Central processing unit
A central processing unit called a central processor or main processor, is the electronic circuitry within a computer that carries out the instructions of a computer program by performing the basic arithmetic, logic and input/output operations specified by the instructions. The computer industry has used the term "central processing unit" at least since the early 1960s. Traditionally, the term "CPU" refers to a processor, more to its processing unit and control unit, distinguishing these core elements of a computer from external components such as main memory and I/O circuitry; the form and implementation of CPUs have changed over the course of their history, but their fundamental operation remains unchanged. Principal components of a CPU include the arithmetic logic unit that performs arithmetic and logic operations, processor registers that supply operands to the ALU and store the results of ALU operations and a control unit that orchestrates the fetching and execution of instructions by directing the coordinated operations of the ALU, registers and other components.
Most modern CPUs are microprocessors, meaning they are contained on a single integrated circuit chip. An IC that contains a CPU may contain memory, peripheral interfaces, other components of a computer; some computers employ a multi-core processor, a single chip containing two or more CPUs called "cores". Array processors or vector processors have multiple processors that operate in parallel, with no unit considered central. There exists the concept of virtual CPUs which are an abstraction of dynamical aggregated computational resources. Early computers such as the ENIAC had to be physically rewired to perform different tasks, which caused these machines to be called "fixed-program computers". Since the term "CPU" is defined as a device for software execution, the earliest devices that could rightly be called CPUs came with the advent of the stored-program computer; the idea of a stored-program computer had been present in the design of J. Presper Eckert and John William Mauchly's ENIAC, but was omitted so that it could be finished sooner.
On June 30, 1945, before ENIAC was made, mathematician John von Neumann distributed the paper entitled First Draft of a Report on the EDVAC. It was the outline of a stored-program computer that would be completed in August 1949. EDVAC was designed to perform a certain number of instructions of various types; the programs written for EDVAC were to be stored in high-speed computer memory rather than specified by the physical wiring of the computer. This overcame a severe limitation of ENIAC, the considerable time and effort required to reconfigure the computer to perform a new task. With von Neumann's design, the program that EDVAC ran could be changed by changing the contents of the memory. EDVAC, was not the first stored-program computer. Early CPUs were custom designs used as part of a sometimes distinctive computer. However, this method of designing custom CPUs for a particular application has given way to the development of multi-purpose processors produced in large quantities; this standardization began in the era of discrete transistor mainframes and minicomputers and has accelerated with the popularization of the integrated circuit.
The IC has allowed complex CPUs to be designed and manufactured to tolerances on the order of nanometers. Both the miniaturization and standardization of CPUs have increased the presence of digital devices in modern life far beyond the limited application of dedicated computing machines. Modern microprocessors appear in electronic devices ranging from automobiles to cellphones, sometimes in toys. While von Neumann is most credited with the design of the stored-program computer because of his design of EDVAC, the design became known as the von Neumann architecture, others before him, such as Konrad Zuse, had suggested and implemented similar ideas; the so-called Harvard architecture of the Harvard Mark I, completed before EDVAC used a stored-program design using punched paper tape rather than electronic memory. The key difference between the von Neumann and Harvard architectures is that the latter separates the storage and treatment of CPU instructions and data, while the former uses the same memory space for both.
Most modern CPUs are von Neumann in design, but CPUs with the Harvard architecture are seen as well in embedded applications. Relays and vacuum tubes were used as switching elements; the overall speed of a system is dependent on the speed of the switches. Tube computers like EDVAC tended to average eight hours between failures, whereas relay computers like the Harvard Mark I failed rarely. In the end, tube-based CPUs became dominant because the significant speed advantages afforded outweighed the reliability problems. Most of these early synchronous CPUs ran at low clock rates compared to modern microelectronic designs. Clock signal frequencies ranging from 100 kHz to 4 MHz were common at this time, limited by the speed of the switching de
The Apple IIe is the third model in the Apple II series of personal computers produced by Apple Computer. The e in the name stands for enhanced, referring to the fact that several popular features were now built-in that were only available as upgrades or add-ons in earlier models. Improved expandability combined with the new features made for a attractive general-purpose machine to first-time computer shoppers; as the last surviving model of the Apple II computer line before discontinuation, having been manufactured and sold for nearly 11 years with few changes, the IIe earned the distinction of being the longest-lived computer in Apple's history. Apple Computer planned to discontinue the Apple II series after the introduction of the Apple III in 1980. Management believed that "once the Apple III was out, the Apple II would stop selling in six months", cofounder Steve Wozniak said. By the time IBM released the rival IBM PC in 1981, the Apple II's technology was four years old. In September 1981 InfoWorld reported—below the PC's announcement—that Apple was secretly developing three new computers "to be ready for release within a year": Lisa, "McIntosh", "Diana".
