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Posted by kaori 03/02/2009 @ 02:03

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IBM PC compatible

The original IBM PC (Model 5150) motivated the production of clones in the early-1980s.

IBM PC compatible computers are those generally similar to the original IBM PC, XT, and AT. Such computers used to be referred to as PC clones, or IBM clones since they almost exactly duplicated all the significant features of the PC architecture, facilitated by various manufacturers' ability to legally reverse engineer the BIOS through clean room design. Columbia Data Products built the first clone of an IBM personal computer through a clean room implementation of its BIOS. Many early IBM PC compatibles used the same computer bus as the original PC and AT models. The IBM AT compatible bus was later named the ISA bus by manufacturers of compatible computers.

The term "IBM PC compatible" became relegated to historical use with the rise of Windows and IBM's loss of dominance in the personal computer market.

Descendants of the IBM PC compatibles make up the majority of microcomputers on the market today, although interoperability with the bus structure and peripherals of the original PC architecture may be limited or non-existent.

The origins of this platform came with the decision by IBM in 1980 to market a low-cost single-user computer as quickly as possible in response to Apple Computer's success in the burgeoning market. On 12 August 1981, the first IBM PC went on sale. There were three operating systems (OS) available for it but the most popular and least expensive was PC DOS, a version of MS DOS licensed from Microsoft. In a crucial concession, IBM's agreement allowed Microsoft to sell its own version, MS-DOS, for non-IBM platforms. The only proprietary component of the PC was the BIOS (Basic Input/Output System).

A number of computers of the time based on the 8086 and 8088 processors were manufactured during this period, but with different architecture to the PC, and which ran under their own versions of DOS and CP/M-86. However, software which addressed the hardware directly instead of making standard calls to MS-DOS was faster. This was particularly relevant to games. The IBM PC was the only machine sold in high enough volumes to justify writing software specifically for it, and this encouraged other manufacturers to produce machines which could use the same programs, expansion cards and peripherals as the PC. The 808x computer marketplace rapidly excluded all machines which were not functionally very similar to the PC. The 640kB limit on "conventional" system memory available to MS-DOS is a legacy of that period; other non-clone machines did not have this limit.

The original "clones" of the IBM Personal Computer were created without IBM's participation or approval. Columbia closely modeled the IBM PC and produced the first "compatible" PC (i.e., more or less compatible to the IBM PC standard) in June 1982 closely followed by Eagle Computer. Compaq Computer Corp. announced its first IBM PC compatible a few months later in November 1982—the Compaq Portable. The Compaq was the first sewing machine-sized portable computer that was essentially 100% PC-compatible. The company could not directly copy the BIOS as a result of the court decision in Apple v. Franklin, but it could reverse-engineer the IBM BIOS and then write its own BIOS using clean room design.

At the same time, many manufacturers such as Xerox, HP, Digital, Sanyo, Texas Instruments, Tulip, Wang and Olivetti introduced personal computers that were MS DOS compatible, but not completely software- or hardware-compatible with the IBM PC.

Microsoft's intention, and that of the industry from 1981 to as late as the mid-1980s, was that application writers would write to the APIs in MS-DOS or the firmware BIOS, and that this would form what would now be called a hardware abstraction layer. Each computer would have its own OEM version of MS-DOS, customized to its hardware. Any software written for MS-DOS would run on any MS-DOS computer, despite variations in hardware design. A similar trend was with the MSX home computer series.

This expectation seemed reasonable in the computer marketplace of the time. Until then Microsoft was primarily focused on computer languages such as BASIC. The established small system operating software was CP/M from Digital Research was in use both at the hobbyist level and at the more professional end of those using microcomputers. To achieve such widespread use, and thus make the product economically viable, the OS had to operate across a range of machines from different vendors that had widely varying hardware. Those customers who needed other applications beyond the starter pack could reasonably expect publishers to offer their products for a variety of computers, on suitable media for each.

Microsoft's competing OS was initially targeted to run on a similar varied spectrum of hardware, although all based on the 8086 processor. Thus, MS-DOS was for many years sold only as an OEM product. There was no Microsoft-branded MS-DOS: MS-DOS could not be purchased directly from Microsoft, and each OEM release was packaged with the trade dress of the given PC vendor. The different versions were in general incompatible with different hardware. Bugs were to be reported to the OEM, not to Microsoft. However, as clones became widespread, it soon became clear that the OEM versions of MS-DOS were virtually identical, except perhaps for the provision of a few utility programs.

At first, few "compatibles" other than Compaq's offered compatibility beyond the DOS/BIOS level. Reviewers and users developed suites of programs to test compatibility; the ability to run Lotus 1-2-3 or Microsoft Flight Simulator became one of the most significant "stress tests". Vendors gradually learned not only how to emulate the IBM BIOS but also where to use identical hardware chips to perform key functions within the system. Eventually, the Phoenix BIOS and similar commercially-available products permitted computer makers to build essentially 100%-compatible clones without having to reverse-engineer the IBM PC BIOS themselves.

Over time, IBM damaged its own market by itself failing to appreciate the importance of "IBM compatibility", introducing products such as the IBM Portable (which underperformed and selled les than the earlier Compaq Portable) and the PCjr, which had significant incompatibilities with the original PC and was quickly discontinued. By the mid to late 1980s buyers began to regard PCs as commodity items, and doubted that the security blanket of the IBM brand warranted the price difference. Meanwhile, MS-DOS-compatible (but not hardware-compatible) systems did not succeed in the marketplace. Being unable to run off-the-shelf high-performance software packages that the IBM PC and true compatibles could made for poor sales and the eventual extinction of this category of systems. Because of hardware incompatibility with the IBM PC design, the 80186 processor released only a year after the IBM PC was never popular in general-purpose personal computers.

However, as the market evolved, despite the failure of MCA, IBM derived a considerable income stream from license fees from companies who paid for licenses to use IBM patents that were in the PC design, to the extent that IBM's focus changed from discouraging PC clones to maximizing its revenue from license sales. IBM finally relinquished its role as a PC manufacturer in April 2005, when it sold its PC division to Lenovo for $1.75 billion.

As of October 2007, Hewlett-Packard and Dell hold the largest shares of the PC market in North America. They are also successful overseas, with Acer, Lenovo, and Toshiba also notable. Worldwide, a huge number of PCs are "white box" systems assembled by a myriad of local systems builders. Despite advances in computer technology, all current IBM PC compatibles remain very much compatible with the original IBM PC computers, although most of the components implement the compatibility in special backward compatibility modes used only during a system boot.

One of the strengths of the PC compatible platform is its modular hardware design. End-users could readily upgrade peripherals and to some degree, processor and memory without modifying the computer's motherboard or replacing the whole computer, as was the case with many of the microcomputers of the time. However, as processor speed and memory width increased, the limits of the original XT/AT bus design were soon reached, particularly when driving graphics video cards. IBM did introduce an upgraded bus in the IBM PS/2 computer that overcame many of the technical limits of the XT/AT bus, but this was rarely used as the basis for IBM compatible computers since it required licence payments to IBM both for the PS/2 bus and any prior AT-bus designs produced by the company seeking a license. This was unpopular with hardware manufacturers and several competing bus standards were developed by consortiums, with more agreeable license terms. Various attempts to standardize the interfaces were made, but in practice, many of these attempts were either flawed or ignored. Even so, there were many expansion options, and the PC compatible platform advanced much faster than other competing platforms of the time, even if only because of its market dominance.

