Universal Serial Bus (III)

USB packets

USB communication taks the form of packets. Initially, all packets are sent from the host, via the root hub and possibly more hubs, to devices. Some of those packets direct a device to send some packets in reply.

After the sync field described above, all packets are made of 8-bit bytes, transmitted least-significant bit first. The first byte is a packet identifier (PID) byte. The PID is actually 4 bits; the byte consists of the 4-bit PID followed by its bitwise complement. This redundancy helps detect errors. (Note also that a PID byte contains at most four consecutive 1 bits, and thus will never need bit-stuffing, even when combined with the final 1 bit in the sync byte. However, the OUT PID byte ends with three consecutive 1 bits, so if the following USB device address begins with three 1 bits, bit-stuffing will be required.)

Packets come in three basic types, each with a different format and CRC (cyclic redundancy check):

Handshake packets

Handshake packets consist of nothing but a PID byte, and are generally sent in response to data packets. The three basic types are ACK, indicating that data was successfully received, NAK, indicating that the data cannot be received at this time and should be retried, and STALL, indicating that the device has an error and will never be able to successfully transfer data until some corrective action (such as device initialization) is performed.

USB 2.0 added two additional handshake packets, NYET which indicates that a split transaction is not yet complete, and an ERR handshake to indicate that a split transaction failed.

The only handshake packet the USB host may generate is ACK; if it is not ready to receive data, it should not instruct a device to send any.

Token packets

Token packets consist of a PID byte followed by 11 bits of address and a 5-bit CRC. Tokens are only sent by the host, never a device.-- IN and OUT tokens contain a 7-bit device number and 4-bit function number (for multifunction devices) and command the device to transmit DATAx packets, or receive the following DATAx packets, respectively.

An IN token expects a response from a device. The response may be a NAK or STALL response, or a DATAx frame. In the latter case, the host issues an ACK handshake if appropriate.

An OUT token is followed immediately by a DATAx frame. The device responds with ACK, NAK, or STALL, as appropriate.

SETUP operates much like an OUT token, but is used for initial device setup.

Every 1 ms (12000 full-speed bit times), the USB host transmits a special SOF (start of frame) token, containing an 11-bit incrementing frame number in place of a device address. This is used to synchronize isochronous data flows. High-speed USB 2.0 devices receive 7 additional duplicate SOF tokens per frame, each introducing a 125 µs "microframe".

USB 2.0 added a PING token, which asks a device if it is ready to receive an OUT/DATA packet pair. The device responds with ACK, NAK, or STALL, as appropriate. This avoids the need to send the DATA packet if the device knows that it will just respond with NAK.

USB 2.0 also added a larger SPLIT token with a 7-bit hub number, 12 bits of control flags, and a 5-bit CRC. This is used to perform split transactions. Rather than tie up the high-speed USB bus sending data to a slower USB device, the nearest high-speed capable hub receives a SPLIT token followed by one or two USB packets at high speed, performs the data transfer at full or low speed, and provides the response at high speed when prompted by a second SPLIT token. The details are complex; see the USB specification.

Data packets

There are two basic data packets, DATA0 and DATA1. Both consist of a DATAx PID field, 0–1023 bytes of data payload (up to 1024 in high speed, at most 8 at low speed), and a 16-bit CRC. They must always be preceded by an address token, and are usually followed by a handshake token from the receiver back to the transmitter. The two packet types provide the 1-bit sequence number required by Stop-and-wait ARQ. If a USB host does not receive a response (such as an ACK) for data it has transmitted, it does not know if the data was received or not; the data might have been lost in transit, or it might have been received but the handshake response was lost.

To solve this problem, the device keeps track of the type of DATAx packet it last accepted. If it receives another DATAx packet of the same type, it is acknowledged but ignored as a duplicate. Only a DATAx packet of the opposite type is actually received.

When a device is reset with a SETUP packet, it expects a DATA0 packet next.

USB 2.0 added DATA2 and MDATA packet types as well. They are used only by high-speed devices doing high-bandwidth isochronous transfers which need to transfer more than 1024 bytes per 125 µs "microframe" (8192 kB/s).

PRE "packet"

Low-speed devices are supported with a special PID value, PRE. This marks the beginning of a low-speed packet, and is used by hubs which normally do not send full-speed packets to low-speed devices. Since all PID bytes include four 0 bits, they leave the bus in the full-speed K state, which is the same as the low-speed J state. It is followed by a brief pause during which hubs enable their low-speed outputs, already idling in the J state, then a low-speed packet follows, beginning with a sync sequence and PID byte, and ending with a brief period of SE0. Full-speed devices other than hubs can simply ignore the PRE packet and its low-speed contents, until the final SE0 indicates that a new packet follows....... Continue

Data and source mostly taken from Wikipedia

Universal Serial Bus (IV)

The connectors specified by the USB committee were designed to support a number of USB's underlying goals, and to reflect lessons learned from the varied menagerie of connectors then in service

Usability

• It is difficult to incorrectly attach a USB connector. Connectors cannot be plugged-in upside down, and it is clear from the appearance and kinesthetic sensation of making a connection when the plug and socket are correctly mated. However, it is not obvious at a glance to the inexperienced user (or to a user without sight of the installation) which way around the connector goes, so it is often necessary to try both ways. More often than not, however, the side of the connector with the "trident" logo should be on top.
• Only a moderate insertion/removal force is needed (by specification). USB cables and small USB devices are held in place by the gripping force from the receptacle (without the need for the screws, clips, or thumbturns that other connectors require). The force needed to make or break a connection is modest, allowing connections to be made in awkward circumstances or by those with motor disabilities.
• The connectors enforce the directed topology of a USB network. USB does not support cyclical networks, so the connectors from incompatible USB devices are themselves incompatible. Unlike other communications systems (e.g. RJ-45 cabling) gender-changers are almost never used, making it difficult to create a cyclic USB network.

Durability

• The connectors are designed to be robust. Many previous connector designs were fragile, with pins or other delicate components prone to bending or breaking, even with the application of only very modest force. The electrical contacts in a USB connector are protected by an adjacent plastic tongue, and the entire connecting assembly is usually further protected by an enclosing metal sheath. As a result USB connectors can safely be handled, inserted, and removed, even by a small child.
• The connector construction always ensures that the external sheath on the plug contacts with its counterpart in the receptacle before the four connectors within are connected. This sheath is typically connected to the system ground, allowing otherwise damaging static charges to be safely discharged by this route (rather than via delicate electronic components). This means of enclosure also means that there is a (moderate) degree of protection from electromagnetic interference afforded to the USB signal while it travels through the mated connector pair (this is the only location when the otherwise twisted data pair must travel a distance in parallel). In addition, the power and common connections are made after the system ground but before the data connections. This type of staged make-break timing allows for safe hot-swapping and has long been common practice in the design of connectors in the aerospace industry.
• The newer USB micro receptacles are designed to allow up to 10,000 cycles of insertion and exertion between the receptacle and plug, compared to 500 for the standard USB and Mini-USB receptacle. This is accomplished by adding a locking device and by moving the leaf-spring connector from the jack to the plug, so that the most-stressed part is on the cable side of the connection. This change was made so that the connector on the (relatively inexpensive) cable would bear the most wear instead of the micro-USB device.

