|Release date||August 12, 1981|
|Discontinued||April 2, 1987|
|Operating system||IBM BASIC / PC-DOS 1.0
|CPU||Intel 8088 @ 4.77 MHz|
|Memory||16 kiB ~ 256 kiB|
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 $20,000 USD. 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 complete circuit schematics, a listing of the ROM BIOS source code, and other engineering and programming information. IBM announced the PC on August 12, 1981. Six weeks later at COMDEX Fall, Tecmar had 20 PC products available for sale. These products included memory expansion, IEEE-488, data acquisition and PC Expansion chassis.    
At the time, Don Estridge and his team considered using the IBM 801 processor (an early RISC CPU) and its operating system that had been developed at the Thomas J. Watson Research Center in Yorktown Heights, New York. 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.
The success of the IBM computer lead other companies to develop IBM Compatibles, which in turn led to brandings like diskettes being in IBM format, or systems complaining about no ROM-BASIC on booting. In essence, during the bulk of the 1980s and early 1990s, the main machines that were talked about in the press and in how-to guides, were IBM ones.
One could to a great deal build a IBM clone with off-the-shelf parts, but the BIOS required some reverse-engineering. Companies like American Megatrends, Award, and others achieved workable clones of this, allowing companies like DELL, Compaq, HP et al, to manufacture PCs that worked like IBM ones. These did not have a ROM-BASIC, so when ROM-Basic was expected to load in the absence of a boot device, a message might appear saying no ROM BASIC.
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 of America.
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 of targeting the office market was the key to higher sales.
|Model name||Model #||Introduced||CPU||Features|
|PC||5150||August 1981||8088||Floppy disk or cassette system|
|XT||5160||March 1983||8088||First IBM PC to come with an internal hard drive as standard.|
|XT/370||5160/588||October 1983||8088||5160 with XT/370 Option Kit and 3278/79 Emulation Adapter|
|3270 PC||5271||October 1983||8088||With 3270 terminal emulation|
|PCjr||4860||November 1983||8088||Floppy-based home computer|
|Portable||5155||February 1984||8088||Floppy-based portable|
|AT||5170||August 1984||80286||Medium-speed hard disk|
|AT/370||5170/599||October 1984||80286||5170 with AT/370 Option Kit and 3278/79 Emulation Adapter|
|3270 AT||5281||June 1985 ||80286||With 3270 terminal emulation|
|Convertible||5140||April 1986||8088||Microfloppy laptop portable|
|XT 286||5162||September 1986||80286||Slow hard disk, but zero wait state memory on the motherboard. This 6 MHz machine was actually faster than the 8 MHz ATs (when using planar memory) because of the zero wait states|
All IBM personal computers are software backwards-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.
The most commonly used storage medium was the floppy disk, though cassette tape was originally envisoned by IBM as a low-budget alternative. Accordingly, the IBM 5150 PC was available with one or two floppy drives or without any drives or storage medium; in the latter case IBM intended for a user to connect their own cassette recorder via the 5150's cassette jack. The cassette tape port was mechanically identical to, and located next to, the keyboard port on the 5150's mainboard. A hard disk could not be installed into the 5150's system unit without retrofitting a stronger power supply, but an "Expansion Unit", a.k.a. the "IBM 5161 Expansion Chassis" was available, which came with one 10MB hard disk and also allowed the installation of a second hard disk. The system unit had five expansion slots; the expansion unit had eight; however, one of the system unit's slots and one of the expansion unit's slots had to be occupied by the Extender Card and Receiver Card, respectively, which were needed to connect the expansion unit to the system unit and make the expansion unit's other slots available, for a total of 11 slots. A working configuration required that some of the slots be occupied by display, disk, and I/O adapters, as none of these were built in to the 5150's motherboard; the only on-board external connectors were the keyboard and cassette ports; the simple PC speaker sound hardware was also on-board. The original PC's maximum memory using IBM parts was 256 kiB, 64 kiB on the main board and three 64 kiB expansion cards. The processor was an Intel 8088 (early 1978 version, later were 1978/81/2 versions of intel chip; second-sourced AMDs were used after 1983) running at 4.77 MHz (4/3 the standard NTSC color burst frequency of 3.579545 MHz). Some owners replaced the 8088 with an NEC V20 for a slight increase in processing speed. An Intel 8087 co-processor could also be added for hardware floating-point arithmetic. IBM sold the PC in configurations with 16 kiB or 64 kiB of RAM preinstalled using either nine or thirty-six 16-kibit DRAM chips. (The ninth bit was used for parity checking of memory.)