In the 1990s, IBM's influence on PC architecture became increasingly irrelevant. An IBM-brand PC became the exception not the rule. Instead of focusing on staying compatible with the IBM PC, vendors began to focus on compatibility with the evolution of Microsoft Windows. In 1993, a version of Windows NT was released that could run on processors other than x86. (It did require that applications be recompiled, a step most developers didn't take.) Still, its hardware independence was taken advantage of by SGI x86 workstations - thanks to NT's HAL, they could run NT (and its vast application library). No mass-market personal computer hardware vendor dares to be incompatible with the latest version of Windows, and Microsoft's annual WinHEC conferences provide a setting in which Microsoft can lobby for and —in some cases— dictate the pace and direction of the hardware side of the PC industry. Microsoft and Windows have become so important to the ongoing development of the PC hardware that industry writers have taken to using the term "Wintel architecture" ("Wintel" being a portmanteau of "Windows" and "Intel") to refer to the combined hardware-software platform. This terminology itself is becoming a misnomer, as on the one hand Intel has lost absolute control over the direction of the hardware platform's development due to the influence of others, and on the other hand non-Windows operating systems running on this hardware platform have established and maintained a notable presence.

Although the IBM PC was designed for expandability, the designers could not anticipate the hardware developments of the '80s. To make things worse, IBM's choice of the Intel 8088 for the CPU introduced several limitations which were hurdles for developing software for the PC compatible platform. For example, the 8088 processor only had a 20-bit memory addressing space. To expand PCs beyond one megabyte, Lotus, Intel, and Microsoft jointly created expanded memory (EMS), a bank-switching scheme to allow more memory provided by add-in hardware, and seen through a set of four 16-Kilobyte "windows" inside the 20-bit addressing. Later, Intel CPUs had larger address spaces and could directly address 16- MiBs (80286) or more, leading Microsoft to develop extended memory (XMS) which did not require additional hardware.

Expanded and extended memory have incompatible interfaces, so anyone writing software that used more than one megabyte had to support both systems for the greatest compatibility until MS-DOS began including EMM386, which simulated EMS memory using XMS memory. A protected mode OS can also be written for the 80286, but DOS application compatibility was harder than expected, not only because most DOS application directly accessed the hardware, but also that most BIOS requests were made via IRQs, hindering multitasking and programmer's predictions of speed.

Video cards suffered from their own incompatibilities. Once video cards advanced to SVGA the standard for accessing them was no longer clear. At the time, PC programming used a memory model that had 64 KB memory segments. The most common VGA graphics mode's screen memory fitted into a single memory segment. SVGA modes required more memory, so accessing the full screen memory was tricky. Each manufacturer developed their own ways of accessing the screen memory, even going so far as not to number the modes consistently. An attempt at creating a standard called VBE was made, but not all manufacturers adhered to it.

Because of the wide number of third-party adapters and no standard for them, programming the PC could be difficult. Professional developers would run a large test-suite of various hardware combinations. Even the PC itself had no clear application interface to the flat memory model the 386 and higher could provide in protected mode.

When the 386 arrived, again a protected mode OS could be written for it. This time, DOS compatibility was much easier because of virtual 8086 mode. Unfortunately programs could not switch directly between them, so eventually, some new memory-model APIs were developed, VCPI and DPMI, the latter becoming the most popular.

Meanwhile, consumers were overwhelmed by the many different combinations of hardware on offer. To give them some idea of what sort of PC they would need to run their software, the Multimedia PC (MPC) standard was set in 1990. A PC that met the minimum MPC standard could be considered, and marketed as, an MPC. Software that could run on the most minimal MPC-compliant PC would be guaranteed to run on any MPC. The MPC level 2 and MPC level 3 standards were later set, but the term "MPC compliant" never caught on. After MPC level 3 in 1996, no further MPC standards were established.

The success of Microsoft Windows had driven nearly all other rival commercial operating systems into near-extinction, and had ensured that the “IBM PC compatible” computer was the dominant computing platform. This meant that if a manufacturer only made their software for the Wintel platform, they would be able to reach out to the vast majority of computer users. By the late 1980s, the only major competitor to Windows with more than a few percentage points of market share was Apple Inc.'s Macintosh. The Mac started out billed as "the computer for the rest of us" but the DOS/Windows/Intel onslaught quickly drove the Macintosh into an education and desktop publishing niche, from which it has only recently begun to emerge. By the mid 1990s Mac marketshare had dwindled to around 5% and introducing a new rival operating system had become too risky a commercial venture. Experience had shown that even if an operating system was technically superior to Windows, it would be a failure in the marketplace (BeOS and OS/2 for example). In 1989 Steve Jobs said of his new NeXT platform, "It will either be the last new hardware platform to succeed, or the first to fail." In 1993 NeXT announced it was ending production of the NeXTcube and porting NeXTSTEP to Intel processors. In 1997, NeXT was acquired by Apple, which then introduced the iMac in 1998, and afterwards the Mac continues to regain marketshare, which is still happening today.

On the hardware front, Intel initially licensed their technology so that other manufacturers could make x86 CPUs. As the "Wintel" platform gained dominance Intel abandoned this practice. Companies such as AMD and Cyrix developed alternative CPUs that were functionally compatible with Intel's. Towards the end of the 1990s, AMD was taking an increasing share of the CPU market for PCs. AMD even ended up playing a significant role in directing the evolution of the x86 platform when its Athlon line of processors continued to develop the classic x86 architecture as Intel deviated with its "Netburst" architecture for the Pentium 4 CPUs and the IA-64 architecture for the Itanium line of server CPUs. AMD developed the first 64 bit extension of the x86 architecture that forced Intel to make a clean-room version of it, in all its latest CPUs. In 2006 Intel began abandoning Netburst with the release of their line of "Core" processors that represented an evolution of the earlier Pentium III.

The term 'IBM PC compatible' is not commonly used for current computers because all the mainstream computers are now PC compatibles. Most competing platforms have either died off or been relegated to niche, enthusiast markets like the Amiga. One notable exception was Apple Macintosh computers, that were running on PowerPC architecture until 2006, when Apple switched its computers to Intel processors and adopted the x86 architecture, which is IBM PC compatible. The processor speed and memory capacity of modern PCs are many orders of magnitude greater than they were on the original IBM PC and yet backwards compatibility has been largely maintained - a 32-bit operating system published in the 2000s can still run many of the simpler programs written for the OS of the early 1980s without needing an emulator.

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IBM Personal Computer

IBM original PC.jpg

The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981. It was created by a team of engineers and designers under the direction of Don Estridge of the IBM Entry Systems Division in Boca Raton, Florida.

Alongside "microcomputer" and "home computer", the term "personal computer" was already in use before 1981. It was used as early as 1972 to characterize Xerox PARC's Alto. However, because of the success of the IBM Personal Computer, the term came to mean more specifically a microcomputer compatible with IBM's PC products.