Compatibility

• The USB standard specifies relatively loose tolerances for compliant USB connectors, intending to minimize incompatibilities in connectors produced by different vendors (a goal that has been very successfully achieved). Unlike most other connector standards, the USB specification also defines limits to the size of a connecting device in the area around its plug. This was done to prevent a device from blocking adjacent ports due to its size. Compliant devices must either fit within the size restrictions or support a compliant extension cable which does.

Two-way communication is also possible. In general, cables have only plugs, and hosts and devices have only receptacles: hosts having type-A receptacles and devices type-B. Type-A plugs only mate with type-A receptacles, and type-B with type-B. However, an extension to USB called USB On-The-Go allows a single port to act as either a host or a device — chosen by which end of the cable plugs into the socket on the unit. Even after the cable is hooked up and the units are talking, the two units may "swap" ends under program control. This facility targets units such as PDAs where the USB link might connect to a PC's host port as a device in one instance, yet connect as a host itself to a keyboard and mouse device in another instance.

Types of USB connector

There are several types of USB connectors, including some that have been added as the specification has progressed. The original USB specification detailed Standard-A and Standard-B plugs and receptacles. The first engineering change notice to the USB 2.0 specification added Mini-B plugs and receptacles. The data slots in the A - Plug are actually farther in the plug than the outside power wires to prevent data errors by powering the device first, then transferring data.

The Standard-A type of USB connectors takes on the appearance of flattened rectangles that plugs into downstream-port sockets on the USB host or a hub. This kind of connector is most frequently seen on cables that are permanently attached to a device, such as one on a cable that connects a keyboard or mouse to the computer. Standard-B connectors looks square with beveled corners, and plugs into upstream sockets on devices and hubs. The Standard-B connector is mainly used only for the device end of a removable cable, such as between a hub and a printer. This two-connector scheme prevents a user from accidentally creating a loop.

The non-standard Mini-USB's, official Mini-B, Micro-A, and Micro-B connectors are used for smaller devices such as PDAs, mobile phones or digital cameras. The Standard-A plug is approximately 4 by 12 mm, the Standard-B approximately 7 by 8 mm, and the Mini-A and Mini-B plugs approximately 2 by 7 mm.

The Micro-USB connector, was announced by the USB-IF on January 4, 2007. It is intended to replace the Mini-USB plugs used in many new smartphones and Personal digital assistants. This Micro-USB plug is rated for 10,000 connect-disconnect cycles. It is about half the height of the mini-USB connector, but features a similar width. In the Universal Serial Bus Micro-USB Cables and Connectors Specification, details have been laid down for Micro-A plugs, Micro-AB receptacles, and Micro-B plugs and receptacles, along with a Standard-A receptacle to Micro-A plug adapter. The carrier led group OMTP have recently endorsed micro-USB as the standard connector for data and power on mobile devices

Data and source mostly taken from Wikipedia

Universal Serial Bus (II)

In the version 1.x age, there were two competing HCD implementations, Open Host Controller Interface (OHCI) and Universal Host Controller Interface (UHCI). OHCI was developed by Compaq, Microsoft and National Semiconductor; UHCI was by Intel.

VIA Technologies licensed the UHCI standard from Intel; all other chipset implementers use OHCI. UHCI is more software-driven, making UHCI slightly more processor-intensive than OHCI but cheaper to implement. The dueling implementations forced operating system vendors and hardware vendors to develop and test on both implementations, which increased cost.

HCD standards are out of the USB specification's scope, and the USB specification does not specify any HCD interfaces. In other words, USB defines the format of data transfer through the port, but not the system by which the USB hardware communicates with the computer it sits in.

During the design phase of USB 2.0, the USB-IF insisted on only one implementation. The USB 2.0 HCD implementation is called the Enhanced Host Controller Interface (EHCI). Only EHCI can support hi-speed (480 Mbit/s) transfers. Most of PCI-based EHCI controllers contain other HCD implementations called 'companion host controller' to support Full Speed (12 Mbit/s) and may be used for any device that claims to be a member of a certain class. An operating system is supposed to implement all device classes so as to provide generic drivers for any USB device. Device classes are decided upon by the Device Working Group of the USB Implementers Forum.

USB implements connections to storage devices using a set of standards called the USB mass storage device class (referred to as MSC or UMS). This was initially intended for traditional magnetic and optical drives, but has been extended to support a wide variety of devices, particularly flash drives. This generality is because many systems can be controlled with the familiar idiom of file manipulation within directories (The process of making a novel device look like a familiar device is also known as extension).

Though most newer computers are capable of booting off USB Mass Storage devices, USB is not intended to be a primary bus for a computer's internal storage: buses such as ATA (IDE), Serial ATA (SATA), and SCSI fulfill that role. However, USB has one important advantage in that it is possible to install and remove devices without opening the computer case, making it useful for external drives. Originally conceived and still used today for optical storage devices (CD-RW drives, DVD drives, etc.), a number of manufacturers offer external portable USB hard drives, or empty enclosures for drives, that offer performance comparable to internal drives. These external drives usually contain a translating device that interfaces a drive of conventional technology (IDE, ATA, SATA, ATAPI, or even SCSI) to a USB port. Functionally, the drive appears to the user just like another internal drive. Other competing standards that allow for external connectivity are eSATA and FireWire.

Another use for USB Mass Storage devices is the portable running of software applications without the need of installation on the host computer, eg. Web Browser, VoIP, etc.

Human-interface devices (HIDs)

Mice and keyboards are frequently fitted with USB connectors, but because most PC motherboards still retain PS/2 connectors for the keyboard and mouse as of 2007, they are often supplied with a small USB-to-PS/2 adaptor, allowing usage with either USB or PS/2 interface. There is no logic inside these adaptors: they make use of the fact that such HID interfaces are equipped with controllers that are capable of serving both the USB and the PS/2 protocol, and automatically detect which type of port they are plugged into. Joysticks, keypads, tablets and other human-interface devices are also progressively migrating from MIDI, PC game port, and PS/2 connectors to USB.

Apple Macintosh computers have been using USB exclusively for all external wired mice and keyboards since January 1999. The original iMac raised public awareness of USB considerably in August 1998, as it discarded legacy ports to use only USB. PCs had USB ports prior to the iMac's introduction, but they were included with a full complement of traditional ports which slowed down USB's adoption. The iMac's influence can be seen in the number of USB peripherals with matching translucent, colored plastic enclosures that were available in the late '90s and early '00s.

