: Data Terminal Ready; Used by the Data Terminal to signal to the Data Set that it is ready for operation, active high.

Serial Port Pin Out

DSR: Data Set Ready; Used by the Data Set to signal to the Data Terminal that it is ready for operation and ready to receive data, active high.

RTS: Request To Send; Used by the Data Terminal to signal the Data Set that it may begin sending data. The Data Set will not send out data with out this signal, active high.

CTS: Clear To Send; Used by the Data Set to signal the Data Terminal that it may begin sending data. The Data Terminal will not send out data with out this signal, active high.

Serial Port Pin Out

CD: Carrier Detect; Used by the Data Set to indicate to the Data Terminal that the Data set has detected a carrier (of another device).

RI: Ring Indicator; Used by the Data Set to indicate to the Data Terminal that a ringing condition has been detected.

GND: Ground; The common return for all signals on the interface.

Serial Port Pin Out

When using Software Flow Control(XON, XOFF); you only need 3 lines, TX (data), RX (data), and GND. XON being equal to ready, XOFF equal to not ready.

2.) Parallel Port

2.) Parallel Port

- was originally developed by IBM to connect a PC to a printer (Centronics)

- takes a byte and sends the entire 8 bits of data at a time

- capable of sending 50 to 100 KB of data in 1 second

- needs 8 data lines in order to send the 8 bits of data simultaneously

-

SPP (Standard Parallel Port)

EPP (Enhanced Parallel Port)

ECP (Extended Capabilities Port)

SPP (Standard Parallel Port)

- completely replaced the original parallel port design

- allows each device to receive and transmit data

- allows full-duplex communication (both directions at the same time)

- made use of pins 18-25

EPP (Enhanced Parallel Port)

- created by Intel, Xircom and Zenith in 1991

- allowed more data to be transferred per second (500 KB to 2MB data transfer)

- aimed at the attachment of non-printer devices, such as storage devices, to the PC

-

ECP (Extended Capabilities Port)

- was designed to provide improved speed and functionality for printers

Basic Operations Involved in Parallel Port Interfacing:

1. Write the byte to the Data Port.

2. Check to see is the printer is busy. If the printer is busy, it will not accept any data, thus any

data which is written will be lost.

3. Take the Strobe (Pin 1) low. This tells the printer that there is the correct data on the data

lines. (Pins 2-9)

4. Put the strobe high again after waiting approximately 5 microseconds after putting the strobe

low. (Step 3)

3.) USB Port

3.) USB Port

- USB (Universal Serial Bus)

- was aimed to allow large number of peripherals to be attached to a PC

- up to 127 devices and peripherals can be attached to the PC

- improved plug and play capabilities by allowing hot swapping

- USB ports can also supply electric power across the cable to devices without their own power source

- allowed many devices to be used without requiring manufacturer-specific device drivers to be installed

- became the standard connection method of most computer devices and peripherals

- The USB industry standard exists in multiple versions including 1.1, 2.0 and 3.0.

USB Port Versions:

USB 1.X

- was introduced in 1995 by Intel, Compaq, Microsoft and several other computer companies

- an external bus standard that supports data transfer rates of 12 Mbps

- effective range is up to 5 meters

USB 2.0

- Hi Speed USB

- was developed by Compaq, Hewlett Packard, Intel, Lucent, Microsoft, NEC and Philips and was introduced in 2001

- is capable of supporting a transfer rate of up to 480 Mbps

- backwards compatible

- effective range is 5 meters.....