The original line of PCs were part of an IBM strategy to get into the small computer market then dominated by the Commodore PET, Atari 8-bit family, Apple II and Tandy Corporation's TRS-80s, and various CP/M machines. IBM's first desktop microcomputer was the IBM 5100, introduced in 1975. It was a complete system - with a built-in monitor, keyboard, and data storage. It was also very expensive - up to US$20,000. It was specifically designed for professional and scientific problem-solvers, not business users or hobbyists. When the PC was introduced in 1981, it was originally designated as the IBM 5150, putting it in the "5100" series, though its architecture wasn't directly descended from the IBM 5100.

Rather than going through the usual IBM design process, a special team was assembled with authorization to bypass normal company restrictions and get something to market rapidly. This project was given the code name Project Chess at the IBM Entry Systems Division in Boca Raton, Florida. The team consisted of twelve people directed by Don Estridge with Chief Scientist Larry Potter and Chief Systems Architect Lewis Eggebrecht. They developed the PC in about a year. To achieve this they first decided to build the machine with "off-the-shelf" parts from a variety of different original equipment manufacturers (OEMs) and countries. Previously IBM had always developed their own components. Secondly for scheduling and cost reasons, rather than developing unique IBM PC monitor and printer designs, project management decided to utilize an existing "off-the-shelf" IBM monitor developed earlier in IBM Japan as well as an existing Epson printer model. Consequently, the unique IBM PC industrial design elements were relegated to the system unit and keyboard. They also decided on an open architecture, so that other manufacturers could produce and sell peripheral components and compatible software without purchasing licenses. IBM also sold an IBM PC Technical Reference Manual which included a listing of the ROM BIOS source code.

At the time, Don Estridge and his team considered using the IBM 801 processor and its operating system that had been developed at the Thomas J. Watson Research Center in Yorktown Heights, New York (The 801 is an early RISC microprocessor designed by John Cocke and his team at Yorktown Heights.) The 801 was at least an order of magnitude more powerful than the Intel 8088, and the operating system many years more advanced than the DOS operating system from Microsoft, that was finally selected. Ruling out an in-house solution made the team’s job much easier and may have avoided a delay in the schedule, but the ultimate consequences of this decision for IBM were far-reaching. IBM had recently developed the Datamaster business microcomputer which used an Intel processor and peripheral ICs; familiarity with these chips and the availability of the Intel 8088 processor was a deciding factor in the choice of processor for the new product. Even the 62-pin expansion bus slots were designed to be similar to the Datamaster slots. Delays due to in-house development of the Datamaster software also influenced the design team to a fast track development process for the PC, with publicly-available technical information to encourage third-party developers.

Other manufacturers soon reverse engineered the BIOS to produce their own non-infringing functional copies. Columbia Data Products introduced the first IBM-PC compatible computer in June 1982. In November 1982, Compaq Computer Corporation announced the Compaq Portable, the first portable IBM PC compatible. The first models were shipped in March 1983.

Once the IBM PC became a commercial success, the product came back under the more usual tight IBM management control. IBM's tradition of "rationalizing" their product lines, deliberately restricting the performance of lower-priced models in order to prevent them from "cannibalizing" profits from higher-priced models, worked against them.

ComputerLand and Sears Roebuck partnered with IBM from the beginning of development. IBM's head of sales and marketing, H.L. ('Sparky') Sparks, relied on these retail partners for important knowledge of the marketplace.

As a natural progression, Computerland and Sears became the main outlets for the new product. More than 190 Computerland stores already existed, while Sears was in the process of creating a handful of in-store computer centers for sale of the new product. This guaranteed IBM widespread distribution across the United States.

Targeting the new PC at the home market, Sears Roebuck sales failed to live up to expectations. This unfavourable outcome revealed that the original strategy - targeting the office market - was the key to higher sales.

All IBM personal computers are software compatible with each other in general, but not every program will work in every machine. Some programs are time sensitive to a particular speed class. Older programs will not take advantage of newer higher-resolution display standards.

The original PC had a version of Microsoft BASIC — IBM Cassette BASIC — in ROM. The CGA (Color Graphics Adapter) video card could use a standard television set or an RGBI monitor for display; IBM's RGBI monitor was their display model 5153. The other option that was offered by IBM was an MDA (Monochrome Display Adapter) and their monochrome display model 5151. It was possible to install both an MDA and a CGA card and use both monitors concurrently, if supported by the application program. For example, AutoCAD allowed use of a CGA card for graphics and a separate monochrome board for text menus. Some model 5150 PCs with CGA monitors and a printer port also included the MDA adapter by default, because IBM provided the MDA port and printer port on the same adapter card; it was in fact an MDA/printer port combo card.

Although the TV-compatible video board, cassette port and FCC Class B certification were all aimed at making it a home computer the original PC proved too expensive for the home market. At introduction a PC with 64 kB of RAM and a single 5 1/4 inch floppy drive and monitor sold for US $3,005, while the cheapest configuration ($1,565) that had no floppy drives, only 16KB RAM, and no monitor (again, the expectation was that users would connect their existing TV sets and cassette recorders) proved too unattractive and low-spec, even for its time (cf. footnotes to the above IBM PC range table). While the 5150 did not become a top selling home computer, its floppy-based configuration became an unexpectedly large success with businesses.

The "IBM Personal Computer XT", IBM's model 5160, was an enhanced machine that was designed for business use. It had 8 expansion slots and a 10 megabyte hard disk (later versions 20MB). Unlike the model 5150 PC, the model 5160 XT no longer had a cassette jack. The XT could take 256 kB of memory on the main board (using 64 kbit DRAM); later models were expandable to 640 kB. (The 384 kB of BIOS ROM, video RAM, and adapter ROM space filled the rest of the one megabyte address space of the 8088 CPU.) It was usually sold with a Monochrome Display Adapter (MDA) video card. The processor was a 4.77 MHz Intel 8088 and the expansion bus 8-bit Industry Standard Architecture (ISA) with XT bus architecture. The XT's expansion slots were placed closer together than with the original PC; this rendered the XT's case and mainboard incompatible with the model 5150's case and mainboard. The slots themselves and the peripheral cards however were compatible. The XT's expansion slot spacing was identical to the one that is still used as of 2008, albeit with different actual slots and bus standards.

The "IBM Personal Computer/AT", announced August 1984, uses an Intel 80286 processor, originally at 6 MHz. It has a 16-bit ISA bus and 20 MB (20 million bytes) hard drive. A faster model, running at 8 MHz, was introduced in 1986. IBM made some attempt at marketing it as a multi-user machine, but it sold mainly as a faster PC for power users. Early PC/ATs were plagued with reliability problems, in part because of some software and hardware incompatibilities, but mostly related to the internal 20 MB hard disk. While some people blamed IBM's hard disk controller card and others blamed the hard disk manufacturer Computer Memories Inc. (CMI), the IBM controller card worked fine with other drives, including CMI's 33-megabyte model. The problems introduced doubt about the computer and, for a while, even about the 286 architecture in general, but after IBM replaced the 20 MB CMI drives, the PC/AT proved reliable and became a lasting industry standard.

