IBM Personal Computer, commonly known as the
IBM PC, is the
original version and progenitor of the
IBM PC compatible
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.
The generic term personal computer was in use before 1981, applied as
early as 1972 to the Xerox PARC's Alto, but because of the success of
IBM Personal Computer, the term "PC" came to mean more
specifically a desktop microcomputer compatible with IBM's Personal
Computer branded products. Within a short time of the introduction,
third-party suppliers of peripheral devices, expansion cards, and
software proliferated; the influence of the
IBM PC on the personal
computer market was substantial in standardizing a platform for
personal computers. "
IBM compatible" became an important criterion for
sales growth; after the 1980s, only the
Apple Macintosh family kept a
significant share of the microcomputer market without compatibility
IBM personal computer.
1.2 Too late?
1.4 Project Chess
1.5 Open standards
1.6.1 The Little Tramp
1.7 Third-party products
IBM PC as standard
3 Third-party distribution
4.1 Original PC
5.2 Peripheral integrated circuits
5.3 Joystick port
5.5 Character set
5.6 Storage media
5.6.1 Cassette tape
5.6.2 Floppy diskettes
5.6.3 Fixed disks
5.6.4 OS support
5.8 Video output
Serial port addresses and interrupts
5.10 Printer port
9 See also
12 Further reading
13 External links
International Business Machines
International Business Machines (IBM), one of the world's largest
companies, had a 62% share of the mainframe computer market in
1981. In the late 1970s the new personal computer industry was
dominated by the Commodore PET, Atari 8-bit family, Apple II, Tandy
Corporation's TRS-80, and various
CP/M machines. With $150 million
in sales by 1979 and projected annual growth of more than 40% in the
early 1980s, the microcomputer market was large enough for IBM's
attention. Other large technology companies such as Hewlett-Packard
Texas Instruments (TI), and
Data General had entered it, and
IBM customers were buying Apples, so the company
saw introducing its own personal computer as both an experiment in a
new market and a defense against rivals, large and small.
In 1980 and 1981 rumors spread of an
IBM personal computer, perhaps a
miniaturized version of the
IBM System/370, while Matsushita
acknowledged that it had discussed with
IBM the possibility of
manufacturing a personal computer for the American company. The
Japanese project, codenamed "Go", ended before the 1981 release of the
IBM PC codenamed "Chess", but two simultaneous
projects further confused rumors about the forthcoming product.
Data General and TI's small computers were not very successful, but
observers expected AT&T to soon enter the computer industry, and
other large companies such as Exxon, Montgomery Ward, Pentel, and Sony
were designing their own microcomputers. Whether
IBM had waited
too long to enter an industry in which Apple and others were already
successful was unclear.
An observer stated that "
IBM bringing out a personal computer would be
like teaching an elephant to tap dance." Successful microcomputer
company Vector Graphic's fiscal 1980 revenue was $12 million. A
IBM computer in the early 1960s cost as much as $9 million,
occupied one quarter acre of air-conditioned space, and had a staff of
60 people; in 1980 its least-expensive computer, the 5120, still
cost about $13,500. The "Colossus of Armonk" only sold through its
internal sales force, had no experience with resellers or retail
stores, and did not introduce the first product
designed to work with non-
IBM equipment until 1980.
Another observer claimed that
IBM made decisions so slowly that, when
tested, "what they found is that it would take at least nine months to
ship an empty box". As with other large computer companies, its
new products typically required about four to five years for
IBM had to learn how to quickly develop,
mass-produce, and market new computers. While the company
traditionally let others pioneer a new market—
IBM released its first
commercial computer a year after Remington Rand's
UNIVAC in 1951, but
within five years had 85% of the market—the personal-computer
development and pricing cycles were much faster than for mainframes,
with products designed in a few months and obsolete quickly.
Many in the microcomputer industry resented IBM's power and wealth,
and disliked the perception that an industry founded by startups
needed a latecomer so staid that it had a strict dress code and
employee songbook. The potential importance to microcomputers
of a company so prestigious, that a popular saying in American
companies stated "No one ever got fired for buying IBM", was
nonetheless clear. InfoWorld, which described itself
as "The Newsweekly for
Microcomputer Users", stated that "for my
grandmother, and for millions of people like her,
IBM and computer are
synonymous". Byte ("The Small Systems Journal") stated in an
editorial just before the announcement of the
Rumors abound about personal computers to come from giants such as
Digital Equipment Corporation
Digital Equipment Corporation and the
General Electric Company. But
there is no contest. IBM's new personal computer ... is far and away
the media star, not because of its features, but because it exists at
all. When the number eight company in the
Fortune 500 enters the
field, that is news ... The influence of a personal computer made by a
company whose name has literally come to mean "computer" to most of
the world is hard to contemplate.
The editorial acknowledged that "some factions in our industry have
IBM as the 'enemy'", but concluded with optimism: "I want
to see personal computing take a giant step."
Desktop sized programmable calculators by HP had evolved into the HP
BASIC language computer by 1972. In 1972–1973 a team led by Dr.
Paul Friedl at the
IBM Los Gatos Scientific Center developed a
portable computer prototype called SCAMP (
Special Computer APL Machine
Portable) based on the
IBM PALM processor with a
cassette drive, small CRT, and full-function keyboard. SCAMP emulated
IBM 1130 minicomputer to run APL1130. In 1973 APL was generally
available only on mainframe computers, and most desktop sized
microcomputers such as the
Wang 2200 or HP 9800 offered only BASIC.
Because it was the first to emulate APL1130 performance on a portable,
PC Magazine in 1983 designated SCAMP a
"revolutionary concept" and "the world's first personal
computer". The prototype is in the Smithsonian Institution. A
non-working industrial design model was also created in 1973 by
industrial designer Tom Hardy illustrating how the SCAMP engineering
prototype could be transformed into a usable product design for the
marketplace. This design model was requested by
IBM executive Bill
Lowe to complement the engineering prototype in his early efforts to
demonstrate the viability of creating a single-user computer.
Successful demonstrations of the 1973 SCAMP prototype led to the IBM
5100 portable microcomputer in 1975. In the late 1960s such a machine
would have been nearly as large as two desks and would have weighed
about half a ton. The 5100 was a complete computer system
programmable in BASIC or APL, with a small built-in CRT monitor,
keyboard, and tape drive for data storage. It was also very expensive,
up to US$20,000; the computer was designed for professional and
scientific customers, not business users or hobbyists. BYTE in
1975 announced the 5100 with the headline "Welcome, IBM, to personal
PC Magazine in 1984 described 5100s as "little
mainframes" and stated that "as personal computers, these machines
were dismal failures ... the antithesis of user-friendly", with no IBM
support for third-party software. Despite news reports that it was
IBM product without a model number, when the PC was
introduced in 1981 it was designated as the
IBM 5150, putting it in
the "5100" series though its architecture was not directly
descended from the
IBM 5100. Later models followed in the trend: For
IBM Portable Personal Computer, PC/XT, and PC AT are IBM
machine types 5155, 5160, and 5170, respectively.