USB 3.0

- Super Speed USB

- introduced on November 2008 by the USB 3.0 Promoter Group (Intel, Microsoft, Hewlett-Packard, Texas Instruments, NEC and NXP Semiconductors)

- ten times faster than USB 2.0 (4.8 Gbps)

- backwards compatible

- cable can have any length depending on the specifications but maximum speed can be achieved at 3 meters or less

USB Connectors

SERIES A

SERIES B

SERIES A AND SERIES B PIN OUTS

PIN WIRE COLOR FUNCTION

1 RED V BUS (+5)

2 WHITE D -

3 GREEN D +

4 BLACK GROUND

MINI USB SERIES A

MINI USB SERIES B

MINI USB SERIES A

PIN WIRE COLOR FUNCTION

1 RED V BUS (+5)

2 WHITE D -

3 GREEN D +

4 JOINED TO PIN 5

5 BLACK GROUND

MINI USB SERIES B

*pin 4 is sometimes joined to pin 5 via resistor

PIN WIRE COLOR FUNCTION

1 RED V BUS (+5)

2 WHITE D -

3 GREEN D +

4 NOT CONNECTED

5 BLACK GROUND

The USB Process

When the host powers up, it queries all of the devices connected to the bus and assigns each one an address. This process is called enumeration -- devices are also enumerated when they connect to the bus. The host also finds out from each device what type of data transfer it wishes to perform:

* Interrupt - A device like a mouse or a keyboard, which will be sending very little data, would choose the interrupt mode.

* Bulk - A device like a printer, which receives data in one big packet, uses the bulk transfer mode. A block of data is sent to the printer (in 64-byte chunks) and verified to make sure it is correct.

* Isochronous - A streaming device (such as speakers) uses the isochronous mode. Data streams between the device and the host in real-time, and there is no error correction.

The host can also send commands or query parameters with control packets.

As devices are enumerated, the host is keeping track of the total bandwidth that all of the isochronous and interrupt devices are requesting. They can consume up to 90 percent of the 480 Mbps of bandwidth that is available. After 90 percent is used up, the host denies access to any other isochronous or interrupt devices. Control packets and packets for bulk transfers use any bandwidth left over (at least 10 percent).

The Universal Serial Bus divides the available bandwidth into frames, and the host controls the frames. Frames contain 1,500 bytes, and a new frame starts every millisecond. During a frame, isochronous and interrupt devices get a slot so they are guaranteed the bandwidth they need. Bulk and control transfers use whatever space is left.

4.) FireWire

4.) FireWire

- also known as the IEEE 1394

- created by Apple

- similar in operation to the USB but is more into digital devices such as camcorders

- intended for devices that handle more data

- peer-to-peer

- efficient implementation for isochronous devices

FireWire 400 (IEEE 1394a)

- was faster than USB

- up to 400 Mbps data transfer rate

- up to 4.5 meters of cable length between devices

- was backwards compatible with IEEE 1394a

- uses a 6-pin configuration (2 for power and 4 for data lines), some only use 4-pin configuration

- power supply can be from 8 volts to 30 volts and a maximum of 1.5 amperes

FireWire 800 (IEEE 1394b)

- introduced in 2002

- up to 800 Mbps data transfer rate

- up to 100 meters cable length between devices

- uses a 9-pin configuration (6 pins are the same with IEEE 1394a, 2 pins are grounded shield, 1 pin is unused)

The FireWire Process When you turn on a computer, it automatically queries all

peripherals attached to it, and assigns each one an address in a process called enumeration. Uses the 64 bit addressing mode (IEEE 1212).

64 bits : 10 bits bus ID determines which FireWire bus

was the data generated from 6 bits physical ID determines which device sent the data 48 bits used for addressing data in every node

The 16 bits of node ID (bus ID and the physical ID) allows a system to have 64,000 nodes.

Allows data to be sent through 16 hops happens when a series of device are connected (daisy chain)

USB VS. FIREWIRE

PRIMARY STORAGE

- ROM (Read - Only Memory)

- RAM (Random - Access Memory)

SECONDARY STORAGE

DISKS

HARD DISK DRIVE

- stores and provides relatively quick access to large amounts of data on an electromagnetically charged surface or set of surfaces

- has data recorded electromagnetically in concentric circles or "tracks" on the disk

- have seek times of about 12 milliseconds or less

- disk drives improve their performance through a technique called caching.