Describing the last as a software-compatible Apple II replacement—"A 6502 machine using custom LSI" and a simpler motherboard—it said that Diana "was ready for release months ago" but decided to improve the design to better compete with the Xerox 820. "Now it appears that when Diana is ready for release, it will offer features and a price that will make the Apple II uncompetitive", the magazine wrote."Apple's plans to phase out the Apple II have been delayed by complications in the design of the Apple III", the article said. After the Apple III struggled, management decided in 1981 that the further continuation of the Apple II was in the company's best interest. After 3 1⁄2 years of the Apple II Plus at a standstill, came the introduction of a new Apple II model — the Apple IIe; the Apple IIe was released in the successor to the Apple II Plus. The Apple IIe was the first Apple computer with a custom ASIC chip, which reduced much of the old discrete IC-based circuitry to a single chip; this change resulted in reducing the size of the motherboard.
Some of the hardware features of the Apple III were borrowed in the design of the Apple IIe, some from incorporating the Apple II Plus Language card. The culmination of these changes led to increased sales and greater market share of home and small business use. One of the most notable improvements of the Apple IIe is the addition of a full ASCII character set and keyboard; the most important addition is the ability to display lower-case letters. Other keyboard improvements include four-way cursor control and standard editing keys, two special Apple modifier keys, a safe off-to-side relocation of the "Reset" key; the auto-repeat function is now automatic, no longer requiring the "REPT" key found on the keyboards of previous models. The machine came standard with 64 KB RAM, with the equivalent of a built-in Apple Language Card in its circuitry, had a new special "Auxiliary slot" for adding more memory via bank-switching RAM cards. Through this slot it includes built-in support for an 80-column text display on monitors and could be doubled to 128 KB RAM by alternatively plugging in Apple's Extended 80-Column Text Card.
As time progressed more memory could be added through third-party cards using the same bank-switching slot or, general-purpose slot cards that addressed memory 1 byte at a time. A new ROM diagnostic routine could be invoked to test the motherboard for faults and test its main bank of memory; the Apple IIe lowered production costs and improved reliability by merging the function of several off-the-shelf ICs into single custom chips, reducing total chip count to 31. The IIe switched to using newer single-voltage 4164 DRAM chips instead of the unreliable triple-voltage 4116 DRAM in the II/II+. For this reason the motherboard design is much cleaner and runs cooler as well, with enough room to add a pin-connector for an external numeric keypad. Added was a backport-accessible DE-9 joystick connector, making it far easier for users to add and remove game and input devices. Improved were port openings for expansion cards. Rather than cutout V-shaped slot openings as in the Apple II and II Plus, the IIe has a variety of different-sized openings, with thumb-screw holes, to accommodate mounting interface cards with DB-xx and DE-xx connectors.
Although the lower IC count improved reliability over previous Apple II models, Apple still retained the practice of socketing all ICs so that servicing and replacement could be performed more easily. Later-production IIe models had the RAM soldered to the system board rather than socketed. Despite the hardware changes, the IIe maintains a high degree of backwards compatibility with the previous models, allowing most hardwa
SuperDrive is a trademark used by Apple Inc. for two different storage drives: from 1988 to 1999 to refer to a high-density floppy disk drive capable of reading all major 3.5″ disk formats. The terms DVD Multi, Super AllWrite and Super Multi are used to describe optical disc drives from non-Apple manufacturers; the term was first used by Apple Computer in 1988 to refer to their 1.44 MB 3.5 inch floppy drive. This replaced the older 800 KB floppy drive, standard in the Macintosh up to but remained compatible in that it could continue to read and write both 800 KB and 400 KB floppy disks, as well as the then-new high-density floppies; this drive was capable of reading and writing MS-DOS formatted disks and FAT12 file formats, using PC Exchange or other software, unlike the 400 KB and 800 KB drives. This was made possible as the SuperDrive now utilitized the same MFM encoding scheme used by the IBM PC, yet still retained backward compatibility with Apple's variable-speed zoned CAV scheme and Group Coded Recording encoding format, so it could continue to read Macintosh MFS, HFS and Apple II ProDOS formats on 400/800 KB disks.