Following SCAMP, the
Boca Raton, Florida
Boca Raton, Florida Laboratory created
several single-user computer design concepts to support Lowe's ongoing
effort to convince
IBM there was a strategic opportunity in the
personal computer business. A selection of these early
concepts created by industrial designer Tom Hardy in the infancy of
personal computing is highlighted in the book DELETE: A Design History
of Computer Vapourware. One such concept in 1977, code-named Aquarius,
was a working prototype utilizing advanced bubble memory cartridges.
While this design was more powerful and smaller than
Apple II launched
the same year, the advanced bubble technology was deemed unstable and
not ready for mass production.
Some employees opposed
IBM entering the market. One said, "Why on
earth would you care about the personal computer? It has nothing at
all to do with office automation." "Besides", he added, "all it can do
is cause embarrassment for IBM". The company studied personal
Walden C. Rhines
Walden C. Rhines of TI, for example, in 1978 met
with a Boca Raton group considering the
TMS9900 for a secret 16-bit
microprocessor-based project—but had determined from studying
the market for years, and building the prototypes during the 1970s,
IBM was unable to internally build a personal computer
John Opel was not among those skeptical of personal
computers. He and CEO
Frank Cary had created more than one dozen
semi-autonomous "Independent Business Units" (IBU) to encourage
innovation; Fortune called them "How to start your own
company without leaving IBM". After Lowe became the first head of
the Entry Level Systems IBU in Boca Raton his team researched the
market. Computer dealers were very interested in selling an IBM
product, but told Lowe that the company could not design, sell, or
service it as
IBM had previously done. An
IBM microcomputer, they
said, must be composed of standard parts that store employees could
repair. While dealers disliked Apple's business practices,
including a shortage of the
Apple II while the company focused on the
more sophisticated Apple III, they saw no alternative because they
doubted that IBM's traditional sales methods and bureaucracy would
Atari in 1980 proposed that it act as original equipment manufacturer
IBM microcomputer. Aware that the company needed to enter the
market quickly—even the schools in Broward County, near Boca
Raton, purchased Apples—in July 1980 Lowe met with Opel, Cary,
and others on the important Corporate Management Committee.
Lowe demonstrated the proposal with an industrial design model by Tom
Hardy based on the Atari 800 platform, and suggested acquiring Atari
"because we can't do this within the culture of IBM".
Cary agreed about the culture, observing that
IBM would need "four
years and three hundred people" to develop its own personal computer;
Lowe, however, promised one in a year if done without traditional IBM
methods. Instead of acquiring Atari, the committee allowed him to
form an independent group of employees—"the Dirty Dozen", led by
engineer Bill Sydnes—which, Lowe promised, could design a prototype
in 30 days. The crude prototype barely worked when he demonstrated it
in August, but Lowe presented a detailed business plan that proposed
that the new computer have an open architecture, use non-proprietary
components and software, and be sold through retail stores, all
The committee agreed that Lowe's approach was the most likely to
succeed. With Opel's strong support, in October it approved turning
the group into another IBU codenamed "Project Chess" to develop
"Acorn", with unusually large funding to help achieve the goal of
introducing the product within one year of the August demonstration.
After Lowe's promotion
Don Estridge became the head of
Chess, and by January 1981 the team made its first
demonstration of the computer within IBM. Other key members
included Sydnes, Lewis Eggebrecht, David Bradley, Mark
Dean, and David O'Connor. Many were already hobbyists who
owned their own computers including Estridge, who had an Apple
II. After the team received permission to expand to 150 by the end
of 1980, it received more than 500 calls in one day from
interested in joining the IBU.
IBM normally was vertically integrated, only purchasing components
like transformers and semiconductors. It internally developed all
important hardware and software and discouraged customers from
purchasing third-party products compatible with
IBM products. For
the PC the company avoided doing so as much as possible; choosing, for
example, to license
Microsoft BASIC despite having a BASIC of its own
for mainframes. (Estridge said that unlike IBM's own version
Microsoft BASIC had hundreds of thousands of users around the world.
How are you going to argue with that?") Although the company
denied doing so, many observers concluded that
emulated Apple when designing the PC. The many Apple II
owners on the team influenced its decision to design the computer with
an open architecture and publish technical information so others
could create software and expansion slot peripherals.
Although the company knew that it could not avoid competition from
third-party software on proprietary hardware—Digital Research
CP/M-86 for the
IBM Displaywriter, for example—it
considered using the
RISC processor and its operating system,
developed at the
Thomas J. Watson Research Center
Thomas J. Watson Research Center in Yorktown Heights,
New York. The 801 processor was more than an order of magnitude more
powerful than the
Intel 8088, and the operating system more advanced
PC DOS 1.0
PC DOS 1.0 operating system from Microsoft. 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
IBM were far-reaching.
IBM had recently developed the Datamaster business microcomputer,
which used a processor and other chips from Intel; familiarity with
them and the immediate availability of the 8088 was a reason for
choosing it for the PC. The 62-pin expansion bus slots were designed
to be similar to the Datamaster slots. Differences from the Datamaster
included avoiding an all-in-one design while limiting the computer's
size so that it would still fit on a standard desktop with the
keyboard (also similar to the Datamaster's), and 5.25" disk drives
instead of 8". Delays due to in-house development of the Datamaster
software was a reason why
available for the 8088—and published available technical information
to encourage third-party developers.
IBM chose the 8088 over
the similar but superior 8086 because
Intel offered a better price on
the former and could provide more units, and the 8088's 8-bit bus
reduced the cost of the rest of the computer.
The design for the computer was essentially complete by April 1981,
when the manufacturing team took over the project.
IBM could not
only use its own hardware and make a profit with "Acorn". To save time
and money, the IBU built the machine with commercial off-the-shelf
parts from original equipment manufacturers whenever possible, with
assembly occurring in Boca Raton. The IBU would decide whether it
would be more economical to "Make or Buy" each manufacturing
IBM divisions for the first time competed
with outsiders to build parts of the new computer; a North Carolina
IBM factory built the keyboard, the
Endicott, New York
Endicott, New York factory had to
lower its bid for printed circuit boards, and a Taiwanese company
built the monitor. The IBU chose an existing monitor from
IBM Japan and an
Epson printer. Because of the off-the-shelf parts
only the system unit and keyboard has unique
IBM industrial design
IBM copyright appears in only the ROM
BIOS and on the
company logo, and the company reportedly received no patents
on the PC, with outsiders manufacturing 90% of it. Because the
product would carry the
IBM logo, the only corporate division the IBU
could not bypass was the Quality Assurance Unit. A component
manufacturer described the process of being selected as a supplier as
rigorous and "absolutely amazing", with
IBM inspectors even testing
solder flux. They stayed after selection, monitoring and helping to
improve the manufacturing process. IBM's size overwhelmed other
companies; "a hundred
IBM engineers" reportedly visited
Mitel to meet
with two of the latter's employees about a problem, according to The
New York Times.