USB signaling

USB supports three data rates:

• The Full Speed rate of 12 Mbit/s (1.5 MB/s) is the basic USB data rate defined by USB 1.0. All USB hubs support Full Speed.
• A Low Speed rate of 1.5 Mbit/s (187.5 kB/s) is also defined by USB 1.0. It is very similar to full speed operation except that each bit takes 8 times as long to transmit. It is intended primarily to save cost in low-bandwidth Human Interface Devices (HID) such as keyboards, mice, and joysticks.
• A High-Speed (USB 2.0) rate of 480 Mbit/s (60 MB/s) was introduced in 2001. All high-speed devices are capable of falling back to full-speed operation if necessary.

Experimental data rate:

• A Super-Speed (USB 3.0) rate of 4.8 Gbit/s (600 MB/s). The USB 3.0 specification was released by Intel and its partners in August 2008, according to early reports from CNET news. Products using the 3.0 specification are likely to arrive in 2009 or 2010.

USB signals are transmitted on a twisted pair data cable with 90Ω ±15% impedance, labeled D+ and D−. These collectively use half-duplex differential signaling to combat the effects of electromagnetic noise on longer lines. Transmitted signal levels are 0.0–0.3 volts for low and 2.8–3.6 volts for high in Full Speed (FS) and Low Speed (LS) modes, and -10–10 mV for low and 360–440 mV for high in High Speed (HS) mode. In FS mode the cable wires are not terminated, but the HS mode has termination of 45Ω to ground, or 90Ω differential to match the data cable impedance.

A USB connection is always between a host or hub at the "A" connector end, and a device or hub's upstream port at the other end. The host includes 15 kΩ pull-down resistors on each data line. When no device is connected, this pulls both data lines low into the so-called "single-ended zero" state (SE0 in the USB documentation), and indicates a reset or disconnected connection.

A USB device pulls one of the data lines high with a 1.5 kΩ resistor. This overpowers one of the pull-down resistors in the host and leaves the data lines in an idle state called "J". The choice of data line indicates a device's speed support; full-speed devices pull D+ high, while low-speed devices pull D− high.

USB data is transmitted by toggling the data lines between the J state and the opposite K state. USB encodes data using the NRZI convention; a 0 bit is transmitted by toggling the data lines from J to K or vice-versa, while a 1 bit is transmitted by leaving the data lines as-is. To ensure a minimum density of signal transitions, USB uses bit stuffing; an extra 0 bit is inserted into the data stream after any appearance of six consecutive 1 bits. Seven consecutive 1 bits is always an error.

A USB frame begins with an 8-bit synchronization sequence 00000001. That is, after the initial idle state J, the data lines toggle KJKJKJKK. The final 1 bit (repeated K state) marks the end of the sync pattern and the beginning of the USB frame proper.

A USB frame's end, called EOP (end-of-packet), is indicated by the transmitter driving 2 bit times of SE0 (D+ and D- both below Vil max) and 1 bit time of J state. After this, the transmitter ceases to drive the D+/D− lines and the aforementioned resistors hold it in the J (idle) state. A receiver may take extra time to decode the SE0 state, and will see the first bit time as a repetition of the last data bit. Since USB frames are always a multiple of 8 bits long, this extra "dribble bit" can be detected and ignored.

A USB bus is reset using a prolonged (10 to 20 milliseconds) SE0 signal.

USB 2.0 devices use a special protocol during reset, called "chirping", to negotiate the High-Speed mode with the host/hub. A Device that is HS capable first connects as an FS device (D+ pulled high), but upon receiving a USB RESET (both D+ and D- driven LOW by host for 10 to 20 mS) it pulls the D- line high. If the host/hub is also HS capable, it chirps (returns alternating J and K states on D- and D+ lines) letting the device know that the hub will operate at High Speed.

Clock tolerance is 480.00 Mbit/s ±500ppm, 12.000 Mbit/s ±2500 ppm, 1.50 Mbit/s ±15000 ppm.

Though Hi-Speed devices are commonly referred to as "USB 2.0" and advertised as "up to 480 Mbit/s", not all USB 2.0 devices are Hi-Speed. The USB-IF certifies devices and provides licenses to use special marketing logos for either "Basic-Speed" (low and full) or Hi-Speed after passing a compliance test and paying a licensing fee. All devices are tested according to the latest spec, so recently-compliant Low-Speed devices are also 2.0 devices.

The actual throughput currently (2006) attained with real devices is about two thirds of the maximum theoretical bulk data transfer rate of 53.248 MB/s. Typical hi-speed USB devices operate at lower speeds, often about 3 MB/s overall, sometimes up to 10–20 MB/s.

.... Continue

Data and source mostly taken from Wikipedia

USB Flash Drive (II)

File system

Most flash drives ship preformatted with the FAT or FAT 32 File system. The ubiquity of this file system allows the drive to be accessed on virtually any host device with USB support. Also, standard FAT maintenance utilities (e.g.ScanDisk ) can be used to repair or retrieve corrupted data. However, because a flash drive appears as a USB-connected hard drive to the host system, the drive can be reformatted to any file system supported by the host operating system.

Flash drives can be defragmented, but this brings little advantage as there is no mechanical head slowed down by having to move from fragment to fragment (flash drives often have very large internal sector size, especially when erasing so defragmenting means accessing fewer sectors to erase a file). Defragmenting shortens the life of the drive by making many unnecessary writes.

Some file systems are designed to distribute usage over an entire memory device without concentrating usage on any part (e.g., for a directory); this prolongs life of simple flash memory devices. USB flash drives, however, have this functionality built into the controller to prolong device life, and use of such a file system brings less advantage

Uses

Personal data transport

The most common use of flash drives is to transport and store personal files such as documents, pictures and videos. Individuals also store medical alert information on MedicTag flash drives for use in emergencies and for disaster preparation.

Secure storage of data, application and software files

With wide deployment(s) of flash drives being used in various environments (secured or otherwise), the issue of data and information security remains of the utmost importance. The use of biometrics and encryption is becoming the norm with the need for increased security for data; OTFE systems such asFreeOTFE and TrueCrypt are particularly useful in this regard, as they can transparently encrypt large amounts of data. In some cases a Secure USB Drive may use a hardware-based encryption mechanism that uses a hardware module instead of software for strongly encrypting data.

System administration

Flash drives are particularly popular among system and network administrators, who load them with configuration information and software used for system maintenance, troubleshooting, and recovery.