Introduced in 1988 under the Trademark name FDHD, the subsequently renamed SuperDrive was known as an internally mounted floppy drive, part of the Macintosh computer. While the external drive worked on both Apple's product lines, it was intended for use on the Apple II series, for which Apple introduced in 1991 a slot-based interface called the Apple II 3.5 Disk Controller Card for Apple IIe and IIGS computers so they too could use 1.40 MB storage and read/write MS-DOS. The controller card as well as the external Superdrive were discontinued in June 1994; the SuperDrive cannot be used with the original four Mac models, as their disk controller doesn't support high density. The next two models to be released shipped with that controller; these two models can gain full use of the SuperDrive. All models shipped with the SWIM; the first Macintosh model to include a SuperDrive floppy drive was the Macintosh IIx. Every Macintosh and PowerBook introduced from 1988–97, had a built-in SuperDrive floppy drive.
The last model to include one was the beige Power Macintosh G3 series, manufactured until January 1999. The PowerBook G3 1998 model had an optional floppy drive module; the PowerBook 190 series, PowerBook 5300 Series, PowerBook 3400c, original PowerBook G3 shared the same interchangeable floppy drive module as a standard feature. The drive as mounted on PowerBooks lacked the auto-inject feature of Apple's initial desktop SuperDrive implementation, requiring the user to manually insert the disk all the way into the drive; the feature was dropped throughout the lineup during 1993–94. The PowerBook 1400 series had a floppy drive module, but was incompatible with the other PowerBooks; the end of the SuperDrive coincides with the demise of Old World ROM Macs. The SuperDrive is not supported in Mac OS X, not on the few Old World ROM machines that can run OS X. Once the use of floppy disks started declining, Apple reused the trademark to refer to the optical drives built into its Macintosh models, which could read and write both DVDs and CDs.
The early 2001 release of the Power Mac G4 was the first Macintosh to include a SuperDrive. SuperDrives featured 6–24x write speeds and supported the DVD±R, DVD±R DL, DVD±RW, CD-R, CD-RW formats along with all normal read-only media. DVD-RAM support, notably, is absent. Apple never supported playback of the format. Unlike tray-style disc holders which have an inner guide ring, slot-style drives will not work with MiniCD or MiniDVD discs unless an 8 cm optical disc adapter is used and extraction of the disc is difficult, requiring tweezers, use of a card with double-sided sticky tape, or complete disassembly of it; the MacBook Air, released in 2008, was the first Macintosh to not include a SuperDrive after it became standard across the line. Apple began phasing SuperDrives out across the Macintosh line beginning in 2011; the updated second generation 13-inch MacBook Pro, released in 2012 and discontinued in 2016, was the last Macintosh to include a SuperDrive. Apple still sells a USB-connected external SuperDrive as an accessory.