Another aspect of
IBM that did not change was its emphasis on
secrecy; employees at Yorktown knew nothing of Boca Raton's
activities. Those working on the project, within and outside of
IBM, were under strict confidentiality agreements. When an individual
mentioned in public on a Saturday that his company was working on
software for a new
IBM security appeared at the company
on Monday to investigate the leak. After an
discovered printouts in a supplier's garbage, the former company
persuaded the latter to purchase a paper shredder. Management Science
America did not know until after agreeing to buy
Peachtree Software in
1981 that the latter was working on software for the PC.
Developers such as
Software Arts received breadboard prototype
computers in boxes lined with lead to block
X-rays and sealed with
solder, and had to keep them in locked, windowless rooms; to
Microsoft emulated the PC on a DEC minicomputer and
used the prototype for debugging. After the PC's debut,
Raton employees continued to decline to discuss their jobs in public.
One writer compared the "silence" after asking one about his role at
the company to "hit[ting] the wall at the Boston Marathon: the
conversation is over".
IBM is proud to announce a product you may have a personal interest
in. It's a tool that could soon be on your desk, in your home or in
your child's schoolroom. It can make a surprising difference in the
way you work, learn or otherwise approach the complexities (and some
of the simple pleasures) of living.
It's the computer we're making for you.
IBM PC advertisement, 1982
After developing it in 12 months—faster than any other hardware
product in company history—
IBM announced the Personal Computer on 12
August 1981. Pricing started at US$1,565 (equivalent to $4,213 in
2017) for a configuration with 16K RAM, Color Graphics Adapter, and no
disk drives. The company intentionally set prices for it and other
configurations that were comparable to those of Apple and other
Dan Bricklin described as "pretty
competitive" pricing surprised him and other
employees; one analyst stated that
IBM "has taken the gloves
off", while the company said "we suggest [the PC's price] invites
comparison". Microsoft, Personal Software, and Peachtree Software
were among the developers of nine launch titles, including EasyWriter
and VisiCalc. In addition to the existing corporate sales force
IBM opened its own Product Center retail stores. After studying
Apple's successful distribution network, the company for the first
time sold through others,
ComputerLand and Sears
Roebuck. Because retail stores receive revenue
from repairing computers and providing warranty service,
IBM broke a
70-year tradition by permitting and training non-
IBM service personnel
to fix the PC.
IBM as having "the strongest marketing organization in
the world", but the PC's marketing also differed from that of
previous products. The company was aware of its strong corporate
reputation among potential customers; an early advertisement began
IBM of Personal Computers". The
advertisements emphasized the novelty of an individual owning an IBM
computer, describing "a product you may have a personal interest
in" and asking readers to think of "'My own
IBM computer. Imagine
that' ... it's yours. For your business, your project, your
department, your class, your family and, indeed, for yourself."
The Little Tramp
After considering Alan Alda, Beverly Sills, Kermit the Frog, and Billy
Martin as celebrity endorsers
IBM chose Charlie Chaplin's The
Little Tramp character—played by Billy Scudder—for a series of
advertisements based on Chaplin's films. The very popular and
award-winning $36-million marketing campaign made the star of Modern
Times—a film that expresses Chaplin's opposition to big business,
mechanization, and technological efficiency—the (as Creative
Computing described him) "warm cuddly" mascot of one of the world's
Chaplin and his character became so widely associated with IBM—Time
stated that "The Tramp ... has given [it] a human face"—that
others used his bowler hat and cane to represent or satirize the
company. Although the Chaplin estate sued those
Otrona who used the trademark without permission, PC Magazine's
April 1983 issue had 12 advertisements that referred to the Little
"We encourage third-part suppliers [for the PC] ... we are delighted
to have them",
IBM stated. It did not sell internally developed PC
software until April 1984, instead relying on already established
software companies. The company contacted
Microsoft even before
the official approval of Chess, and it and others received
cooperation that was, one writer said, "unheard of" for IBM. Such
openness surprised observers; BYTE called it "striking" and
"startling", and one developer reported that "it's a very
different IBM". Another said "They were very open and helpful
about giving us all the technical information we needed. The feeling
was so radically different—it's like stepping out into a warm
breeze." He concluded, "After years of hassling—fighting the
Not-Invented-Here attitude—we're the gods."
Most other personal-computer companies did not disclose technical
details; TI, for example, intentionally made developing
TI 99/4A software difficult, even requiring a
lockout chip in cartridges.
IBM itself kept its mainframe
technology so secret that rivals were indicted for industrial
espionage. For the PC, however,
IBM immediately released detailed
information. The US$36
IBM PC Technical Reference Manual included
complete circuit schematics, commented ROM
BIOS source code, and other
engineering and programming information for all of IBM's PC-related
hardware, plus instructions on designing third-party peripherals.
It was so comprehensive that one reviewer suggested that the manual
could serve as a university textbook, and so clear that a
developer claimed that he could design an expansion card without
seeing the physical computer.
IBM marketed the technical manual in full-page color print
advertisements, stating that "our software story is still being
written. Maybe by you". Sydnes stated that "The definition of a
personal computer is third-party hardware and software." Estridge said
IBM did not keep software development proprietary because it
would have to "out-
VisiCalc VisiCorp and out-Peachtree Peachtree—and
you just can't do that".
Another advertisement told developers that the company would consider
publishing software for "Education. Entertainment. Personal finance.
Data management. Self-improvement. Games. Communications. And yes,
business." Estridge explicitly invited small, "cottage" amateur
and professional developers to create products "with", he said,
"our logo and our support".
IBM sold the PC at a large discount to
employees, encouraged them to write software, and distributed a
catalog of inexpensive software written by individuals that might not
otherwise appear in public.
BYTE was correct in predicting that an
IBM personal computer would
receive much public attention. Its rapid development amazed
observers, as did the willingness of the Colossus of
sell as a launch title
Microsoft Adventure (a video game that, its
press release stated, brought "players into a fantasy world of caves
and treasures"); the company even offered an optional
joystick port. Future Computing estimated that "IBM's Billion
Dollar Baby" would have $2.3 billion in hardware sales by 1986.
David Bunnell, an editor at Osborne/McGraw-Hill, recalled that
None of my associates wanted to talk about the
Apple II or the Osborne
I computer anymore, nor did they want to fantasize about writing the
next super-selling program ... All they wanted to talk about was the
IBM Personal Computer—what it was, its potential and limitations,
and most of all, the impact
IBM would have on the business of personal
Within seven weeks Bunnell helped found PC Magazine, the first
periodical for the new computer.
Competitors were more skeptical.