Computer repair

Flash drives enjoy notable success in the PC repair field as a means to transfer recovery and antivirus software to infected PCs, while allowing a portion of the host machine's data to be archived in case of emergency. As the drives have increased in storage space, they have also replaced the need to carry a number of CD ROMs and installers which were needed when reinstalling or updating a system.

Application carriers

Flash drives are used to carry applications that run on the host computer without requiring installation. While any standalone application can in principle be used this way, many programs store data, configuration information, etc. on the hard drive and registry of the host computer

The U3 company works with drive makers (parent company SanDisk as well as others) to deliver custom versions of applications designed for Microsoft Windows from a special flash drive; U3-compatible devices are designed to autoload a menu when plugged into a computer running Windows. Applications must be modified for the U3 platform and not to leave any data on the host machine. U3 also provides a software framework for ISVs interested in their platform.

Ceedo is an alternative product with the key difference that it does not require Windows applications to be modified in order for them to be carried and run on the drive.

Similarly, other application virtualization solutions, such as VMware ThinApp can be used to run software from a flash drive without installation.

A range of portable applications which are all free of charge and able to run off a computer running Windows without storing anything on the host computer's drives or registry is available from portableapps.com; unlike U3 programs which run from a special U3-compatible USB stick, the PortableApps menu will run from a standard device, but does not use the WindowsAutoRun feature.

Computer forensics and law enforcement

A recent development for the use of a USB Flash Drive as an application carrier is to carry the Computer Online Forensic Evidence Extractor (COFEE) application developed by Microsoft. COFEE is a set of applications designed to search for and extract digital evidence on computers confiscated from suspects. Forensic software should not alter the information stored on the computer being examined in any way; other forensic suites run from CD-ROM or DVD-ROM, but cannot store data on the media they are run from (although they can write to other attached devices such as external drives or memory sticks).

Booting operating systems

Most current PC firmware permits booting from a USB drive, allowing the launch of an operating system from a bootable flash drive. Such a configuration is known as a Live USB.

While a Live USB could be used for general-purpose applications, size and memory wear make them poor choices compared to alternatives. They are more suited to special-purpose or temporary tasks, such as:

• Loading a minimal, hardened kernel for embedded applications (e.g. network router, firewall).
• Bootstrapping an operating system install or disk cloning operation, often across a network.
• Maintenance tasks, such as virus scanning or low-level data repair, without the primary host operating system loaded.

Audio players

Many companies make small solid-state digital audio players, essentially producing flash drives with sound output and a simple user interface. Examples include the Creative MuVo and the iPod shuffle. Some of these players are true USB flash drives as well as music players; others do not support general-purpose data storage.

Many of the smallest players are powered by a permanently fitted rechargeable battery, charged from the USB interface.

Data and source mostly taken from Wikipedia

Sound Card (II)

Since digital sound playback has become available and provided better performance than synthesis, modern soundcards with hardware polyphony don't actually use DACs with as many channels as voices, but rather perform voice mixing and effects processing in hardware (eventually performing digital filtering and conversions to and from the frequency domain for applying certain effects) inside a dedicated DSP. The final playback stage is performed by an external (in reference to the DSP chip(s)) DAC with significantly fewer channels than voices (e.g., 8 channels for 7.1 audio, which can be divided among 32, 64 or even 128 voices).

Sound cards for computers compatible with the IBM PC were very uncommon until 1988, which left the single internal PC speaker as the only way early PC software could produce sound and music. The speaker hardware was typically limited to square waves, which fit the common nickname of "beeper". The resulting sound was generally described as "beeps and boops". Several companies, most notably Access Software, developed techniques for digital sound reproduction over the PC speaker; the resulting audio, while baldly functional, suffered from distorted output and low volume, and usually required all other processing to be stopped while sounds were played. Other home computer models of the 1980s included hardware support for digital sound playback, or music synthesis (or both), leaving the IBM PC at a disadvantage to them when it came to multimedia applications such as music composition or gaming.

It is important to note that the initial design and marketing focuses of sound cards for the IBM PC platform were not based on gaming, but rather on specific audio applications such as music composition (AdLib Personal Music System, Creative Music System, IBM Music Feature Card) or on speech synthesis (Digispeech DS201, Covox Speech Thing, Street Electronics Echo). Only until Sierra and other game companies became involved in 1988 was there a switch toward gaming.

One of the first manufacturers of sound cards for the IBM PC was AdLib, who produced a card based on the Yamaha YM3812 sound chip, aka the OPL2. The AdLib had two modes: A 9-voice mode where each voice could be fully programmed, and a less frequently used "percussion" mode with 3 regular voices producing 5 independent percussion-only voices for a total of 11. (The percussion mode was considered inflexible by most developers; it was used mostly by AdLib's own composition software.)

Creative Labs also marketed a sound card about the same time called the Creative Music System. Although the C/MS had twelve voices to AdLib's nine, and was a stereo card while the AdLib was mono, the basic technology behind it was based on the Philips SAA 1099 chip which was essentially a square-wave generator. It sounded much like twelve simultaneous PC speakers would have, and failed to sell well, even after Creative renamed it the Game Blaster a year later, and marketed it through Radio Shack in the US. The Game Blaster retailed for under $100 and included the hit gameSlipheed.

A large change in the IBM PC compatible sound card market happened with Creative Labs' introduced the Sound Blaster card. The Sound Blaster cloned the AdLib, and added a sound coprocessor for recording and play back of digital audio (likely to have been an Intel microcontroller relabeled by Creative). It was incorrectly called a "DSP" to suggest it was a digital signal processor), a game port for adding a joystick, and capability to interface to MIDI equipment (using the game port and a special cable). With more features at nearly the same price, and compatibility as well, most buyers chose the Sound Blaster. It eventually outsold the AdLib and dominated the market.

The Sound Blaster line of cards, together with the first inexpensive CD-ROM drives and evolving video technology, ushered in a new era of multimedia computer applications that could play back CD audio, add recorded dialogue to computer games, or even reproduce motion video (albeit at much lower resolutions and quality in early days). The widespread decision to support the Sound Blaster design in multimedia and entertainment titles meant that future sound cards such as Media Vision's Pro Audio Spectrum and the Gravis Ultrasound had to be Sound Blaster compatible if they were to sell well. Until the early 2000s (by which the AC'97 audio standard became more widespread and eventually usurped the SoundBlaster as a standard due to its low cost and integration into many motherboards), Sound Blaster compatibility is a standard that many other sound cards still support to maintain compatibility with many games and applications released.

Data and source mostly taken from Wikipedia

Keyboard (II)

Key types

Alphanumeric keys

Alphabetical, numeric, and punctuation keys are used in the same fashion as a typewriter keyboard to enter their respective symbol into a word processing program, text editor, data spreadsheet, or other program. Many of these keys will produce different symbols when modifier keys or shift keys are pressed. The alphabetic characters become uppercase when the shift key or Caps Lock key is depressed. The numeric characters become symbols or punctuation marks when the shift key is depressed. The alphabetical, numeric, and punctuation keys can also have other functions when they are pressed at the same time as some modifier keys.