The main circuit board in an IBM PC is called the motherboard (IBM terminology calls it a planar). This carries the CPU and memory, and has a bus with slots for expansion cards.

The bus used in the original PC became very popular, and was subsequently named ISA. It is in use to this day in computers for industrial use. Later, requirements for higher speed and more capacity forced the development of new versions. IBM introduced the MCA bus with the PS/2 line. The VESA Local Bus allowed for up to three, much faster 32-bit cards, and the EISA architecture was developed as a backward compatible standard including 32-bit card slots, but it only sold well in high-end server systems. The lower-cost and more general PCI bus was introduced in 1994 and has now become ubiquitous.

The motherboard is connected by cables to internal storage devices such as hard disks, floppy disks and CD-ROM drives. These tend to be made in standard sizes, such as 3.5" (90 mm) and 5.25" (133.4 mm) widths, with standard fixing holes. The case also contains a standard power supply unit (PSU) which is either an AT or ATX standard size.

Intel 8086 and 8088-based PCs require expanded memory (EMS) boards to work with more than one megabyte of memory. The original IBM PC AT used an Intel 80286 processor which can access up to 16 megabytes of memory (though standard DOS applications cannot use more than one megabyte without using additional APIs.) Intel 80286-based computers running under OS/2 can work with the maximum memory.

The original 1981 IBM PC's keyboard at the time was an extremely reliable and high quality electronic keyboard originally developed in North Carolina for the Datamaster system . Each key was rated to be reliable to over 100 million keystrokes. For the IBM PC, a separate keyboard housing was designed with a novel usability feature that allowed users to adjust the keyboard angle for personal comfort. Compared with the keyboards of other small computers at the time, the IBM PC keyboard was far superior and played a significant role in establishing a high quality impression. For example, the industrial design of the keyboard, together with the system unit, was recognized with a major design award. Byte magazine in the fall of 1981 went so far as to state that the keyboard was 50 percent of the reason to buy an IBM PC. The importance of the keyboard was definitely established when the 1983 IBM PCjr flopped, in very large part for having a much different and mediocre Chiclet keyboard that made a poor impression on customers. Oddly enough, the same thing almost happened to the original IBM PC when in early 1981 management seriously considered substituting a cheaper but lower quality keyboard. This mistake was narrowly avoided by the advice of one of the original development engineers.

However, the original 1981 IBM PC's keyboard was severely criticized by typists for its non-standard placement of the Return and left Shift keys. In 1984, IBM corrected this on its AT keyboard, but shortened the 'backspace' key, making it harder to reach. In 1987, it introduced the enhanced keyboard, which relocated all the function keys and the Ctrl keys. The Esc key was also relocated to the opposite side of the keyboard.

Another criticism of the original keyboard was the relatively loud "clack" sound each key made when pressed. Since typewriter users were accustomed to keeping their eyes on the hardcopy they were typing from and had come to rely on the mechanical sound that was made as each character was typed onto the paper to ensure that they had pressed the key hard enough (and only once), the PC keyboard electronic "clack" feature was intended to provide that same reassurance. However, it proved to be very noisy and annoying, especially if many PCs were in use in the same room, and later keyboards were significantly quieter.

An "IBM PC compatible" may have a keyboard that does not recognize every key combination a true IBM PC does, such as shifted cursor keys. In addition, the "compatible" vendors sometimes used proprietary keyboard interfaces, preventing the keyboard from being replaced.

Although the PC/XT and AT used the same style of keyboard connector, the low-level protocol for reading the keyboard was different between these two series. An AT keyboard could not be used in an XT, nor the reverse. Third-party keyboard manufacturers provided a switch to select either AT-style or XT-style protocol for the keyboard.

The serial port is an 8250 or a derivative (such as the 16450 or 16550), mapped to eight consecutive IO addresses and one interrupt request line.

Only COM1: and COM2: addresses were defined by the original PC. Attempts to share IRQ 3 and IRQ4 to use additional ports require special measures in hardware and software, since shared IRQs were not defined in the original PC design.

The original IBM PC used the 7-bit ASCII alphabet as its basis, but extended it to 8 bits with nonstandard character codes. This character set was not suitable for some international applications, and soon a veritable cottage industry emerged providing variants of the original character set in various national variants. In IBM tradition, these variants were called code pages. These codings are now obsolete, having been replaced by more systematic and standardized forms of character coding, such as ISO 8859-1, Windows-1251 and Unicode. The original character set is known as code page 437.

As mentioned above, IBM equipped the model 5150 with a cassette port for connecting a cassette drive, and originally intended compact cassettes to become the 5150's most common storage medium. However, adoption of the floppy- and monitor-less configuration was low; few (if any) IBM PCs left the factory without a floppy disk drive installed. Also, DOS was not available on cassette tape, only on floppy disks (hence "Disk Operating System"). 5150s with just external cassette recorders for storage could only use the built-in ROM BASIC as their operating system. As DOS saw increasing adoption, the incompatibility of DOS programs with PCs that used only cassettes for storage made this configuration even less attractive.

Most or all 5150 PCs had one or two 5¼ inch floppy disk drives. These floppy drives were either single-sided double-density drives (SS/DD, aka SSDD), or double-sided double-density drives (DS/DD, aka DSDD). The IBM PC never used single density floppy drives. The drives and disks were commonly referred to by capacity, e.g. "160KB floppy disk" or "360KB floppy drive", but because this is not entirely unambiguous, they are here referred to using the less commonly used but more accurate SSDD and DSDD terminology. DSDD drives were backwards compatible; they could read and write SSDD floppies. The same type of physical diskette could be used for both drives, however to convert a 5¼" SSDD disk to a DSDD disk, it needed to be reformatted, at which point SSDD drives could no longer read it.

The disks were Modified Frequency Modulation (MFM) coded in 512-byte sectors, and were soft-sectored. They contained 40 tracks per side at the 48 track per inch (TPI) density, and initially were formatted to contain 8 sectors per track. This meant that SSDD disks initially had a formatted capacity of 160 KB, while DSDD disks had a capacity of 320 KB. However, the DOS operating system was later updated to allow formatting the disks with 9 sectors per track. This yielded a formatted capacity of 180 KB with SSDD disks/drives, and 360 KB with DSDD disks/drives. The unformatted capacity of the floppy disks was advertised as 250KB (SSDD) and 500KB (DSDD), however these "raw" 250/500KB were not the same thing as the usable formatted capacity; under DOS, the maximum capacity for SSDD and DSDD disks was 180KB and 360KB, respectively. Regardless of type, the file system of all floppy disks was FAT12.

While the SSDD drives initially were the only floppy drives available for the model 5150 PC, IBM later switched to DSDD drives, and the majority of 5150 PCs sold eventually shipped with one or two DSDD drives. The 5150's successor, the model 5160 IBM XT, never shipped with SSDD drives; it generally had one double-sided 360 kB drive (next to its internal hard disk). While it was technically possible to retrofit more advanced floppy drives such as the high-density drive (released in 1984) into the original IBM PC, this was not an option offered by IBM for the 5150 model, and the move to high-density 5.25" floppies in particular was notoriously fraught with compatibility problems.