Adam Osborne said "when you buy a
computer from IBM, you buy a la carte. By the time you have a computer
that does anything, it will cost more than an Apple. I don't think
Apple has anything to worry about." Apple's
Mike Markkula agreed that
IBM's product was more expensive than the Apple II, and claimed that
Apple III "offers better performance". He denied that the
offered more memory, stating that his company could offer more than
128K "but frankly we don't know what anyone would do with that
memory". At Tandy, John Roach said "I don't think it's that
Jon Shirley admitted that
IBM had a "legendary service
reputation" but claimed that its thousands of
Radio Shack stores "can
provide better service", while predicting that the
IBM PC's "major
market will be
IBM addicts"; another executive claimed that Tandy
could undersell a $3,000
IBM computer by $1,000. Many criticized the
PC's design as not innovative and outdated, and believed that its
alleged weaknesses, such as the use of single-sided, single-density
disks with less storage than the computer's RAM, existed because the
company was uncertain about the market and was experimenting before
releasing a better computer. (Estridge later boasted,
"Many ... said that there was nothing technologically new in this
machine. That was the best news we could have had; we actually had
done what we had set out to do.")
Rivals such as Apple, Tandy, and Commodore—together with more than
50% of the personal-computer market—had many advantages. While
IBM began with one microcomputer, little available hardware or
software, and a couple of hundred dealers,
Radio Shack had 14
million customers and 8,000 stores—more than McDonald's—that
only sold its broad range of computers and accessories. Apple had five
times as many dealers in the US as IBM, an established international
distribution network, and an installed base of more than 250,000
customers. Hundreds of independent developers produced software and
peripherals for both companies' computers; at least ten Apple
databases and ten word processors were available, while the PC had no
databases and one word processor. The computer had very
limited graphics capability, and customers who wanted both color and
high-quality text had to purchase two graphics cards and two
Steve Jobs at Apple ordered a team to examine an
IBM PC. After finding
Chris Espinosa called the computer "a half-assed,
hackneyed attempt"—the company confidently purchased a full-page
The Wall Street Journal
The Wall Street Journal with the headline "Welcome,
Bill Gates was at Apple headquarters
the day of IBM's announcement and later said "They didn't seem to
care. It took them a full year to realize what had happened".
IBM PC was immediately successful. BYTE reported a rumor that more
than 40,000 were ordered on the day of the announcement; John
Dvorak recalled that one dealer that day praised the computer as an
"incredible winner, and
IBM knows how to treat us — none of the
Apple arrogance". One dealer received 22 $1,000 deposits from
customers although he could not promise a delivery date. The
company could have sold its entire projected first-year production to
IBM customers that were reluctant to purchase Apples
were glad to buy microcomputers from its traditional supplier. The
computer began shipping in October, ahead of schedule; by then
some referred to it simply as the "PC".
BYTE estimated that 90% of the 40,000 first-day orders were from
software developers. By
COMDEX in November
Tecmar developed 20
products including memory expansion and expansion chassis,
surprising even IBM.
Jerry Pournelle reported after attending the
West Coast Computer Faire
West Coast Computer Faire in early 1982 that because
amateurs" with "documents that tell all", "an explosion of
[third-party] hardware and software" was visible at the
convention. Many manufacturers of professional business
application software, who had been planning/developing versions for
the Apple II, promptly switched their efforts over to the
IBM PC when
it was announced. Often, these products needed the capacity and speed
of a hard-disk. Although
IBM did not offer a hard-disk option for
almost two years following introduction of its PC, business sales were
nonetheless catalyzed by the simultaneous availability of hard-disk
subsystems, like those of
Tallgrass Technologies which sold in
Computerland stores alongside the
IBM 5150 at the introduction in
One year after the PC's release, although
IBM had sold fewer than
PC World counted 753 software packages for the
PC—more than four times the number available for the Apple Macintosh
one year after its 1984 release—including 422 applications and
almost 200 utilities and languages.
InfoWorld reported that "most
of the major software houses have been frantically adapting their
programs to run on the PC", with new PC-specific developers composing
"an entire subindustry that has formed around the PC's open system",
which Dvorak described as a "de facto standard microcomputer". The
magazine estimated that "hundreds of tiny garage-shop operations" were
in "bloodthirsty" competition to sell peripherals, with 30 to 40
companies in a price war for memory-expansion cards, for example.
PC Magazine renamed its planned "1001 Products to Use with Your IBM
PC" special issue after the number of product listings it received
exceeded the figure.
Tecmar and other companies that benefited
from IBM's openness rapidly grew in size and importance, as did PC
Magazine; within two years it expanded from 96 bimonthly to 800
monthly pages, including almost 500 pages of advertisements.
Gates estimated that
IBM would sell "not far from 200,000" PCs in
1982. By the end of that year the company was selling one every
minute of the business day. It estimated that 50 to 70% of PCs
sold in retail stores went to the home, and the publicity from
selling a popular product to consumers caused
IBM to, a spokesman
said, "enter the world" by familiarizing them with the Colossus of
Armonk. Although the PC only provided two to three percent of sales
the company found that it had underestimated demand by as much as
800%. Because its prices were based on forecasts of much lower
volume—250,000 over five years, which would have made the PC a very
IBM product—the PC became very profitable; at times the
company sold almost that many computers per month.
Estridge claimed in 1983 that from October 1982 to March 1983 customer
demand quadrupled. He stated that the company had increased production
three times in one year, and warned of a component shortage if demand
continued to increase. Many small suppliers' sales to
rapidly, both pleasing their executives and causing them to worry
about being overdependent on it. Miniscribe, for example, in 1983
received 61% of its hard drive orders from IBM; the company's stock
price fell by more than one third in one day after
IBM reduced orders
in January 1984. Suppliers often found, however, that the prestige of
IBM as a customer led to additional sales elsewhere.
Yankee Group estimated that ten new
products appeared every day. In August 1983 the Chess IBU, with
4,000 employees, became the Entry Systems Division, which observers
believed indicated that the PC was significantly important to IBM
overall, and no longer an experiment. The PC surpassed the
Apple II as the best-selling personal computer with more than
750,000 sold by the end of the year, while DEC only sold 69,000
microcomputers in the first nine months of the year despite offering
three models for different markets. Retailers also benefited, with
65% of BusinessLand's revenue coming from the PC. Demand still so
exceeded supply two years after its debut that, despite
40,000 PCs a month, dealers reportedly received 60% or less of their
desired quantity. Pournelle received the PC he paid for in early
July 1983 on 1 November, and
IBM Boca Raton employees and
neighbors had to wait five weeks to buy the computers assembled
Yankee Group also stated that the PC had by 1983 "destroyed the market
for some older machines" from companies like Vector Graphic, North
Star, and Cromemco. inCider wrote "This may be an Apple magazine,
but let's not kid ourselves,
IBM has devoured competitors like a cloud
of locusts". By February 1984 BYTE reported on "the phenomenal
market acceptance of the
IBM PC", and by fall concluded that the
company "has given the field its third major standard, after the Apple
II and CP/M". Some rivals speculated that the government might
IBM for antitrust, and
Ben Rosen claimed that the
company's dominance "is having a chilling effect on new ventures, a
By that time, Apple was less welcoming of the rival that inCider
stated had a "godlike" reputation. Its focus on the III had
delayed improvements to the II, and the sophisticated Lisa was
unsuccessful in part because, unlike the II and the PC, Apple
discouraged third-party developers. The head of a retail chain said
"It appears that
IBM had a better understanding of why the Apple II
was successful than had Apple." Jobs, after trying to recruit
Estridge to become Apple's president, admitted that in two years
IBM had joined Apple as "the industry's two strongest competitors". He
warned in a speech before previewing the forthcoming "1984" Super Bowl
commercial: "It appears
IBM wants it all ... Will Big Blue dominate
the entire computer industry? The entire information age? Was George
Orwell right about 1984?"