The Space bar is a horizontal bar in the lowermost row, which is significantly wider than other keys. Like the alphanumeric characters, it is also descended from the mechanical typewriter. Its main purpose is to enter the space between words during typing. It is large enough so that a thumb from either hand can use it easily. Depending on the operating system, when the space bar is used with a modifier key such as the control key, it may have functions such as resizing or closing the current window, half-spacing, or backspacing. In computer games and other applications the key has myriad uses in addition to its normal purpose in typing, such as jumping and adding marks to check boxes. In certain programs for playback of digital video, the space bar is used for pausing and resuming the playback.

Special keys

Modifier keys are special keys on a computer keyboard that modify the normal action of another key when the two are pressed in combination. For example, + in Microsoft Windows will close the program in an active window. In contrast, pressing just will probably do nothing unless assigned a specific function in a particular program. By themselves, modifier keys usually do nothing. The most widely-used modifier keys include the Control key, Shift key, and the Alt key. The AltGr key is used to access additional symbols for keys that have three symbols printed on them. On the Macintosh and Apple keyboards, the modifier keys are the Option key and Command key, respectively. On MIT computer keyboards, the Meta key is used as a modifier, and for Windows keyboards, there is a Windows key. Compact keyboard layouts often use a Fn key. "Dead keys" allow placement of diacritic mark such as an accent) on the following letter (e.g., the Compose key).

The Return key / Enter key (rarely "Execute") typically causes a command line, window form or dialog box to operate its default function, which is typically to finish an "entry" and begin the desired process. In word processing applications, pressing the enter key ends a paragraph and starts a new one.

Navigation and typing mode keys

Navigation keys include a variety of keys which move the cursor to different positions on the screen. Arrow keys are programmed to move the cursor in a specified direction; Page scroll keys, such as Page up key and the Page down key, scroll the page up and down. The Home key is used to return the cursor to the beginning of the line where the cursor is located; the End key puts the cursor at the end of the line. The Tab key advances the cursor to the next tab stop.

The Insert key is mainly used to switch between overtype mode, in which the cursor overwrites any text that is present on and after its current location, and insert mode, where the cursor inserts a character at its current position, forcing all characters past it one position further. The Delete key discards the character ahead of the cursor's position, moving all following characters one position "back" towards the freed place. On many notebook computer keyboards the key labeled Delete (sometimes Delete and Backspace are printed on the same key) serves the same purpose as a Backspace key. The Backspace key deletes the preceding character.

Lock keys lock part of a keyboard, depending on the settings selected. The lock keys are scattered around the keyboard. Most styles of keyboards have three LEDs indicating which locks are enabled, in the upper right corner above the numpad. The lock keys include Scroll lock, Num lock (which allows the use of the numeric keypad), and Caps lock.

System command keys

The SysRq / Print screen commands often share the same key. SysRq was used in earlier computers as a "panic" button to recover from crashes. The Print screen command typically captures the entire screen and sends it to the printer. The Break/Pause key no longer has a well-defined purpose. Its origins go back to teletype users, who wanted a key that would temporarily interrupt the communications line. The Break key can be used by software in several different ways, such as to switch between multiple login sessions, to terminate a program, or to interrupt a modem connection. In programming, especially old DOS-style BASIC, Pascal and C, Break is used (in conjunction with Ctrl) to stop program execution. In addition to this, Linux and variants, as well as many DOS programs, treat this combination the same as Ctrl+C. On modern keyboards, the break key is usually labeled Pause/Break. In most Windows environments, the key combination Windows key+Pause brings up the system properties.

The Escape key (often abbreviated Esc) is used to initiate an escape sequence. As most computer users no longer are concerned with the details of controlling their computer's peripherals, the task for which the escape sequences were originally designed, the escape key was appropriated by application programmers, most often to mean Stop. This use continues today in Microsoft Windows's use of escape as a shortcut in dialog boxes for No, Quit, Exit, Cancel, or Abort. A common application today of the Esc key is as a shortcut key for the Stop button in many web browsers. On machines running Microsoft Windows, prior to the implementation of the Windows key on keyboards, the typical practice for invoking the "start" button was to hold down the control key and press escape. This process still works in Windows XP and Windows Vista.

The Menu key or Application key is a key found on Windows-oriented computer keyboards. It is used launch a context menu with the keyboard rather than with the usual right mouse button. The key's symbol is a small icon depicting a cursor hovering above a menu. This key was created at the same time as the Windows key. This key is normally used when the right mouse button is not present on the mouse. Some Windows public terminals do not have a Menu key on their keyboard to prevent users from right-clicking (however, in many windows applications, a similar functionality can be invoked with the Shift+F10 keyboard shortcut).

Miscellaneous keys

Many, but not all computer keyboards have a numeric keypad to the right of the alphabetic keyboard. On Japanese/Korean keyboards, there may be Language input keys. Some keyboards have power management keys (e.g., Power key, Sleep key, and Wake key); Internet keys to access a web browser or E-mail; and/or multimedia keys such as volume controls.

Data and source mostly taken from Wikipedia

Bluetooth (II)

Bluetooth protocol stack

“Bluetooth is defined as a layer protocol architecture consisting of core protocols, cable replacement protocols, telephony control protocols, and adopted protocols”.

Mandatory protocols for all bluetooth stacks are: LMP, L2CAP and SDP

Additionally, these protocols are almost universally supported: HCI and RFCOMM

LMP (Link Management Protocol)

Used for control of the radio link between two devices. Implemented on the controller.

HCI (Host/Controller Interface)

Standardised communication between the Host stack (e.g. a PC or mobile phone OS) and the Controller (the bluetooth I.C.) This standard allows the host stack or controller I.C. to be swapped with minimal adaptation.

There are several HCI transport layer standards, each using a different hardware interface to transfer the same command, event and data packets. The most commonly used are USB (in PCs) and UART (in mobile phones and PDAs).

In bluetooth devices with simple functionality, e.g. headsets, the host stack and controller can be implemented on the same microprocessor. In this case the HCI is optional, although often implemented as an internal software interface.

L2CAP (Logical Link Control & Adaptation Protocol)

Used to multiplex multiple logical connections between two devices using different higher level protocols. Provides segmentation and reassembly of on-air packets. In basic mode, L2CAP provides reliable sequenced packets with a payload configurable up to 64kB, with 672 bytes as the minimum mandatory supported size. In retransmission & flow control modes, L2CAP can be configured for reliable or isochronous data per channel by configuring the number of retransmissions and flush timeout.

The EL2CAP specification adds an additional "enhanced mode" to the core specification, which is an improved version of retransmission & flow control modes.