IBM's original floppy disk controller card also included an external 37-pin D-shell connector. This allowed users to connect additional external floppy drives by third party vendors. IBM themselves did not offer external floppy drives.

The 5150 could not itself power hard drives without retrofitting a stronger power supply, but IBM later offered the 5161 Expansion Unit, which not only provided more expansion slots, but also included a 10MB (later 20MB) hard drive powered by the 5161's own separate 130-watt power supply.

The first IBM PC that shipped with an internal, fixed, non-removable hard disk was IBM's model 5160, the XT. However, as other IBM-compatible PCs started to appear, hard disks with larger storage capacities than the 5160's and 5161's initial 10MB (later 20MB) also became available, and could — space permitting — be installed into either the IBM PC's Expansion Unit or into PSU-upgraded model 5150 IBM PCs (or into XTs). Adding a third-party hard disk sometimes required plugging in a new controller board, because some of these hard drives were not compatible with the existing disk controller. Some third party hard disks for IBM PCs even sold as kits including a controller card and replacement power supply. Finally, some hard disks were integrated with their controller in a single expansion board, commonly called a "Hard Card".

The IBM PC's ROM BASIC supported cassette tape storage. DOS itself did not support cassette tape storage. PC-DOS version 1.00 supported only 160KB SSDD floppies, but version 1.1, which was released 9 months after the PC's introduction, supported 160KB SSDD and 320KB DSDD floppies. Support for the slightly larger 9 sector per track 180KB and 360KB formats arrived 10 further months later in March 1983.

All IBM PCs include a relatively small piece of software stored in ROM. The original IBM PC 40 KB ROM included 8 KB for power-on self-test (POST) and basic input/output system (BIOS) functions plus 32 KB BASIC in ROM (Cassette BASIC). The ROM BASIC interpreter was the default user interface if no DOS boot disk was present. BASICA was distributed on floppy disk and provided a way to run the ROM BASIC under PC-DOS control.

In addition to PC-DOS, buyers could choose either CP/M-86 or UCSD p-System as operating systems. Due to their higher prices, they never became very popular and PC-DOS or MS-DOS came to be the dominant operating system.

While the IBM PC technology is largely obsolete by today's standards, many are still in service. As of June 2006, IBM PC and XT models are still in use at the majority of U.S. National Weather Service upper-air observing sites. The computers are used to process data as it is returned from the ascending radiosonde, attached to a weather balloon. They are being phased out over a several year period, to be replaced by the Radiosonde Replacement System.

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History of IBM

T-REX Corporate Center was originally one of IBM's research labs where the IBM PC was created.

For issues and trends that span particular time periods, see major events, trends, and technologies, below.

IBM's history dates back decades before the development of electronic computers. It originated as the Tabulating Machine Company in 1896, founded by Herman Hollerith, and specialized in the development of punched card data processing equipment. Hollerith's series of patents on tabulating machine technology, first applied for in 1884, drew on his work at the U.S. Census Bureau from 1879–82. Hollerith was initially trying to reduce the time and complexity needed to tabulate the 1890 Census. His transition to the use of punch cards in 1886 laid a foundation for generations of equipment and a core component of what would become IBM.

The company originally sold some machines to a railway company but quickly focused on the easy profits of the 1900 US Census. The census sustained the company for another 3 years, but as the census wound down it returned to targeting private businesses, including automatic punching, tabulating and sorting machines in 1908. By 1911 Hollerith was 51 and his health was failing and the business was sold to Charles Flint for $2.3 million (of which Hollerith got $1.2 million) to form the Computing Tabulating Recording (CTR) Corporation. This was incorporated on June 16, 1911 in Endicott, New York, United States of America.

CTR was formed through a merger of three different companies: the Tabulating Machine Company, the International Time Recording Company (founded 1900 in Endicott), and the Computing Scale Corporation (founded 1901 in Dayton, Ohio, USA). Flint was the financier and key person behind the merger and remained a member of the board of CTR until his retirement in 1930.

The companies that merged to form CTR manufactured a wide range of products, including employee time-keeping systems, weighing scales, automatic meat slicers, and most importantly for the development of the computer, punched card equipment.

Thomas J. Watson Sr. became General Manager of CTR in 1914 and President in 1915. In 1917, CTR entered the Canadian market under the name of International Business Machines Co., Limited. On February 14, 1924, CTR changed its name to International Business Machines Corporation, or IBM. At the helm during this period, Watson played a central role in establishing what would become the IBM organization and culture.

During the next twenty-five years, IBM's organization and product lines grew steadily. Despite the Great Depression of the 1930s, IBM continued to develop and manufacture new products, and after the Social Security Act of 1935 secured a major government contract to maintain employment data for 26 million people. IBM's archive website describes this as "the biggest accounting operation of all time," and it opened the door for a variety of other government contracts.

In 1928, IBM introduced a new 80 column rectangular-hole punched card. This format became the standard "IBM Card" that was used by the company's tabulators and computers for many decades.

The rise of Nazi Germany and the onset of World War II had a profound impact on IBM. Like many U.S. businesses, IBM had relationships and contracts with the German military/industrial technocracy. This topic is addressed in more detail below (see IBM's role in WWII and the Holocaust).

When World War II began, all IBM facilities were placed at the disposal of the U.S. government. IBM's product line expanded to include bombsights, rifles and engine parts – in all, more than three dozen major ordnance items. Thomas Watson, Sr., set a nominal one percent profit on those products and used the money to establish a fund for widows and orphans of IBM war casualties.

In particular, IBM manufactured the Browning Automatic Rifle and the M1 Carbine. Allied military forces widely utilized IBM's tabulating equipment for military accounting, logistics, and other war-related purposes. There was extensive use of IBM punch-card machines for calculations made at Los Alamos during the Manhattan Project for developing the first atomic bombs; this has been notably discussed by Richard Feynman in his book, Surely You're Joking, Mr. Feynman!. During the War IBM also built the Harvard Mark I for the U.S. Navy, the first large-scale automatic digital computer in the U.S.

In the 1950s, IBM became a chief contractor for developing computers for the United States Air Force's automated defense systems. Working on the SAGE interceptor control system, IBM gained access to crucial research being done at Massachusetts Institute of Technology, working on the first real-time, digital computer (which included many other advancements such as an integrated video display, magnetic core memory, light guns, the first effective algebraic computer language, analog-to-digital and digital-to-analog conversion techniques, digital data transmission over telephone lines, duplexing, multiprocessing, and networks). IBM built fifty-six SAGE computers at the price of US$30 million each, and at the peak of the project devoted more than 7,000 employees (20% of its then workforce) to the project. More valuable to the company in the long run than the profits, however, was the access to cutting-edge research into digital computers being done under military auspices. IBM neglected, however, to gain an even more dominant role in the nascent industry by allowing the RAND Corporation to take over the job of programming the new computers, because, according to one project participant, Robert P. Crago, "we couldn't imagine where we could absorb two thousand programmers at IBM when this job would be over some day, which shows how well we were understanding the future at that time." IBM would use its experience designing massive, integrated real-time networks with SAGE to design its SABRE airline reservation system, which met with much success.