IBM had $4 billion in annual PC revenue by 1984, more than twice that
of Apple and as much as the sales of Apple, Commodore, HP, and Sperry
combined, and 6% of total revenue. A Fortune survey found
that 56% of American companies with personal computers used
compared to Apple's 16%. A 1983 study of corporate customers
similarly found that two thirds of large customers standardizing on
one computer chose the PC, compared to 9% for Apple. IBM's own
documentation described the PC as inferior to competitors'
less-expensive products, but the company generally did not compete on
price; rather, the study found that they preferred "IBM's hegemony"
because of its support. Most companies with mainframes used their PCs
with the larger computers, which likely benefited IBM's mainframe
sales and discouraged their purchasing non-
IBM introduced the PC/AT, unlike its predecessor the most
sophisticated personal computer from any major company. By
1985 the PC family had more than doubled Future Computing's 1986
revenue estimate, with more than 12,000 applications and 4,500 dealers
and distributors worldwide. In his obituary that year, The New
York Times wrote that Estridge had led the "extraordinarily successful
entry of the
International Business Machines
International Business Machines Corporation into the
personal computer field". The Entry Systems Division had 10,000
employees and by itself would have been the world's third-largest
computer company behind
IBM and DEC, with more revenue than IBM's
minicomputer business despite its much later start.
IBM was the only
major company with significant minicomputer and microcomputer
businesses, in part because rivals like DEC and Wang did not
adjust to the retail market.
Rumors of "lookalike", compatible computers, created without IBM's
approval, began almost immediately after the
IBM PC's release.
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
Compaq Computer Corporation
Compaq Computer Corporation announced the Compaq
Portable, the first portable
IBM PC compatible. The first models were
shipped in January 1983.
IBM PC as standard
Main article: Influence of the
IBM PC on the personal computer market
The success of the
IBM computer led other companies to develop IBM
Compatibles, which in turn led to branding like diskettes being
advertised as "
IBM format". An
IBM PC clone could be built with
off-the-shelf parts, but the
BIOS required some reverse engineering.
Companies like Compaq, Phoenix
Software Associates, American
Megatrends, Award, and others achieved fully functional versions of
the BIOS, allowing companies like Dell, Gateway and HP to manufacture
PCs that worked like IBM's product. The
IBM PC became the industry
IBM had no retail experience, the retail chains ComputerLand
Sears Roebuck provided important knowledge of the
marketplace. They 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
distribution across the U.S.
Targeting the new PC at the home market,
Sears Roebuck sales failed to
live up to expectations. This unfavorable outcome revealed that the
strategy of targeting the office market was the key to higher sales.
IBM Personal Computer
IBM 5150 PC with
IBM 5151 monitor
IBM PC line
Floppy disk or cassette system. One or two internal floppy drives
IBM PC to come with an internal hard drive as standard.
5160 with XT/370 Option Kit and 3277 Emulation Adapter
3270 terminal emulation, 20 function key keyboard
Floppy-based home computer, but also used ROM cartridges; infrared
Faster processor, faster system bus (6 MHz, later 8 MHz, vs
4.77 MHz), jumperless configuration, real-time clock
5170 with AT/370 Option Kit and 3277 Emulation Adapter
3270 terminal emulation
Microfloppy laptop portable
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
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 and
higher-color display standards, while some newer programs require
newer display adapters. (Note that as the display adapter was an
adapter card in all of these
IBM models, newer display hardware could
easily be, and often was, retrofitted to older models.) A few
programs, typically very early ones, are written for and require a
specific version of the
BIOS ROM. Most
BASICA which was dependent on the
BIOS ROM had a sister
GW-BASIC which supported more functions, was 100%
backwards compatible and could run independently from the
IBM PC 5150.
The CGA video card, with a suitable modulator, could use an NTSC
television set or an RGBi monitor for display; IBM's RGBi monitor was
their display model 5153. The other option that was offered by
an MDA 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
Lotus 1-2-3 and others allowed use of a CGA Monitor
for graphics and a separate monochrome monitor 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
Although cassette tape was originally envisioned by
IBM as a
low-budget storage alternative, the most commonly used medium was the
floppy disk. The 5150 was available with one or two 5 1⁄4"
floppy drives – with two drives the program disc(s) would be in
drive A, while drive B would hold the disc(s) for working files; with
one drive the user had to swap program and file discs into the single
drive. For models without any drives or storage medium,
users to connect their own cassette recorder via the 5150's cassette
socket. The cassette tape socket was physically the same
DIN plug as
the keyboard socket and next to it, but electrically completely
A hard disk could not be installed into the 5150's system unit without
changing to a higher-rated power supply (although later drives with
lower power consumption have been known to work with the standard 63.5
Watt unit). The "
IBM 5161 Expansion Chassis" came with its own power
supply and one 10 MB hard disk and allowed the installation of a
second hard disk. The system unit had five expansion slots, and
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 into the 5150's
motherboard; the only motherboard external connectors were the
keyboard and cassette ports.
PC speaker sound hardware was also on board.
The original PC's maximum memory using
IBM parts was 256 kB,
achievable through the installation of 64 kB on the motherboard
and three 64 kB expansion cards. The processor was an
running at 4.77 MHz, 4/3 the standard NTSC color burst frequency
of 315/88 = 3.57954[a] MHz. (In early units, the Intel
8088 used was a 1978 version, later were 1978/81/2 versions of the
Intel chip; second-sourced AMDs were used after 1983)[citation
needed]. Some owners replaced the 8088 with an
NEC V20 for a slight
increase in processing speed and support for real mode 80186
instructions. The V20 gained its speed increase through the use of a
hardware multiplier which the 8088 lacked. An
Intel 8087 coprocessor
could also be added for hardware floating-point arithmetic.
IBM sold the first
IBM PCs in configurations with 16 or 64 kB of
RAM preinstalled using either nine or thirty-six 16-kilobit DRAM
chips. (The ninth bit was used for parity checking of memory.) After
IBM XT shipped, the
IBM PC motherboard was redesigned with the
same RAM configuration as the
IBM XT. (64 kB in one bank, expandable
to 256kB by populating the other 3 banks.)
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 kB of RAM and a
single 5.25-inch floppy drive and monitor sold for US $3,005
(equivalent to $8,089 in 2017), while the cheapest configuration (US
$1,565) that had no floppy drives, only 16 kB 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.