Although the TV-compatible video board, cassette port and Federal Communications Commission 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 kiB of RAM and a single 5 1/4 inch floppy drive and monitor sold for 3005 US$, while the cheapest configuration (1565 US$) that had no floppy drives, only 16kiB RAM, and no monitor (again, under the expectation 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 diskette and hard drive storage introduced two years after the introduction of the "IBM Personal Computer". 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 kiB of memory on the main board (using 64 kibit DRAM); later models were expandable to 640 kiB. (The 384 kiB of BIOS ROM, video RAM, and adapter ROM and RAM 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, unless a rare card designed for the PC happened to use the extra width of the 5150's slots. The XT's expansion slot spacing was identical to the one that was later used in the IBM PC AT and is still used as of 2009, though with different actual slots and bus standards.
The "IBM Personal Computer/AT" (model 5170), announced August 1984, used an Intel 80286 processor, originally running at 6 MHz. It had a 16-bit ISA bus and 20 MB 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 mainly carries the CPU and RAM, and it has a bus with slots for expansion cards. On the motherboard are also the ROM subsystem, DMA and IRQ controllers, coprocessor socket, sound (PC speaker, tone generation) circuitry, and keyboard interface.
The bus used in the original PC became very popular, and it was subsequently named ISA. While it was popular, it was more commonly known as the PC-bus or XT-bus; the term ISA arose later when industry leaders chose to continue manufacturing machines based on the IBM PC AT architecture rather than license the PS/2 architecture and its MCA bus from IBM. The XT-bus was then retroactively named 8-bit ISA or XT ISA, while the unqualified term ISA usually refers to the 16-bit AT-bus (as better defined in the ISA specifications.) The AT-bus is an extension of the PC-/XT-bus and is in use to this day in computers for industrial use.
A monitor and any floppy or hard disk drives are connected to the motherboard through cables connected to graphics adapter and disk controller cards, respectively, installed in expansion slots. Memory expansion beyond the amount installable on the motherboard was also done with boards installed in expansion slots, and I/O devices such as parallel, serial, or network ports were likewise installed as individual expansion boards. For this reason, it was easy to fill the five expansion slots of the PC, or even the eight slots of the XT, even without installing any special hardware. Companies like Quadram and AST addressed this with their popular multi-I/O cards which combined several peripherals on one adapter card that uses only one slot; Quadram offered the QuadBoard and AST the SixPak.
Intel 8086 and 8088-based PCs require expanded memory (EMS) boards to work with more than 640 kiB of memory. (Though the 8088 can address one megabyte of memory, the last 384 kiB of that is used or reserved for the BIOS ROM, BASIC ROM, extension ROMs installed on adapter cards, and memory address space used by devices including display adapter RAM and even the 64kiB EMS page frame itself.) The original IBM PC AT used an Intel 80286 processor which can access up to 16 MiB 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 and lower quality keyboard. This mistake was narrowly avoided on 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.
On the technical side, the IBM PC keyboard is very robust and flexible. At the electrical and low-level interface level, every key is equal in basic capability: each key sends a signal when it is pressed and another signal when it is released. An integrated microcontroller in the keyboard manages electrical scanning of the keyboard and encodes a "scan code" and "release code" for each key as it is pressed and released separately. Therefore, any key can be used as a shift key, and a large number of keys can be held down simultaneously and separately sensed. The controller in the keyboard handles typematic operation, issuing periodic repeat scan codes for a depressed key and then a single release code when the key is finally released.