IBM was the largest of the eight major computer companies (with UNIVAC, Burroughs, NCR, Control Data Corporation, General Electric, RCA and Honeywell) through most of the 1960s. People in this business would talk jokingly of "IBM and the seven dwarfs," given the much smaller size of the other companies' computer divisions (IBM produced approximately 70 % of all computers in 1964).

The major technical development of the 1960s was IBM's System/360 series.

In 1970, GE sold most of its computer business to Honeywell and in 1971, RCA sold its computing division to Sperry Rand. With only Burroughs, UNIVAC, NCR, Control Data, and Honeywell producing mainframes, people then talked, but by now in humorless, blistering criticism, of "IBM and the BUNCH." In April 1973 Honeywell v. Sperry Rand, a landmark U.S. federal court case, was decided. That decision invalidated the 1964 patent for the ENIAC, the world's first general-purpose electronic digital computer, thus putting the invention of the electronic digital computer into the public domain.

Most of those companies are now long gone as IBM competitors, except for Unisys, which is the result of multiple mergers that included Sperry Rand, UNIVAC and Burroughs, and General Electric, which has re-entered the business in recent years. NCR and Honeywell dropped out of the general mainframe and mini sector and concentrated on lucrative niche markets, NCR's being cash registers (hence the name, National Cash Register), and Honeywell becoming the market leader in thermostats. The IBM computer, the IBM mainframe, that earned it its position in the market at that time is still growing today. It was originally known as the IBM System/360 and, in far more modern 64-bit form, is now known as the IBM System z10.

IBM's success in the mid-1960s led to inquiries as to IBM antitrust violations by the U.S. Department of Justice, which filed a complaint for the case U.S. v. IBM in the United States District Court for the Southern District of New York, on January 17, 1969. The suit alleged that IBM violated the Section 2 of the Sherman Act by monopolizing or attempting to monopolize the general purpose electronic digital computer system market, specifically computers designed primarily for business. Litigation continued until 1983, and had a significant impact on the company's practices. In 1973, IBM was ruled to have created a monopoly via its 1956 patent-sharing agreement with Sperry-Rand in the decision of Honeywell v. Sperry Rand, a decision that invalidated the patent on the ENIAC.

A key event at IBM in 1969 was the decision to "unbundle" software from hardware sales. See unbundling of software and services, below.

The major technical development of the 1970s was IBM's System/370 series.

Between 1971 and 1975, IBM investigated the feasibility of a new revolutionary line of products designed to make obsolete all existing products in order to re-establish its technical supremacy. This effort, known as the Future Systems project, was terminated by IBM's top management in 1975, but had consumed most of the high-level technical planning and design resources during five years, thus jeopardizing progress of the existing product lines (although some elements of FS were later incorporated into actual products).

In the 1980s, IBM consolidated its mainframe business, and expanded the scope of mainframes with the S/390 and ESA/390 series. Importantly, during this time, the company embarked on the practice of converting its large rental base of mainframes to lease agreements. This financial strategy created the perception that IBM's revenues and profits were much stronger than they really were as in the mid to latter part of the decade, management scrambled to react to the spending shift towards distributed computing, which threatened the monopoly IBM held within the technology business.

The company hired Don Estridge at the IBM Entry Systems Division in Boca Raton, Florida. With a team known as "Project Chess," they built the IBM PC, launched on August 12, 1981. Although not cheap, at a base price of US$1,565 it was affordable for businesses — and many businesses purchased PCs. Typically, these purchases were not by corporate computer departments, as the PC was not seen as a "proper" computer. Purchases were often instigated by middle managers and senior staff who saw the potential — once the revolutionary VisiCalc spreadsheet, the killer app, had been surpassed by a far more powerful and stable product, Lotus 1-2-3. Reassured by the IBM name, they began buying microcomputers on their own budgets aimed at numerous applications that corporate computer departments did not, and in many cases could not, accommodate.

Up to this point in its history, IBM relied on a vertically integrated strategy, building most key components of its systems itself, including processors, operating systems, peripherals, databases and the like. In an attempt to speed time to market for the PC, IBM chose not to build the operating system and microprocessor internally, rather it sourced these vital components from Microsoft and Intel respectively. Ironically, in a decade which marked the end of IBM's monopoly, it was this fateful decision by IBM that passed the sources of its monopolistic power (operating system and processor architecture) to Microsoft and Intel, paving the way for the creation of hundreds of billions of dollars of market value outside of IBM.

In the midrange arena, IBM consolidated the market position its General Systems Division had built in the 1970s with the System/3, System/32 and System/34. The System/38, with its radical architecture, had experienced delays to its first customer shipment since announcement in 1978. In 1982, IBM disbanded the organization that had meant the Data Processing Division sold only mainframes to large customers while the General Systems Division sold only S/3x machines to small and medium-sized customers. Instead, the new ISM (for small and medium customers) and ISAM divisions (large customers) could sell from the entire IBM portfolio.

As the decade ended, it was clear that competition and innovation in the computer industry was now taking place along segmented, versus vertically integrated lines, where leaders emerged in their respective domains. Examples included Intel in microprocessors, Microsoft in desktop software, Novell in networking, HP in printers, Seagate in disk drives and Oracle in database software. Soon IBM's dominance in personal computers would be challenged by the likes of Compaq and later Dell. Recognizing this trend, CEO John Akers, with the support of the Board of Directors, began to split IBM into increasingly autonomous business units (e.g. processors, storage, software, services, printers, etc.) to compete more effectively with competitors that were more focused and nimble and had lower cost structures.

IBM's traditional mainframe business underwent major changes in the 1990s, as customers increased their emphasis on departmental and desktop computing. However, the decade of the 1990s began with IBM posting record profits up to that point in its history. This proved illusory as the rental to lease conversion was tapping out, demand for mainframes was waning and corporate downsizing was in full swing. Corporate spending shifted from high profit margin mainframes to lower margin microprocessor-based systems and the growth in IBM's PC business was not nearly enough to offset the company's mainframe revenue decline.

On October 5, 1992, at the COMDEX computer expo, IBM announced the first ThinkPad laptop computer, the 700c. The computer, which then cost US$4350, included a 25 MHz Intel 80486SL processor, a 10.4-inch active matrix display, removable 120 MB hard drive, 4 MB RAM (expandable to 16 MB) and a TrackPoint II pointing device.

A decade of steady acceptance and widening corporate growth of local area networking technology, a trend headed by Novell Inc. and other vendors, and its logical counterpart, the ensuing decline of mainframe sales, brought about a wake-up call for IBM: after two consecutive years of reporting losses in excess of $1 billion, on January 19, 1993, IBM announced a US$8.10 billion loss for the 1992 financial year, which was then the largest single-year corporate loss in U.S. history.

That same year, Louis V. Gerstner, Jr. joined IBM and he is widely credited with turning the company around. His strategy to reverse the decision of his predecessor and re-integrate IBM's major divisions to focus on services first and products second, is often heralded as the decision that led the company from the brink of disaster and remains the fundamental underpinning of IBM's strategy today. A byproduct of that decision was a shift in focus significantly away from components and hardware and towards software and services.