IBM Personal Computer XT
IBM Personal Computer XT",
IBM model 5160, was introduced two
years after the PC and featured a 10 megabyte hard drive. It had eight
expansion slots but the same processor and clock speed as the PC. The
XT had no cassette jack, but still had the Cassette Basic interpreter
The XT could take 256 kB of memory on the main board (using
64 kbit DRAM); later models were expandable to 640 kB. The
remaining 384 kilobytes of the 8088 address space were used for the
BIOS ROM, adapter ROM and RAM space, including video RAM space. It was
usually sold with a
Monochrome Display Adapter
Monochrome Display Adapter (MDA) video card or a
CGA video card.
The eight expansion slots were the same as the model 5150 but were
spaced closer together. Although rare, a card designed for the 5150
could be wide enough to obstruct the adjacent slot in an XT.
Because of the spacing, an XT motherboard would not fit into a case
designed for the PC motherboard, but the slots and peripheral cards
were compatible. The XT expansion bus (later called "8-bit Industry
Standard Architecture" (ISA) by competitors) was retained in the IBM
AT, which added connectors for some slots to allow 16-bit transfers;
8-bit cards could be used in an AT.
Main article: PC-based IBM-compatible mainframes
IBM Personal Computer XT/370" was an XT with three custom 8-bit
cards: the processor card (370PC-P) contained a modified Motorola
68000 chip, microcoded to execute
System/370 instructions, a second
68000 to handle bus arbitration and memory transfers, and a modified
8087 to emulate the S/370 floating point instructions. The second card
(370PC-M) connected to the first and contained 512 kB of memory. The
third card (PC3277-EM), was a
3270 terminal emulator necessary to
install the system software for the VM/PC software to run the
The computer booted into DOS, then ran the VM/PC Control
IBM PCjr" was IBM's first attempt to enter the market for
relatively inexpensive educational and home-use personal computers.
IBM model number 4860, retained the
IBM PC's 8088 CPU and
BIOS interface for compatibility, but its cost and differences in the
PCjr's architecture, as well as other design and implementation
decisions, eventually led to the PCjr, and the related
IBM JX, being
IBM Portable Personal Computer
IBM Portable Personal Computer" 5155 model 68 was an early
portable computer developed by
IBM after the success of Compaq's
suitcase-size portable machine (the
Compaq Portable). It was released
in February 1984, and was eventually replaced by the
The Portable was an XT motherboard, transplanted into a Compaq-style
luggable case. The system featured 256 kilobytes of memory (expandable
to 512 KB), an added CGA card connected to an internal monochrome
(amber) composite monitor, and one or two half-height 5.25"
360 KB floppy disk drives. Unlike the
Compaq Portable, which used
a dual-mode monitor and special display card,
IBM used a stock CGA
board and a composite monitor, which had lower resolution. It could
however, display color if connected to an external monitor or
IBM Personal Computer/AT
IBM Personal Computer/AT" (model 5170), announced August 15,
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 and sporting a 30-megabyte hard disk was
introduced in 1986.
The AT was designed to support multitasking; the new SysRq (system
request) key, little noted and often overlooked, is part of this
design, as is the 80286 itself, the first
Intel 16-bit processor with
multitasking features (i.e. the 80286 protected mode).
IBM made some
attempt at marketing the AT as a multi-user machine, but it sold
mainly as a faster PC for power users. For the most part,
were used as more powerful
DOS (single-tasking) personal computers, in
the literal sense of the PC name.
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, and High Density Floppy 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-MB 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.
IBM AT's Drive parameter table listed the CMI-33 as having 615
cylinders instead of the 640 the drive was designed with, as to make
the size an even 30 MB. Those who re-used the drives mostly found that
the 616th cylinder was bad due to it being used as a landing area.
IBM Personal Computer AT/370" was an AT with two custom 16-bit
cards, running almost exactly the same setup as the XT/370.
IBM PC Convertible
IBM PC Convertible, released April 3, 1986, was IBM's first laptop
computer and was also the first
IBM computer to utilize the 3.5"
floppy disk which went on to become the standard. Like modern laptops,
it featured power management and the ability to run from batteries. It
was the follow-up to the
IBM Portable and was model number 5140. The
concept and the design of the body was made by the German industrial
designer Richard Sapper.
It utilized an
Intel 80c88 CPU (a CMOS version of the
running at 4.77 MHz, 256 kB of RAM (expandable to 640 kB), dual
720 kB 3.5" floppy drives, and a monochrome CGA-compatible LCD screen
at a price of $2,000. It weighed 13 pounds (5.9 kg) and featured
a built-in carrying handle.
The PC Convertible had expansion capabilities through a proprietary
ISA bus-based port on the rear of the machine. Extension modules,
including a small printer and a video output module, could be snapped
into place. The machine could also take an internal modem, but there
was no room for an internal hard disk.
IBM PS/2 line was introduced in 1987. The Model 30 at the bottom
end of the lineup was very similar to earlier models; it used an 8086
processor and an ISA bus. The Model 30 was not "
IBM compatible" in
that it did not have standard 5.25-inch drive bays; it came with a
3.5-inch floppy drive and optionally a 3.5-inch-sized hard disk. Most
models in the PS/2 line further departed from "
IBM compatible" by
replacing the ISA bus completely with Micro Channel Architecture.
IBM Personal Computer motherboard,
IBM 5150. It has five
expansion slots (an interface later called "PC/XT-bus" by
"8-bit ISA" by other manufacturers of compatible computers), and two
DIN connectors for keyboard and cassette interface.
The main circuit board in an 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 original PC also has a cassette 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
AT architecture rather than license the PS/2 architecture and its
Micro Channel 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, where its relatively low speed, 5 volt
signals, and relatively simple, straightforward design (all by year
2011 standards) give it technical advantages (e.g. noise immunity for
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. Each
expansion slot on the motherboard has a corresponding opening in the
back of the computer case through which the card can expose
connectors; a blank metal cover plate covers this case opening (to
prevent dust and debris intrusion and control airflow) when no
expansion card is installed. 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 combine 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 kB of memory. (Though the 8088 can
address one megabyte of memory, the last 384 kB 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 64 kB EMS page frame itself.)
IBM PC AT used an
Intel 80286 processor which can access
up to 16 MB 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
Peripheral integrated circuits
The set of peripheral chips selected for the original
IBM PC defined
the functionality of an
IBM compatible. These became the de facto base
for later application specific integrated circuits (ASICs) used in
The original system chips were one
Intel 8259 programmable interrupt
controller (PIC) (at I/O address 0x20), one
Intel 8237 direct memory
access (DMA) controller (at I/O address 0x00), and an
programmable interval timer (PIT) (at I/O address 0x40). The PIT
provides the 18.2 Hz clock ticks, dynamic memory refresh timing, and
can be used for speaker output; one DMA channel is used to
perform the memory refresh.
The mathematics coprocessor was the
Intel 8087 using I/O address 0xF0.
This was an option for users who needed extensive floating-point
arithmetic, such as users of computer-aided drafting.