The drive has faced criticism and mixed customer reviews, with 410 reviews averaging two and a half stars. Criticism includes the lack of support for Blu-ray or mini optical discs and malfunctions that make the drive inoperable, with no way to eject a stuck disc. DVD Multi is a specification for optical media drives; the specification was approved by the DVD Forum and was claimed to enable disc compatibility for all formats created by the DVD Forum. However, DVD+R / RW is not included. LiteOn uses the term Super AllWrite instead, which has the same meaning. Drives with DVD Multi logo can read and write DVD-R/RW and DVD-RAM discs, read DVD-Video and DVD-ROM
The Macintosh Plus computer is the third model in the Macintosh line, introduced on January 16, 1986, two years after the original Macintosh and a little more than a year after the Macintosh 512K, with a price tag of US$2599. As an evolutionary improvement over the 512K, it shipped with 1 MB of RAM standard, expandable to 4 MB, an external SCSI peripheral bus, among smaller improvements, it had the same beige-colored case as the original Macintosh, but in 1987, the case color was changed to the long-lived, warm gray "Platinum" color. It is the earliest Macintosh model able to run System 7 OS. Bruce Webster of BYTE reported a rumor in December 1985: "Supposedly, Apple will be releasing a Big Mac by the time this column sees print: said Mac will come with 1 megabyte of RAM... the new 128K-byte ROM... and a double-sided disk drive, all in the standard Mac box". Introduced as the Macintosh Plus, it was the first Macintosh model to include a SCSI port, which launched the popularity of external SCSI devices for Macs, including hard disks, tape drives, CD-ROM drives, Zip Drives, monitors.
The SCSI implementation of the Plus was engineered shortly before the initial SCSI spec was finalized and, as such, is not 100% SCSI-compliant. SCSI ports remained standard equipment for all Macs until the introduction of the iMac in 1998, which replaced most of Apple's "legacy ports" with USB; the Macintosh Plus was the last classic Mac to have a phone cord-like port on the front of the unit for the keyboard, as well as the DE-9 connector for the mouse. The Mac Plus was the first Apple computer to utilize SIMM memory modules instead of single DIP DRAM chips. Four slots were provided and the computer shipped with four 256k SIMMs for 1MB total. By replacing them with 1MB SIMMs, it was possible to have 4MB of RAM. Although 30-pin SIMMs could support up to 16MB total RAM, the motherboard had only 22 address lines connected for 4MB, it has what was a new 3 1⁄2-inch double-sided 800 KB floppy drive, offering double the capacity of floppy disks for previous Macs, along with backward compatibility.
The then-new drive is controlled by the same IWM chip as in previous models, implementing variable speed GCR. The drive was still incompatible with PC drives; the 800 KB drive has two read/write heads, enabling it to use both sides of the floppy disk and thereby double storage capacity. Like the 400 KB drive before it, a companion Macintosh 800K External Drive was an available option. However, with the increased storage capacity combined with 2-4x the available RAM, the external drive was less of a necessity than it had been with the 128K and 512K; the Mac Plus has 128 KB of ROM on the motherboard, double the amount of ROM that's in previous Macs. For programmers, the fourth Inside Macintosh volume details how to use HFS and the rest of the Mac Plus's new system software; this new filing system allows it to use the first hard drive Apple developed for the 512K, the IWM floppy disk-based Hard Disk 20 and the then-new ROMs allow the Macintosh to use the drive as a startup disk for the first time.
The Plus still did not include provision for an internal hard drive and it would be over nine months before Apple would offer a SCSI drive replacement for the slow Hard Disk 20. It would be well over a year before Apple would offer the first internal hard disk drive in any Macintosh. A compact Mac, the Plus has a 9-inch 512×342 pixel monochrome display with a resolution of 72 PPI, identical to that of previous Macintosh models. Unlike earlier Macs, the Mac Plus's keyboard includes a numeric keypad and directional arrow keys and, as with previous Macs, it has a one-button mouse and no fan, making it quiet in operation; the lack of a cooling fan in the Mac Plus led to frequent problems with overheating and hardware malfunctions. The applications MacPaint and MacWrite were bundled with the Mac Plus. After August 1987, HyperCard and MultiFinder were bundled. Third-party software applications available included MacDraw, Microsoft Word and PowerPoint, as well as Aldus's PageMaker. Microsoft Excel and PowerPoint were developed and released first for the Macintosh, Microsoft Word 1 for Macintosh was the first time a GUI version of that software was introduced on any personal computer platform.