SDP (Service Discovery Protocol)

Used to allow devices to discover what services each other support, and what parameters to use to connect to them. For example, when connecting a mobile phone to a bluetooth headset, SDP will be used to determine which bluetooth profiles are supported by the headset (Headset Profile, Hands Free Profile, Advanced Audio Distribution Profile etc) and the protocol multiplexor settings needed to connect to each of them. Each service is identified by a Universally Unique Identifier (UUID), with official services (Bluetooth profiles) assigned a short form UUID (16 bits rather than the full 128)

RFCOMM (Cable replacement protocol)

Radio frequency communications (RFCOMM) is the cable replacement protocol used to create a virtual serial port used to make replacement of cable technologies transparent through minimal modification of existing devices. RFCOMM provides for binary data transport and emulates EIA-232 (formerly RS-232) control signals over the Bluetooth baseband layer.

RFCOMM provides a simple reliable data stream to the user, similar to TCP. It is used directly by many telephony related profiles as a carrier for AT commands, as well as being a transport layer for OBEX over bluetooth.

Many bluetooth applications use RFCOMM because of its widespread support and publicly available API on most operating systems. Additionally, applications that used a serial port to communicate can be quickly ported to use RFCOMM.

BNEP (Bluetooth Network Encapsulation Protocol)

BNEP is used to transfer another protocol stack's data via an L2CAP channel. Its main purpose is the transmission of IP packets in the Personal Area Networking Profile. BNEP performs a similar function to SNAP in Wireless LAN.

AVCTP (Audio/Visual Control Transport Protocol)

Used by the remote control profile to transfer AV/C commands over an L2CAP channel. The music control buttons on a stereo headset use this protocol to control the music player

AVDTP (Audio/Visual Data Transport Protocol)

Used by the advanced audio distribution profile to stream music to stereo headsets over an L2CAP channel. Intended to be used by video distribution profile.

Telephony control protocol

Telephony control protocol-binary (TCS BIN) is the bit-oriented protocol that defines the call control signaling for the establishment of voice and data calls between Bluetooth devices. Additionally, “TCS BIN defines mobility management procedures for handling groups of Bluetooth TCS devices”

TCS-BIN is only used by the cordless telephony profile, which failed to attract implementers. As such it is only of historical interest.

Adopted protocols

Adopted protocols are defined by other standards-making organizations and incorporated into Bluetooth’s protocol stack, allowing Bluetooth to create protocols only when necessary. The adopted protocols include:

Point-to-Point Protocol (PPP) – Internet standard protocol for transporting IP datagrams over a point-to-point link

TCP/IP/UDP – Foundation Protocols for TCP/IP protocol suite

Object Exchange Protocol (OBEX) – Session-layer protocol for the exchange of objects, providing a model for object and operation representation

Wireless Application Environment / Wireless Application Protocol (WAE/WAP) – WAE specifies an application framework for wireless devices and WAP is an open standard to provide mobile users access to telephony and information services.

Data and source mostly taken from Wikipedia.

BIOS (II)

The size of the BIOS, and the capacities of the ROM, EEPROM and other media it may be stored on, has increased over time as new features have been added to the code; BIOS versions now exist with sizes up to 8 megabytes. Some modern motherboards are including even bigger NAND Flash ROM ICs on board which are capable of storing whole compact operating system distribution like some Linux distributions. For example, some recent ASUS motherboards including SplashTop Linux embedded into their NAND Flash ROM ICs.

EEPROM chips are advantageous because they can be easily updated by the user; hardware manufacturers frequently issue BIOS updates to upgrade their products, improve compatibility and remove bugs. However, this advantage had the risk that an improperly executed or aborted BIOS update could render the computer or device unusable. To avoid these situations, more recent BIOSes use a "boot block"; a portion of the BIOS which runs first and must be updated separately. This code verifies if the rest of the BIOS is intact (using hash checksums or other methods) before transferring control to it. If the boot block detects any corruption in the main BIOS, it will typically warn the user that a recovery process must be initiated by booting from removable media (floppy, CD or USB memory) so the user can try flashing the BIOS again. Some motherboards have a backup BIOS (sometimes referred to as DualBIOS boards) to recover from BIOS corruptions. In 2007, Gigabyte began offering motherboards with a QuadBIOS recovery feature

A computer system can contain several BIOS firmware chips. The motherboard BIOS typically contains code to access fundamental hardware components such as the keyboard, floppy drives, ATA (IDE) hard disk controllers, USB human interface devices, and storage devices. In addition, plug-in adapter cards such as SCSI, RAID, Network interface cards, and video boards often include their own BIOS, complementing or replacing the system BIOS code for the given component.

In some devices that can be used by add-in adapters and actually directly integrated on the motherboard, the add-in ROM may also be stored as separate code on the main BIOS flash chip. It may then be possible to upgrade this "add-in" BIOS (sometimes called an option ROM) separately from the main BIOS code.

Add-in cards usually only require such an add-in BIOS if they:

Need to be used prior to the time that the operating system loads (e.g. they may be used as part of the process which loads (bootstraps) the operating system), and:

Are not sufficiently simple, or generic in operation to be handled by the main BIOS directly

PC operating systems such as DOS, including all DOS-based versions of MS Windows, as well as bootloaders, may continue to make use of the BIOS to handle input and output. However, other modern operating systems will interact with hardware devices directly by using their own device drivers to directly access the hardware. Occasionally these add-in BIOSs are still called by these operating systems, in order to carry out specific tasks such as preliminary device initialization.

To find these memory mapped expansion ROMs during the boot process, PC BIOS implementations scan real memory from 0xC0000 to 0xF0000 on 2 kibibyte boundaries looking for the ROM signature bytes of 55h followed by AAh (0xAA55). For a valid expansion ROM, its signature is immediately followed by a single byte indicating the number of 512-byte blocks it occupies in real memory. The BIOS then jumps to the offset located immediately after this size byte; at which point the expansion ROM code takes over, using the BIOS services to register interrupt vectors for use by post-boot applications and provide a user configuration interface, or display diagnostic information.

There are many methods and utilities for dumping the contents of various motherboard BIOS and expansion ROMs. Under a Microsoft OS, DEBUG can be used to examine 64 KiB segments of memory and save the contents to a file. For UNIX systems the dd command can be used by a user with root privileges: "dd if=/dev/mem bs=1k skip=768 count=256 2>/dev/null | strings -n 8".

Data and source mostly taken from Wikipedia.

Computer Operating System (III)

LINUX

Linux is a term commonly used to refer to Unix-like computer Operating system which uses the Linux kernel. Linux is one of the most prominent examples of free software and open source development; typically all the underlying source code can be freely modified, used, and redistributed by anyone.