Starting in 1995 with its acquisition of Lotus Development Corp., IBM built up the Software Group from one brand, DB2, to five: DB2, Lotus, WebSphere, Tivoli, and Rational.

In 2002, IBM strengthened its business advisory capabilities by acquiring the consulting arm of professional services firm PricewaterhouseCoopers. The company has increasingly focused on business solution-driven consulting, services and software, with emphasis also on high-value chips and hardware technologies; as of 2005 it employs about 195,000 technical professionals. That total includes about 350 «Distinguished Engineers» and 60 IBM Fellows, its most-senior engineers. It should be noted, however, that IBM and some other U.S. firms use the term 'engineer' in a broad sense, applying it to technicians from diverse disciplines who may not be graduates from Engineering Schools of recognized Universities.

In 2002, IBM announced the beginning of a US$10 billion program to research and implement the infrastructure technology necessary to be able to provide supercomputer-level resources "on demand" to all businesses as a metered utility. The program has since then been implemented.

In the same year its hard disk operations was sold to Hitachi.

IBM has steadily increased its patent portfolio since the early 1990s, which is valuable for cross-licensing with other companies. In every year from 1993 to 2005, IBM has been granted significantly more U.S. patents than any other company. The thirteen-year period has resulted in over 31,000 patents for which IBM is the primary assignee. In 2003, IBM earned 3415 patents, breaking the US record for patents in a single year.

Protection of the company's intellectual property has grown into a business in its own right, generating over $10 billion dollars to the bottom line for the company during this period. A 2003 Forbes article quotes Paul Horn, head of IBM Research, saying that IBM has generated $1 billion in profit by licensing intellectual property.

In 2004, IBM announced the proposed sale of its PC business to Chinese computer maker Lenovo Group, which is partially owned by the Chinese government, for US$650 million in cash and US$600 million in Lenovo stock. The deal was approved by the Committee on Foreign Investment in the United States in March 2005, and completed in May 2005. IBM acquired a 19% stake in Lenovo, which moved its headquarters to New York State and appointed an IBM executive, Steve Ward, as its chief executive officer. The company retained the right to use certain IBM brand names for an initial period of five years. As a result of the purchase, Lenovo inherited a product line that features the ThinkPad, a line of laptops that had been one of IBM's most successful products.

As of 2004, IBM had shifted much of its focus to the provision of business consulting & re-engineering services from its hardware & technology focus. The new IBM has enhanced global delivery capabilities in consulting, software and technology based process services—and this change is reflected in its top-line.

On June 20, 2006, IBM and Georgia Institute of Technology jointly announced a new record in silicon-based chip speed at 500 GHz. This was done by freezing the chip to 4.5 K (−269 °C; −452 °F) using liquid helium and is not comparable to CPU speed. The chip operated at about 350 GHz at room temperature.

A significant part of IBM's operations were dedicated to the support of the U.S. Federal Government, with a wide range of projects ranging from the U.S. Census Bureau to the Department of Defense to the National Security Agency. These projects spanned mundane administrative processing to ultra-secret supercomputing.

IBM's early dominance of the computer industry was in part due to its strong professional services activities. IBM's advantage in building software for its own computers eventually was seen as monopolistic, leading to antitrust proceedings. As a result, a complex, artificial "arms-length" relationship was created separating IBM's computer business from its service organizations. This situation persisted for decades. An example was IBM Global Services, a huge services firm that competed with the likes of Electronic Data Systems or Computer Sciences Corporation.

At the time, the unbundling of services was perhaps the most contentious point, involving antitrust issues that had recently been widely debated in the press and the courts. However, IBM's unbundling of software had long-term impacts. After the unbundling event, IBM software was divided into two main categories: System Control Programming (SCP), which remained free to customers; and Program Products (PP), which were subject to a separate cost. This transformed the customer's value proposition for computer solutions, giving a significant monetary value to something that, before, had essentially been free. This helped enable the creation of a software industry.

Early IBM computer systems, like those from many other vendors, were programmed using assembly language. Computer science efforts through the 1950s and early 1960s led to the development of many new high-level languages for programming. IBM played a complicated role in this process. Hardware vendors were naturally concerned about the implications of portable languages that would allow customers to pick and choose among vendors without compatibility problems. IBM, in particular, helped create barriers that tended to lock customers into a single platform.

IBM developed a schizophrenic relationship with the UNIX and Linux worlds. The importance of IBM's large computer business placed strange pressures on all of IBM's attempts to develop other lines of business. All IBM projects faced the risk of being seen as competing against company priorities. This was because, if a customer decided to build an application on an RS/6000 platform, this also meant that a decision had been made against a mainframe platform. So despite having some excellent technology, IBM often placed itself in a compromised position. For UNIX zealots, this meant that IBM lagged behind leaders like Sun Microsystems.

A case in point is IBM's GFIS products for infrastructure management and GIS applications. Despite long having a dominant position in such industries as electric, gas, and water utilities, IBM stumbled badly in the 1990s trying to build workstation-based solutions to replace its old mainframe-based products. Customers were forced to move on to new technologies from other vendors; many felt betrayed by IBM.

IBM embraced open source technologies in the 1990s. It later became embroiled in a complex litigation with SCO group over intellectual property rights related to the UNIX and Linux platforms.

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IBM System i

i5 Model 570 (2006)

The IBM System i is IBM's previous generation of systems designed for IBM i users, and was subsequently replaced by the IBM Power Systems in April 2008.

In 2006, the platform was rebranded to System i as part of IBM's Systems branding initiative. Previously it was known as eServer iSeries in 2000 and before that it was introduced as AS/400 in 1988.

In April 2008 IBM announced its integration with the System p platform. The unified product line is called IBM Power Systems and features support for the IBM i (previously known as i5/OS or OS/400), AIX and Linux operating systems. Previous hardware ran OS/400 exclusively.

The AS/400 was introduced in 1988 by IBM as a minicomputer for general business and departmental use. It underwent several rebrandings until its last rebrand in 2006 to the name of IBM System i. It remained in production until April 2008 when it was replaced by the IBM Power Systems line. It uses an object-based library-based operating system called IBM i. The operating system also underwent rebrandings in accordance with the name changes of the general line. At first it was called OS/400 (following the name schema that gave birth to OS/2 and OS/390). Later on became known as i5/OS in line with the introduction of the powerful eServer i5 servers featuring POWER5 processors. Finally, it was called just IBM i coinciding with the 6.1 release.

Features include a DBMS (DB2/400), a menu-driven interface, multi-user support, dumb terminal support (IBM 5250), printers, as well as security, communications and web-based; which could be programmed either inside the (optional) IBM WebSphere application server or in PHP/MySQL using a native port of the Apache web server.

While in Unix-like systems everything is a file, on the System i everything is an object, with built-in persistence and garbage collection. It also offers Unix-like file directories using the Integrated File System . Java compatibility is implemented through a native port of the Java virtual machine.