For a time, the exclusive availability of Excel and PageMaker on the Macintosh were noticeable drivers of sales for the platform. The case design is identical to the original Macintosh, it debuted in beige and was labeled Macintosh Plus on the front, but Macintosh Plus 1 MB on the back, to denote the 1 MB RAM configuration with which it shipped. In January 1987 it transitioned to Apple's long-lived platinum-gray color with the rest of the Apple product line, the keyboard's keycaps changed from brown to gray. In January 1988, with reduced RAM prices, Apple began shipping 2- and 4- MB configurations and rebranded it as "Macintosh Plus." Among other design changes, it included the same trademarked inlaid Apple logo and recessed port icons as the Apple IIc and IIGS before it, but it retained the original design. An upgrade kit was offered for the earlier Macintosh 128K and Macintosh 512K/enhanced, which includes a new motherboard, floppy disk drive and rear case; the owner retained the front case, monitor, a
A monochromic image is composed of one color. The term monochrome comes from the Ancient Greek: translit. Monochromos, lit.'having one color'. A monochromatic object or image reflects colors in shades of limited hues. Images using only shades of grey are called black-and-white. However, scientifically speaking, monochromatic light refers to visible light of a narrow band of wavelengths. Of an image, the term monochrome is taken to mean the same as black and white or, more grayscale, but may be used to refer to other combinations containing only tones of a single color, such as green-and-white or green-and-red, it may refer to sepia displaying tones from light tan to dark brown or cyanotype images, early photographic methods such as daguerreotypes and tintypes, each of which may be used to produce a monochromatic image. In computing, monochrome has two meanings: it may mean having only one color, either on or off, allowing shades of that color. A monochrome computer display is able to display only a single color green, red or white, also shades of that color.
In film photography, monochrome is the use of black-and-white film. All photography was done in monochrome. Although color photography was possible in the late 19th century used color films, such as Kodachrome, were not available until the mid-1930s. In digital photography, monochrome is the capture of only shades of black by the sensor, or by post-processing a color image to present only the perceived brightness by combining the values of multiple channels; the weighting of individual channels may be selected to achieve a desired artistic effect. If the red channel is eliminated and the green and blue combined the effect will be similar to that of orthochromatic film or the use of a cyan filter on panchromatic film; the selection of weighting thus allows a wide range of artistic expression in the final monochromatic image. For production of an anaglyph image the original color stereogram source may first be reduced to monochrome in order to simplify the rendering of the image; this is sometimes required in cases where a color image would render in a confusing manner given the colors and patterns present in the source image and the selection filters used.
In physics, monochromatic light is electromagnetic radiation of a single frequency. In the physical sense, no source of electromagnetic radiation is purely monochromatic, since that would require a wave of infinite duration as a consequence of the Fourier transform's localization property. Controlled sources such as lasers operate in a range of frequencies. In practice, filtered light, diffraction grating separated light and laser light are all referred to as monochromatic. Light sources can be compared and one be labeled as “more monochromatic”. A device which isolates a narrow band of frequencies from a broader-bandwidth source is called a monochromator though the bandwidth is explicitly specified, thus a collection of frequencies is understood. Duotone – the use of two ink colors in printing Halftone – the use of black and white in a pattern, perceived as shades of grey Polychrome – of multiple colors, the opposite of monochrome Monochromacy Monochromatic color Selective color – use of monochrome and color selectively within an image Monochrome painting – monochromes in art
The Apple IIGS, the fifth and most powerful of the Apple II family, is a 16-bit personal computer produced by Apple Computer, Inc. While featuring the Macintosh look and feel, resolution and color similar to the Commodore Amiga and Atari ST, it remains compatible with earlier Apple II models; the "GS" in the name stands for "Graphics and Sound," referring to its enhanced multimedia hardware its state of the art audio. The microcomputer is a radical departure from any previous Apple II, with its 65C816 16-bit microprocessor, direct access to megabytes of RAM, mouse, it was the first computer produced by Apple to use a color graphical user interface and Apple Desktop Bus interface for keyboards and other input devices. It is the first personal computer to have a wavetable synthesis chip, utilizing technology from Ensoniq; the IIGS set forth a promising future and evolutionary advancement of the Apple II line, but Apple focused on the Macintosh platform. The IIGS clock speed was intentionally limited below the maximum for the 65C816 so the system would not outperform the Macintosh.