Predominantly known for its use in servers, it is installed on a wide variety of computer hardware, ranging from embedded devices and mobile phones to supercomputers.

The name "Linux" comes from the Linux kernel, originally written in 1991 byLinus Torvalds. The system's utilities and libraries usually come from the GNU operating system, announced in 1983 by Richard Stallman. The GNU contribution is the basis for the alternative name GNU/Linux

The Unix operating system was conceived and implemented in the 1960s and first released in 1970. Its wide availability and portability meant that it was widely adopted, copied and modified by academic institutions and businesses, with its design being influential on authors of other systems.

The GNU Project, started in 1984 by Richard Stallman, had the goal of creating a "complete Unix-compatible software system" made entirely of free software. The next year Stallman created the Free Software Foundation and wrote the GNU General Public License (GNU GPL) in 1989. By the early 1990s, many of the programs required in an operating system (such as libraries, compilers, text editors, a Unix shell, and a windowing system) were completed, although low-level elements such as device drivers, daemons, and the kernel were stalled and incomplete. Linus Torvalds has said that if the GNU kernel had been available at the time (1991), he would not have decided to write his own

MINIX, a Unix-like system intended for academic use, was released by Andrew S. Tanenbaum in 1987. While source code for the system was available, modification and redistribution were restricted (that is not the case today). In addition, MINIX's 16-bit design was not well adapted to the 32-bit design of the increasingly cheap and popular Intel 386 architecture for personal computers.

In 1991, Torvalds began to work on a non-commercial replacement for MINIX while he was attending the University of Helsinki,which would eventually become the Linux kernel.

In 1992, Tanenbaum posted an article on Usenet claiming Linux was obsolete. In the article, he criticized the operating system as being monolithic in design and being tied closely to the x86 architecture and thus not portable, as he described "a fundamental error." Tanenbaum suggested that those who wanted a modern operating system should look into one based on the microkernel model. The posting elicited the response of Torvalds, which resulted in a well known debate over the microkernel and monolithic kernel designs.

Linux was dependent on the MINIX user space at first. With code from the GNU system freely available, it was advantageous if this could be used with the fledgling OS. Code licensed under the GNU GPL can be used in other projects, so long as they also are released under the same or a compatible license. In order to make the Linux kernel compatible with the components from the GNU Project, Torvalds initiated a switch from his original license (which prohibited commercial redistribution) to the GNU GPL.Linux and GNU developers worked to integrate GNU components with Linux to make a fully functional and free operating system.

Today Linux is used in numerous domains, from embedded systems to supercomputers, and has secured a place in server installations with the popular LAMP application stack.Linux use in home desktop and enterprise desktop has been rapidly expanding and now claims a significant share of the desktop market.

Linux has also become popular with the newly founded netbook market, with many devices such as the ASUS Eee PC and Acer Aspire One shipping with customized Linux distributions pre-installed.

Linux is a modular Unix-like operating system. It derives much of its basic design from principles established in Unix during the 1970s and 1980s. Linux uses a monolithic kernel, the Linux kernel, which handles process control, networking, and peripheral and file system access. Device drivers are integrated directly with the kernel.

Much of Linux's higher-level functionality is provided by separate projects which interface with the kernel. The GNU userland is an important part of most Linux systems, providing the shell and Unix tools which carry out many basic operating system tasks. On top these tools form a Linux system with a graphical user interface that can be used, usually running in the X Window System.

Linux can be controlled by one or more of a text-based command line interface (CLI), graphical user interface (GUI) (usually the default for desktop), or through controls on the device itself (common on embedded machines).

On desktop machines, KDE, GNOME and Xface are the most popular user interfaces,^[though a variety of other user interfaces exist. Most popular user interfaces run on top of the X Window System (X), which provides network transparency, enabling a graphical application running on one machine to be displayed and controlled from another.

Other GUIs include X window managers such as FVWM, Enlightenment and Window Maker. The window manager provides a means to control the placement and appearance of individual application windows, and interacts with the X window system.

A Linux system typically provides a CLI of some sort through a shell, which is the traditional way of interacting with a Unix system. A Linux distribution specialized for servers may use the CLI as its only interface. A “headless system” run without even a monitor can be controlled by the command line via a protocol such as SSH or telnet.

Most low-level Linux components, including the GNU Userland, use the CLI exclusively. The CLI is particularly suited for automation of repetitive or delayed tasks, and provides very simple inter-process communication. A graphical terminal emulator program is often used to access the CLI from a Linux desktop.

The primary difference between Linux and many other popular contemporary operating systems is that the Linux kernel and other components are free and open source software. Linux is not the only such operating system, although it is the best-known and most widely used. Some free and open source software licences are based on the principle of copyleft, a kind of reciprocity: any work derived from a copyleft piece of software must also be copyleft itself. The most common free software license, the GNU GPL, is a form of copyleft, and is used for the Linux kernel and many of the components from the GNU project.

As an operating system underdog competing with mainstream operating systems, Linux cannot rely on a monopoly advantage; in order for Linux to be convenient for users, Linux aims for interoperability with other operating systems and established computing standards. Linux systems adhere to POSIX, SUS, ISO and ANSI standards where possible, although to date only one Linux distribution has been POSIX.1 certified, Linux-FT.

Free software projects, although developed in a collaborative fashion, are often produced independently of each other. However, given that the software licenses explicitly permit redistribution, this provides a basis for larger scale projects that collect the software produced by stand-alone projects and make it available all at once in the form of a Linux distribution.

A Linux distribution, commonly called a “distro”, is a project that manages a remote collection of Linux-based software, and facilitates installation of a Linux operating system. Distributions are maintained by individuals, loose-knit teams, volunteer organizations, and commercial entities. They include system software and application software in the form of packages, and distribution-specific software for initial system installation and configuration as well as later package upgrades and installs. A distribution is responsible for the default configuration of installed Linux systems, system security, and more generally integration of the different software packages into a coherent whole.

Data and source mostly taken from Wikipedia.

Computer Operating System (II)

MAC OS

Mac OS is the trademarked name for a series of graphical user interface-based operating systems developed by Apple Inc. (formerly Apple Computer, Inc.) for their Macintosh line of computer systems. The Macintosh user experience is credited with popularizing the graphical user interface. The original form of what Apple would later name the "Mac OS" was the integral and unnamed system software first introduced in 1984 with the original Macintosh, usually referred to simply as the System software.

Apple deliberately downplayed the existence of the operating system in the early years of the Macintosh to help make the machine appear more user-friendly and to distance it from other operating systems such as MS-DOS, which was more arcane and technically challenging. Much of this early system software was held in ROM, with updates typically provided free of charge by Apple dealers on floppy disk. As increasing disk storage capacity and performance gradually eliminated the need for fixing much of an advanced GUI operating system in ROM, Apple explored cloning while positioning major operating system upgrades as separate revenue-generating products, first with System 7 and System 7.5, then with Mac OS 7.6 in 1997.