- often called Winchester drive - name of one of the first popular hard disk drive technologies developed by IBM in 1973

PARTS OF HARD DISK

The Platters - The platters are the actual disks inside the drive that store the

magnetized data. Traditionally platters are made of a light aluminum alloy and coated with a magnetizable material such as a ferrite compound that is applied in liquid form and spun evenly across the platter or thin metal film plating that is applied to the platter through electroplating, the same way that chrome is produced. Newer technology uses glass and/or ceramic platters because they can be made thinner and also because they are more efficient at resisting heat. The magnetic layer on the platters has tiny domains of magnetization that are oriented to store information that is transferred through the read/write heads. Most drives have at least two platters, and the larger the storage capacity of the drive, the more platters there are. Each platter is magnetized on each side, so a drive with 2 platters has 4 sides to store data.

The Spindle and Spindle Motor

- The platters in a drive are separated by disk spacers and are clamped to a rotating spindle that turns all the platters in unison. The spindle motor is built right into the spindle or mounted directly below it and spins the platters at a constant set rate ranging from 3,600 to 7,200 RPM. The motor is attached to a feedback loop to ensure that it spins at precisely the speed it is supposed to.

The Read/Write Heads

- The read/write heads read and write data to the platters. There is typically one head per platter side, and each head is attached to a single actuator shaft so that all the heads move in unison. When one head is over a track, all the other heads are at the same location over their respective surfaces. Typically, only one of the heads is active at a time, i.e., reading or writing data. When not in use, the heads rest on the stationary platters, but when in motion the spinning of the platters create air pressure that lifts the heads off the platters. The space between the platter and the head is so minute that even one dust particle or a fingerprint could disable the spin. This necessitates that hard drive assembly be done in a clean room. When the platters cease spinning the heads come to rest, or park, at a predetermined position on the heads, called the landing zone.

The Head Actuator

- All the heads are attached to a single head actuator, or actuator arm, that moves the heads around the platters. Older hard drives used a stepper motor actuator, which moved the heads based on a motor reacting to stepper pulses. Each pulse moved the actuator over the platters in predefined steps. Stepper motor actuators are not used in modern drives because they are prone to alignment problems and are highly sensitive to heat. Modern hard drives use a voice coil actuator, which controls the movement of a coil toward or away from a permanent magnet based on the amount of current flowing through it. This guidance system is called a servo.

OPTICAL DISK DRIVE

- uses laser light or some electromagnetic light waves near the light spectrum in reading and writing data data on an optical disk.

- optical disk writers (burners) create bumps and dips on optical disks in order to signify ones and zeros when read by optical disk readers

- CD-ROM

- CD-RW

- DVD-ROM

- DVD?RW

- Blu-ray drives

DVD-R versus DVD+R

DVD-R

The DVD-R (pronounced "DVD dash R") and -RW media formats are officially approved by the standards group DVD Forum. The DVD Forum was founded by Mitsubishi, Sony, Hitachi, and Time Warner, so it has tremendous industry support for its technical standards.

DVD+R

DVD+R ("DVD plus R) and +RW formats are not approved by the DVD Forum standards group, but are instead supported by the DVD+RW Alliance. The DVD+RW Alliance is supported by Sony, Yamaha, Philips, Dell, and JP, so it also has tremendous industry support for its technical standards. Note that Sony supports both organizations.

What DVD+R can do (According to DVD Alliance):

1. Instantly eject without having to wait for finalized formatting.

2. Ability to record one DVD disc partially on PC and partially on television.

3. Background formatting: while the disc is being formatted, you can simultaneously record on already-formatted portions of the same disc.

4. Enhanced ability to edit filenames, movie and song titles, and playlists.

5. 100% compatibility with all other DVD players, while still enjoying these extra recording features.

BLU-RAY

is an optical disc storage medium designed to supersede the DVD format. Conventional Blu-ray Discs contain 25 GB per layer, with dual layer discs (50 GB) being the industry standard for feature-length video discs. The name Blu-ray Disc refers to the blue laser used to read the disc, which allows information to be stored at a greater density than is possible with the longer-wavelength red laser used for DVDs.

SINGLE LAYER VS DOUBLE LAYER