The IIGS outsold all other Apple products, including the Macintosh, during its first year in production. Apple ceased IIGS production in December 1992; the Apple IIGS made significant improvements over previous machines from the line such as the Apple IIe and Apple IIc. It emulates its predecessors by utilizing a custom chip called the Mega II and used the new Western Design Center 65C816 16-bit microprocessor running at 2.8 MHz, faster than the 8-bit NMOS 6502 and CMOS 65C02 processors used in the earlier Apple II models. Use of the 65C816 allows the IIGS to address more RAM; the use of a 2.8 MHz clock was a marketing decision intended to limit the IIGS's performance to a level lower than that of the Macintosh, a decision that had a critical effect on the IIGS's success. The IIGS includes enhanced graphics and sound, which led to its GS name, its graphical capabilities are the best of the Apple II series, with new higher resolution video modes. These include a 640×200-pixel mode with 2-bit color and a 320×200-pixel mode with 4-bit color, both of which can select 4 or 16 colors at a time from a palette of 4,096 colors.
By changing the palette on each scanline, it is possible to display up to 256 colors or more per screen, quite seen within games and graphic design software during this computer's heyday. Through some clever programming, it is possible to make the IIGS display as many as 3,200 colors at once; when first introduced, Apple's user interface known as MouseDesk and the IIGS System Demo were both in black and white only. Users did not see color until an application. Audio is generated by a built-in sound-and-music synthesizer in the form of the Ensoniq 5503 digital oscillator chip, which has its own dedicated RAM and 32 separate channels of sound; these channels are paired to produce 15 voices in stereo audio. Although Apple had hoped that the IIc would outsell the IIe, the latter was more popular because of its slots; the IIGS supports both 5.25-inch and 3.5-inch floppy disks and, like the IIe before it, has several expansion slots. These include seven general-purpose expansion slots compatible with those on the Apple II, II+, IIe, plus a memory expansion slot that can be used to add up to 8 MB of RAM.
The IIGS, like the IIc has dedicated ports for external devices. These include a port to attach more floppy disk drives, two serial ports for devices such as printers and modems, an Apple Desktop Bus port to connect the keyboard and mouse, composite and RGB video ports; these ports are associated with the slots, so for example using a card in slot 1 means that the printer port is disabled. The machine features a user-adjustable control panel and real-time clock, which are maintained by a built-in battery; the IIGS supports booting from an AppleShare server, via the AppleTalk protocol, over LocalTalk cabling. When the Apple IIe Workstation Card was introduced, this capability was given to the IIe; this was over a decade before NetBoot offered the same capability to computers running Mac OS 8 and beyond. In addition to supporting all graphics modes of previous Apple II models, the Apple IIGS introduced several new ones through a custom video graphics chip, all of which use a 12-bit palette for a total of 4,096 possible colors, though not all colors can appear onscreen at the same time.
320×200 pixels with a single palette of 16 colors. 320×200 pixels with up to 16 palettes of 16 colors. In this mode, the VGC holds 16 separate palettes of 16 colors in its own memory; each of the 200 scan lines can be assigned any one of these palettes allowing for up to 256 colors on the screen at once. 320×200 pixels with up to 200 palettes of 16 colors. In this mode, the CPU assists the VGC in swapping palettes into and out of the video memory so that each scan line can have its own palette of 16 colors allowing for up to 3,200 colors on the screen at once. 320×200 pixels with 15 colors per palette, plus a fill-mode color. In this mode, color 0 in the palette is replaced by the last non-zero color pixel displayed on the scan line, allowing fast solid-fill graphics. 640×200 pixels with 4 pure colors. 640×200 pixels with up to 16 palettes of 4 pure colors. In this mode, the VGC holds 16 separate palettes of 4 pure colors in its own memory; each of the 200 scan lines can be assigned any one of these palett
In computing, the expansion card, expansion board, adapter card or accessory card is a printed circuit board that can be inserted into an electrical connector, or expansion slot, on a computer motherboard, backplane or riser card to add functionality to a computer system via the expansion bus. An expansion bus is a computer bus which moves information between the internal hardware of a computer system and peripheral devices, it is a collection of protocols that allows for the expansion of a computer. Vacuum-tube based computers had modular construction, but individual functions for peripheral devices filled a cabinet, not just a printed circuit board. Processor, memory and I/O cards became feasible with the development of integrated circuits. Expansion cards allowed a processor system to be adapted to the needs of the user, allowing variations in the type of devices connected, additions to memory, or optional features to the central processor. Minicomputers, starting with the PDP-8, were made of multiple cards, all powered by and communicating through a passive backplane.