The IBM System i platform extended the System/38 architecture of an object-based system with an integrated DB2 database that was designed to implement E. F. Codd's relational database model, which is based on Codd's 12 rules, in the operating system and hardware. Equally important were the virtual machine and single-level storage concepts which established the platform as an advanced business computer.

One feature that contributes to the longevity of the IBM System i platform is its high-level instruction set (called TIMI for "Technology Independent Machine Interface" by IBM), which allows application programs to take advantage of advances in hardware and software without recompilation. TIMI is a virtual instruction set; it is not the instruction set of the underlying CPU. User-mode programs contain both TIMI instructions and the machine instructions of the CPU, thus ensuring hardware independence. This is conceptually somewhat similar to the virtual machine architecture of programming environments such as Smalltalk, Java and .NET. The key difference is that it is embedded so deeply into the AS/400's design as to make applications effectively binary-compatible across different processor families.

Note that, unlike some other virtual-machine architectures in which the virtual instructions are interpreted at runtime, TIMI instructions are never interpreted. They constitute an intermediate compile time step and are translated into the processor's instruction set as the final compilation step. The TIMI instructions are stored within the final program object, in addition to the executable machine instructions. This is how application objects compiled on one processor family (e.g., the original CISC AS/400 48-bit processors) could be moved to a new processor (e.g., PowerPC 64-bit) without re-compilation. An application was saved from the older 48-bit platform and restored onto the new 64-bit platform, where the operating system discarded the old machine instructions and re-translated the TIMI instructions into 64-bit instructions for the new processor.

The IBM System i's instruction set defines all pointers as 128-bit. This was an original design feature of the System/38 (S/38) in the mid 1970s. For PowerPC processors, the virtual address resides in the rightmost 64 bits of a pointer (48 bits in the S/38 and CISC AS/400), leaving room for addresses to be expanded past 64 bits in future processors. The 64-bit address space addresses all of main memory and disk (the single-level storage concept).

The original AS/400 CISC models used the same 48-bit address space as the S/38. This was expanded to 64-bits in 1995 when the PowerPC RISC 64-bit CPU processor replaced the 48-bit CISC processor.

The IBM System i includes an extensive library-based operating system, i5/OS, and is also capable of supporting multiple instances of AIX, Linux, Lotus Domino, Microsoft Windows 2000 and Windows Server 2003. While i5/OS, AIX, Linux and Lotus Domino are supported on the POWER processors, Windows is supported with either single-processor internal blade servers (IXS) or externally-linked multiple-processor servers (IXA and iSCSI). iSCSI also provides support for attachment of IBM Bladecenters. Windows, Linux, and VMWare ESX(VI3) are supported on iSCSI attached servers.

LPAR (Logical PARtitioning), a feature introduced from IBM's mainframe computers, facilitates running multiple operating systems simultaneously on one IBM System i unit. A system configured with LPAR can run various operating systems on separate partitions while ensuring that one OS cannot run over the memory or resources of another. Each LPAR is given a portion of system resources (memory, hard disk space, and CPU time) via a system of weights that determines where unused resources are allocated at any given time. The operating systems supported (and commonly used) under the LPAR scheme are i5/OS, AIX, and Linux.

Other features include an integrated DB2 database management system, a menu-driven interface, multi-user support, non-programmable terminals (IBM 5250) and printers, security, communications, client-server and web-based applications. Much of the software necessary to run the IBM System i is included and integrated into the base operating system.

The IBM System i also supports common client-server-based technologies such as ODBC and JDBC for accessing its database from client software such as Java, Microsoft .NET languages and others.

The IBM System i also provides an environment for AIX applications to run natively on i5/OS without the need for an AIX LPAR.

AIX programs are binary compatible with OS/400 when using OS/400's PASE (Portable Applications System Environment). PASE is essentially "an operating system within an operating system", supporting the most recent stable version of AIX. Binaries need to be re-compiled on the AIX system, with 16-byte (quadword) pointer alignment enabled. Once the program is compiled with this option, it can be executed under the PASE Korn Shell.

Programming languages available for the AS/400 include RPG, assembly language, C, C++, Pascal, Java, EGL, Perl, Smalltalk, COBOL, SQL, BASIC, PHP, PL/I, Python and REXX. Several CASE tools are available: AllFusion Plex (see *Plex Wiki), ADELIA, Synon, AS/SET, IBM Rational Business Developer Extension, LANSA and ProGen Plus.

The IBM System i fully supports the Java language, including a 32-bit Java Virtual Machine (JVM) and a 64-bit JVM.

Commands in the Control Language (CL) are promptable using the keyboard F4 function key, and most provide cursor-sensitive help to make specifying command parameters simpler. All command names and parameter keywords are based upon uniform standardized and mostly 3-letter abbreviations for verbs and subjects, making for easy rendering and interpretation by the application developer, as opposed to other operating systems with often cryptic or inconsistent command names for related functions or command parameter switches. For instance, the parameter keyword to apply a text description to any object to be created or changed is spelled the same way for all such commands.

For traditional business programming languages such as RPG, COBOL, and C, the IBM System i provides an interface to the integrated database that allows these languages to treat database files much like other platforms treat ISAM or VSAM files.

Support for 5250 display operations is provided via display files, an interface between workstations, keyboards and displays, and interactive applications, as opposed to batch processing with little or no user interaction. ASCII terminals and PC workstations are equally and well supported, also via internet or LAN network access supplemented by either IBM or non-IBM communication software, for example TELNET or TELNET 5250.

The IBM System i, then known as the AS/400, was the continuation of the System/38 database machine architecture (announced by IBM in October 1978 and delivered in August 1979). The AS/400 removed capability-based addressing. The AS/400 added source compatibility with the System/36 combining the two primary computers manufactured by the IBM Rochester plant. The System/36 was IBM's most successful mini-computer but the architecture had reached its limit. The first AS/400 systems (known by the development code names Silverlake and Olympic) were delivered in 1988, and the product line has been refreshed continually since then. The programmers who worked on OS/400, the operating system of the AS/400, did not have a UNIX background. Dr Frank Soltis, the chief architect, says that this is the main difference between this and any other operating system.

The AS/400 was the first general-purpose computer system to attain a C2 security rating from the NSA, and in 1995 was extended to employ a 64-bit processor and operating system.

In 2000 IBM renamed the AS/400 to iSeries, as part of its e-Server branding initiative. The product line was further extended in 2004 with the introduction of the i5 servers, the first to use the IBM POWER5 processor. The architecture of the system allows for future implementation of 128-bit processors when they become available. Existing applications can use the new hardware without modification.

The AS/400 was originally based on a custom IBM CISC CPU which used a CPU architecture known as Internal MicroProgrammed Interface (IMPI) and an instruction set similar to the IBM 370. It was later migrated to a POWER-based RISC CPU family eventually known as RS64.

The System i5 uses IBM POWER CPUs. These CPUs are developed and manufactured by IBM. The POWER 4/5/5+ chips contain two cores. There are Multi-Chip Modules (MCM) available. They have 2 CPUs (4 cores) or 4 CPUs (8 cores) in one MCM.

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Source : Wikipedia