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. The AT keyboard uses a bidirectional interface which allows the computer to send commands to the keyboard. An AT keyboard could not be used in an XT, nor the reverse. Third-party keyboard manufacturers provided a switch on some of their keyboards to select either AT-style or XT-style protocol for the keyboard.
|COM Port||IRQ||Base port address [Hex]|
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. The ROM BIOS supported cassette operations.
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, a.k.a. SSDD), or double-sided double-density drives (DS/DD, a.k.a. 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 kiB, while DSDD disks had a capacity of 320 kiB. 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 kiB with SSDD disks/drives, and 360 kiB with DSDD disks/drives. The unformatted capacity of the floppy disks was advertised as "250KB" for SSDD and "500KB" for DSDD ("KB" ambiguously referring to either 1000 or 1024 bytes; essentially the same for rounded-off values), however these "raw" 250/500 kB were not the same thing as the usable formatted capacity; under DOS, the maximum capacity for SSDD and DSDD disks was 180 kiB and 360 kiB, respectively. Regardless of type, the file system of all floppy disks (under DOS) 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 kiB 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 5¼″ floppies in particular was notoriously fraught with disk 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. As other IBM-compatible PCs started to appear, hard disks with larger storage capacities also became available. Space permitting, these could be installed into either the IBM PC's Expansion Unit or into PSU-upgraded PCs or 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 were sold as kits including a controller card and replacement power supply. Finally, some hard disks were integrated with their controller into a single expansion card, commonly called a "Hard Card".
The IBM PC's ROM BASIC and BIOS supported cassette tape storage. DOS itself did not support cassette tape storage. PC-DOS version 1.00 supported only 160kiB SSDD floppies, but version 1.1, which was released 9 months after the PC's introduction, supported 160 kiB SSDD and 320 kiB DSDD floppies. Support for the slightly larger 9 sector per track 180 kiB and 360 kiB 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 kiB ROM included 8 kiB for power-on self-test (POST) and basic input/output system (BIOS) functions plus 32 kiB BASIC in ROM (Cassette BASIC). The 40 kiB of ROM was present in five 8 kiB ROM DIP chip packages installed in sockets. (A sixth empty socket was provided for a customer's own custom ROM, and some vendors resold special-purpose PC units with the BASIC ROMs and even the BIOS ROM replaced with specialized custom ROMs.) While small compared to the maximum and even the common RAM size, the 40 kiB of ROM was large compared to subsequent PC clones, which omitted the ROM BASIC and provided only the POST and BIOS ROM programs. 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 (a.k.a. IBM-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 were still in use at the majority of U.S. National Weather Service upper-air observing sites. The computers were used to process data as it is returned from the ascending radiosonde, attached to a weather balloon. They were phased out over a several year period, replaced by the Radiosonde Replacement System. Factors that have contributed to the 5150 PC's longevity are its flexible modular design, open technical standard making information needed to adapt, modify, and repair it readily available, use of few special nonstandard parts, and rugged high-standard IBM manufacturing, the last of which provided for exceptional long-term reliability and durability. Most newer PCs, by contrast, use special-purpose chips (ASICs) implementing trend-driven technology which becomes obsolete in a few years—after which the parts become unavailable.
|IBM Personal Computers||Succeeded by
IBM Personal Computer XT
IBM Portable Personal Computer
IBM Personal Computer/AT
IBM PC Convertible
The PC (short for Personal Computer) is a versatile gaming platform, but it is generally not classed as a console.
PCs can run a variety of operating systems, but because of Microsoft's dominance of the operating system market almost all PC games are only released for Windows; prior to Windows 95 and the advent of DirectX, games were instead primarily produced for MS-DOS. As a result, terms such as "PC games" and "computer games" have come to refer specifically to games programmed for DOS and/or Windows. Game releases for other PC operating systems (such as Linux or Mac OS) are instead referred to by the name of the operating system itself ("Linux games", "Mac games", etc.)