The first commercial microcomputer to feature expansion slots was the Micral N, in 1973. The first company to establish a de facto standard was the Altair 8800, developed 1974-1975, which became a multi-manufacturer standard, the S-100 bus. Many of these computers were passive backplane designs, where all elements of the computer, plugged into a card cage which passively distributed signals and power between the cards. Proprietary bus implementations for systems such as the Apple II co-existed with multi-manufacturer standards. IBM introduced what would retroactively be called the Industry Standard Architecture bus with the IBM PC in 1981. At that time, the technology was called the PC bus; the IBM XT, introduced in 1983, used the same bus. The 8-bit PC and XT bus was extended with the introduction of the IBM AT in 1984; this used a second connector for extending the address and data bus over the XT, but was backward compatible. Industry Standard Architecture became the designation for the IBM AT bus after other types were developed.
Users of the ISA bus had to have in-depth knowledge of the hardware they were adding to properly connect the devices, since memory addresses, I/O port addresses, DMA channels had to be configured by switches or jumpers on the card to match the settings in driver software. IBM's MCA bus, developed for the PS/2 in 1987, was a competitor to ISA their design, but fell out of favor due to the ISA's industry-wide acceptance and IBM's licensing of MCA. EISA, the 32-bit extended version of ISA championed by Compaq, was used on some PC motherboards until 1997, when Microsoft declared it a "legacy" subsystem in the PC 97 industry white-paper. Proprietary local buses and the VESA Local Bus Standard, were late 1980s expansion buses that were tied but not exclusive to the 80386 and 80486 CPU bus; the PC/104 bus is an embedded bus. Intel launched their PCI bus chipsets along with the P5-based Pentium CPUs in 1993; the PCI bus was introduced in 1991 as a replacement for ISA. The standard is found on PC motherboards to this day.
The PCI standard supports bus bridging: as many as ten daisy chained PCI buses have been tested. Cardbus, using the PCMCIA connector, is a PCI format that attaches peripherals to the Host PCI Bus via PCI to PCI Bridge. Cardbus is being supplanted by ExpressCard format. Intel introduced the AGP bus in 1997 as a dedicated video acceleration solution. AGP devices are logically attached to the PCI bus over a PCI-to-PCI bridge. Though termed a bus, AGP supports only a single card at a time. From 2005 PCI-Express has been replacing both PCI and AGP; this standard, approved in 2004, implements the logical PCI protocol over a serial communication interface. PC/104 or Mini PCI are added for expansion on small form factor boards such as Mini-ITX. For their 1000 EX and 1000 HX models, Tandy Computer designed the PLUS expansion interface, an adaptation of the XT-bus supporting cards of a smaller form factor; because it is electrically compatible with the XT bus, a passive adapter can be made to connect XT cards to a PLUS expansion connector.
Another feature of PLUS cards is. Another bus that offered stackable expansion modules was the "sidecar" bus used by the IBM PCjr; this may have been electrically comparable to the XT bus. Again, PCjr sidecars are not technically expansion cards, but expansion modules, with the only difference being that the sidecar is an expansion card enclosed in a plastic box. Most other computer lines, including those from Apple Inc. Tandy, Commodore and Atari, offered their own expansion buses; the Amiga used Zorro II. Apple used a proprietary system with seven 50-pin-slots for Apple II peripheral cards later used the NuBus for its Macintosh series until 1995, when they switched to a PCI Bus. PCI expansion cards will function on any CPU platform if there is a software driver for that type. PCI video cards and other cards that contain a BIOS are problematic, although video cards conforming to VESA Standards may be used for secondary monitors. DEC Alpha, IBM PowerPC, NEC MIPS workstations used PCI bus connectors.