IBM PC AT added a second, slave 8259 PIC (at I/O address 0xA0), a
second 8237 DMA controller for 16-bit DMA (at I/O address 0xC0), a DMA
address register (implemented with a 74LS612 IC) (at I/O address
0x80), and a
Motorola MC146818 real-time clock (RTC) with
nonvolatile memory (NVRAM) used for system configuration (replacing
the DIP switches and jumpers used for this purpose in PC and PC/XT
models (at I/O address 0x70). On expansion cards, the
programmable peripheral interface (PPI) (at I/O addresses 0x378 is
used for parallel I/O controls the printer, and the 8250
universal asynchronous receiver/transmitter (UART) (at I/O address
0x3F8 or 0x3E8) controls the serial communication at the
IBM offered a
Game Control Adapter
Game Control Adapter for the PC, which supported
analog joysticks similar to those on the Apple II. Although analog
controls proved inferior for arcade-style games, they were an asset in
certain other genres such as flight simulators. The joystick port on
IBM PC supported two controllers, but required a Y-splitter cable
to connect both at once. It remained the standard joystick interface
IBM compatibles until being replaced by
USB during the 2000s.
IBM PC keyboard
The original keyboard for the
IBM 5150 and XT
The keyboard that came with the
IBM 5150 was an extremely reliable and
high-quality electronic keyboard originally developed in North
Carolina for the Datamaster. 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
adjustable 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% of the reason to buy an
IBM PC. The importance of the keyboard was definitely established when
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
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
However, the original 1981
IBM PC 83-key keyboard was criticized by
typists for its non-standard placement of the Return and left ⇧
Shift keys, and because it did not have separate cursor and numeric
pads that were popular on the pre-PC DEC
VT100 series video terminals.
Key Tronic introduced the now standard 101-key PC keyboard.
IBM corrected the Return and left ⇧ Shift keys on its AT
keyboard, but shortened the Backspace key, making it harder to reach.
IBM changed to the 101 key enhanced keyboard, which added the
separate cursor and numeric key pads, relocated all the function keys
and the Ctrl keys, and the
Esc key was also relocated to the opposite
side of the keyboard.
Another feature of the original keyboard is the relatively loud
"click" 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 used a keyswitch that
produced a click and tactile bump intended to provide that same
IBM PC keyboard is very robust and flexible. The low-level
interface for each key is the same: each key sends a signal when it is
pressed and another signal when it is released. An integrated
microcontroller in the keyboard scans the keyboard and encodes a "scan
code" and "release code" for each key as it is pressed and released
separately. 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.
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 the AT-style or XT-style protocol for the keyboard.
See also: Keyboard layout
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.
IBM cassette tape
IBM equipped the model 5150 with a cassette port for connecting a
cassette drive and assumed that home users would purchase the low-end
model and save files to cassette tapes as was typical of home
computers of the time. 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
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
IBM PC cassette interface encodes data using frequency modulation
with a variable data rate. Either a one or a zero is represented by a
single cycle of a square wave, but the square wave frequencies differ
by a factor of two, with ones having the lower frequency. Therefore,
the bit periods for zeros and ones also differ by a factor of two,
with the unusual effect that a data stream with more zeros than ones
will use less tape (and time) than an equal-length (in bits) data
stream containing more ones than zeros, or equal numbers of each.
IBM also had an exclusive license agreement with
Microsoft to include
BASIC in the ROM of the PC; clone manufacturers could not have ROM
BASIC on their machines, but it also became a problem as the XT, AT,
and PS/2 eliminated the cassette port and
IBM was still required to
install the (now useless) BASIC with them. The agreement finally
expired in 1991 when
Microsoft replaced BASICA/
GW-BASIC with QBASIC.
The main core BASIC resided in ROM and "linked" up with the
RAM-resident BASIC.COM/BASICA.COM included with PC-
DOS (they provided
disk support and other extended features not present in ROM BASIC).
Because BASIC was over 50 kB in size, this served a useful function
during the first three years of the PC when machines only had 64–128
kB of memory, but became less important by 1985. For comparison, clone
makers such as
Compaq were forced to include a version of BASIC that
resided entirely in RAM.
Tandon 5.25-inch Diskette Drive with a partially inserted
double-density diskette containing
Most or all 5150 PCs had one or two 5.25-inch floppy disk drives.
These were either single-sided double-density (SSDD) or double-sided
double-density (DSDD) drives. The
IBM PC never used single density
floppy drives. The drives and disks were commonly referred to by
capacity, such as "160KB floppy disk" or "360KB floppy drive". DSDD
drives were backwards compatible; they could read and write SSDD
floppies. The same type of physical diskette media could be used for
both drives, but a disk formatted for double-sided use could not be
read on a single-sided drive.
The disks were
Modified Frequency Modulation
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 eight 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
nine 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" 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 kB and 360 kB,
respectively. Regardless of type, the file system of all floppy disks
(under DOS) was FAT12.
IBM PCs had only single-sided floppy drives until
double-sided drives became available in the spring of 1982. After the
upgraded 64k-256k motherboard PCs arrived in early 1983, single-sided
drives and the cassette model were discontinued.
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, but
IBM did not offer
their own external floppies until 1986.
The industry-standard way of setting floppy drive numbers was via
setting jumper switches on the drive unit, however
IBM chose to
instead use a method known as the "cable twist" which had a floppy
data cable with a bend in the middle of it that served as a switch for
the drive motor control. This eliminated the need for users to adjust
jumpers while installing a floppy drive.
20MB Seagate ST-225 with a controller card by Western Digital
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
10 MB (later 20 MB) hard drive powered by the 5161's own
separate 130-watt power supply. The
IBM 5161 Expansion Unit was
released in early 1983.
During the first year of the
IBM PC, it was commonplace for users to
install third-party Winchester hard disks which generally connected to
the floppy controller and required a patched version of PC-
treated them as a giant floppy disk (there was no subdirectory
IBM began offering hard disks with the XT, however the original PC was
never sold with them. Nonetheless, many users installed hard disks and
upgraded power supplies in them.
After floppy disks became obsolete in the early 2000s, the letters A
and B became unused. But for 25 years, virtually all DOS-based PC
software assumed the program installation drive was C, so the primary
HDD continues to be "the C drive" even today. Other operating system
families (e.g. Unix) are not bound to these designations.
IBM Disk Operating System version 1.1 by Microsoft
Which operating system
IBM customers would choose was at first
unclear. Although the company expected that most would use PC
IBM supported using CP/M-86—which became available six
months after DOS—or UCSD p-System as operating systems. IBM
promised that it would not favor one operating system over the others;
CP/M-86 support surprised Gates, who claimed that
"blackmailed into it".
IBM was correct, nonetheless, in its
expectation; one survey found that 96.3% of PCs were ordered with the
DOS compared to 3.4% for the $240 CP/M-86.