Earlier versions of the Mac OS were compatible only with Motorola 68000-based Macintoshes. As Apple introduced computers with PowerPC hardware, the OS was upgraded to support this architecture as well. Mac OS X, which has superseded the "Classic" Mac OS, is compatible with both PowerPC and Intel processors.

Versions

The early Macintosh operating system initially consisted of two pieces of software, called "System" and "Finder", each with its own version number. System 7 was the first to include the Mac OS logo (a variation on the original Happy Mac startup icon), and Mac OS 7.6 was the first to be named "Mac OS" (to ensure that users would still identify it with Apple, even when used in "clones" from other companies).

Until the advent of the later PowerPC G3-based systems, significant parts of the system were stored in physical ROM on the motherboard. The initial purpose of this was to avoid using up the limited storage of floppy disks on system support, given that the early Macs had no hard disk. (Only one model of Mac was ever actually bootable using the ROM alone, the 1991 Mac Classic model.) This architecture also allowed for a completely graphical OS interface at the lowest level without the need for a text-only console or command-line mode. A fatal software error, or even a low-level hardware error discovered during system startup (such as finding no functioning disk drives), was communicated to the user graphically using some combination of icons, alert box windows, buttons, a mouse pointer, and the distinctive Chicago bitmap font. Mac OS depended on this core system software in ROM on the motherboard, a fact that later helped to ensure that only Apple computers or licensed clones (with the copyright-protected ROMs from Apple) could run Mac OS.

The Mac OS can be divided into two families of operating systems:

• "Classic" Mac OS, the system which shipped with the first Macintosh in 1984 and its descendants, culminating with Mac OS 9.
• The newer Mac OS X (where the X is 10 written as a Roman numeral). Mac OS X incorporates elements of OpenStep(thus also BSD Unix and Mach) and Mac OS 9. Its low-level BSD-based foundation, Darwin, is free software/open source software.

"Classic" Mac OS (1984–2001)

The "classic" Mac OS is characterized by its total lack of a command line; it is a completely graphical operating system. Noted for its ease of use and its cooperative multitasking, it was criticized for its very limited memory management, lack of protected memory, and susceptibility to conflicts among operating system "extensions" that provide additional functionality (such as networking) or support for a particular device. Some extensions may not work properly together, or work only when loaded in a particular order. Troubleshooting Mac OS extensions can be a time-consuming process of trial and error.

The Macintosh originally used the Macintosh File System (MFS), a flat file system with only one level of folders. This was quickly replaced in 1985 by the Hierarchical File System (HFS), which had a true directory tree. Both file systems are otherwise compatible.

Most file systems used with DOS, Unix, or other operating systems treat a file as simply a sequence of bytes, requiring an application to know which bytes represented what type of information. By contrast, MFS and HFS gave files two different "forks". The data fork contained the same sort of information as other file systems, such as the text of a document or the bitmaps of an image file. The resource fork contained other structured data such as menu definitions, graphics, sounds, or code segments. A file might consist only of resources with an empty data fork, or only a data fork with no resource fork. A text file could contain its text in the data fork and styling information in the resource fork, so that an application, which didn’t recognize the styling information, could still read the raw text. On the other hand, these forks provided a challenge to interoperability with other operating systems; copying a file from a Mac to a non-Mac system would strip it of its resource fork, necessitating such encoding schemes as BinHex andMacBinary.

Classic Application Support was shipped with Mac OS X with PowerPC Macs until early 2006. However, Intel-based Macintoshes cannot run the Classic system or applications, nor can PowerPC models while they are running Mac OS X v10.5 Leopard.

Mac OS X (2000–present)

Mac OS X brought Unix-style memory management and pre-emptive multitasking to the Mac platform. It is based on the Mach kernel and the BSD implementation of UNIX, which were incorporated intoNeXTSTEP, the object-oriented operating system developed by Steve Job' next company. The new memory management system allowed more programs to run at once and virtually eliminated the possibility of one program crashing another. It is also the second Macintosh operating system to include a command line (the first is the now-discontinued A/UX, which supported classic Mac OS applications on top of a UNIX kernel), although it is never seen unless the user launches a terminal emulator.

However, since these new features put higher demands on system resources, Mac OS X only officially supported the PowerPC G3 and newer processors, and now has the additional requirement of built-in USB (10.3) and FireWire (10.4)).

For over three years, Mac OS X has become faster with every release - faster on the same hardware.

PowerPC builds of Mac OS X include a compatibility layer for running older Mac applications, the Classic Environment. This runs a full copy of the older Mac OS, version 9.1 or later, in a Mac OS X process. PowerPC-based Macs shipped with Mac OS 9.2 as well as Mac OS X. Mac OS 9.2 had to be installed by the user — it was not installed by default on hardware revisions released after the release of Mac OS X 10.4 Tiger. Most well-written "classic" applications function properly under this environment, but compatibility is only assured if the software was written to be unaware of the actual hardware, and to interact solely with the operating system. The Classic Environment is not available on Intel-based Macintoshes due to the incompatibility of Mac OS 9 with the x86 hardware, and was removed completely on Mac OS X 10.5 Leopard.

Users of the classic Mac OS generally upgraded to Mac OS X, but many criticized it as being more difficult and less user-friendly than the original Mac OS, for the lack of certain features that had not been re-implemented in the new OS, or for being slower on the same hardware (especially older hardware), or other, sometimes serious incompatibilities with the older OS. Because drivers (for printers, scanners, tablets, etc.) written for the older Mac OS are not compatible with Mac OS X, and due to the lack of Mac OS X support for older Apple machines, a significant number of Macintosh users have still continued using the older classic Mac OS. But by 2005, it has been reported that almost all users of systems capable of running Mac OS X are doing so, with only a small fraction still running the classic Mac OS.

In June 2005, Steve Jobs announced at the Worldwide Developers Conference keynote that Apple computers would be transitioning from PowerPC to Intel processors. At the same conference, Jobs announced Developer Transition Kits that included beta versions of Apple software including Mac OS X that developers could use to test their applications as they ported them to run on Intel-powered Macs. In January 2006, Apple released the first Macintosh computers with Intel processors, an iMac and theMacBookPro , and in February 2006, Apple released a Mac mini with an Intel Core Solo and Duo processor. On May 16, 2006, Apple released theMacBook, before completing the Intel transition on August 7 with the Mac Pro. To ease the transition for early buyers of the new machines, Intel-based Macs include an emulation technology called Rosetta, which allows them to run (at reduced speed) pre-existing Mac OS X native application software that was compiled only for PowerPC-based Macintoshes......... Continue

Data and source mostly taken from Wikipedia.