IBM PC's ROM BASIC and
BIOS supported cassette tape storage. PC
DOS itself did not support cassette tape storage. PC
DOS version 1.00
supported only 160 kB SSDD floppies, but version 1.1, which was
released nine months after the PC's introduction, supported
160 kB SSDD and 320 kB DSDD floppies. Support for the
slightly larger nine sector per track 180 kB and 360 kB
formats arrived 10 months later in March 1983.
BIOS (Basic Input/Output System) provided the core ROM code for
the PC. It contained a library of functions that software could call
for basic tasks such as video output, keyboard input, and disk access
in addition to interrupt handling, loading the operating system on
boot-up, and testing memory and other system components.
BIOS was 8k in size and occupied four 2k ROM chips
on the motherboard, with a fifth and sixth empty slot left for any
extra ROMs the user wished to install.
IBM offered three different
BIOS revisions during the PC's lifespan. The initial
BIOS was dated
April 1981 and came on the earliest models with single-sided floppy
drives and PC
DOS 1.00. The second version was dated October 1981 and
arrived on the "Revision B" models sold with double-sided drives and
DOS 1.10. It corrected some bugs, but was otherwise unchanged.
Finally, the third
BIOS version was dated October 1982 and found on
IBM PCs with the newer 64k-256k motherboard. This revision was
more-or-less identical to the XT's BIOS. It added support for
detecting ROMs on expansion cards as well as the ability to use 640k
of memory (the earlier
BIOS revisions had a limit of 544k). Unlike the
XT, the original PC remained functionally unchanged from 1983 until
its discontinuation in early 1987 and did not get support for 101-key
keyboards or 3.5" floppy drives, nor was it ever offered with
IBM initially offered two video adapters for the PC, the
Color/Graphics Adapter and the Monochrome Display and Printer Adapter.
CGA was intended to be a typical home computer display; it had NTSC
output and could be connected to a composite monitor or a
TV set with
an RF modulator in addition to RGB for digital RGBI-type monitors,
IBM did not offer their own RGB monitor until 1983. Supported
graphics modes were 40 or 80×25 color text with 8×8 character
resolution, 320×200 bitmap graphics with two fixed 4-color palettes,
or 640×200 monochrome graphics.
The MDA card and its companion 5151 monitor supported only 80×25 text
with a 9×14 character resolution (total pixel resolution was
720×350). It was mainly intended for the business market and so also
included a printer port.
During 1982, the first third-party video card for the PC appeared when
Hercules Computer Technologies released a clone of the MDA that could
use bitmap graphics. Although not supported by the BIOS, the Hercules
Graphics Adapter became extremely popular for business use due to
allowing sharp, high resolution graphics plus text and itself was
widely cloned by other manufacturers.
In 1985, after the launch of the
IBM AT, the new Enhanced Graphics
Adapter became available which could support 320×200 or 640×200 in
16 colors in addition to high-resolution 640×350 16 color graphics.
IBM also offered a video board for the PC, XT, and AT known as the
Professional Graphics Adapter
Professional Graphics Adapter during 1984–86, mainly intended for
CAD design. It was extremely expensive, required a special monitor,
and was rarely ordered by customers.
VGA graphics cards could also be installed in
IBM PCs and XTs,
although they were introduced after the computer's discontinuation.
Serial port addresses and interrupts
The serial port is an 8250 or a derivative (such as the 16450 or
16550), mapped to eight consecutive IO addresses and one interrupt
Base port address [Hex]
Only COM1: and COM2: addresses were defined by the original PC.
Attempts to share IRQ3 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 most typical devices plugged
into the serial port were modems and mice. Plotters and serial
printers were also among the more commonly used serial peripherals,
and there were numerous other more unusual uses such as operating cash
registers, factory equipment, and connecting terminals.[citation
IBM made a deal with Japan-based
Epson to produce printers for the PC
and all IBM-branded printers were manufactured by that company (Epson
of course also sold printers with their own name). There was a
considerable amount of controversy when
IBM included a printer port on
the PC that did not follow the industry-standard
and it was rumored that this had been done to prevent customers from
IBM printers with their machines (plugging a
Centronics printer into an
IBM PC could damage the printer, the
parallel port, or both). Although third-party cards were available
Centronics ports on them, PC clones quickly copied the IBM
printer port and by the late 80s, it had largely displaced the
BYTE wrote in October 1981 that the
IBM PC's "hardware is impressive,
but even more striking are two decisions made by IBM: to use outside
suppliers already established in the microcomputer industry, and to
provide information and assistance to independent, small-scale
software writers and manufacturers of peripheral devices". It praised
the "smart" hardware design and stated that its price was not much
higher than the 8-bit machines from Apple and others. The reviewer
admitted that the computer "came as a shock. I expected that the giant
would stumble by overestimating or underestimating the capabilities
the public wants and stubbornly insisting on incompatibility with the
rest of the microcomputer world. But
IBM didn't stumble at all;
instead, the giant jumped leagues in front of the competition ... the
only disappointment about the
IBM Personal Computer is its dull
In a more detailed review in January 1982, BYTE called the
IBM PC "a
synthesis of the best the microcomputer industry has offered to date
... as well designed on the inside as it is on the outside". The
magazine praised the keyboard as "bar none, the best ... on any
microcomputer", describing the unusual
Shift key locations as "minor
[problems] compared to some of the gigantic mistakes made on almost
every other microcomputer keyboard". The review also complimented
IBM's manuals, which it predicted "will set the standard for all
microcomputer documentation in the future. Not only are they well
packaged, well organized, and easy to understand, but they are also
complete". Observing that detailed technical information was available
"much earlier ... than it has been for other machines", the magazine
predicted that "given a reasonable period of time, plenty of hardware
and software will probably be developed for" the computer. The review
stated that although the
IBM PC cost more than comparably configured
Apple II and
TRS-80 computers, and the insufficient number of slots
for all desirable expansion cards was its most serious weakness, "you
get a lot more for your money" and concluded, "In two years or so, I
think [it] will be one of the most popular and best-supported ... IBM
should be proud of the people who designed it".
In a special 1984 issue dedicated to the
IBM PC, BYTE concluded that
the PC had succeeded both because of its features like an 80-column
screen, open architecture, and high-quality keyboard, and "the failure
of other major companies to provide these same fundamental features
earlier. In retrospect, it seems
IBM stepped into a void that
remained, paradoxically, at the center of a crowded market".
IBM PCs have remained in service long after their technology
became largely obsolete. In June 2006,
IBM PC and XT models were still
in use at the majority of U.S.
National Weather Service
National Weather Service upper-air
observing sites, used to process data as it is returned from the
ascending radiosonde, attached to a weather balloon, although they
have been slowly phased out. Factors that have contributed to the 5150
PC's longevity are its flexible modular design, its open technical
standard (making information needed to adapt, modify, and repair it
readily available), use of few special nonstandard parts, and rugged
IBM manufacturing, which provided for exceptional
long-term reliability and durability.
Some of the mechanical aspects of the slot specifications are still
used in current PCs. A few systems still come with PS/2 style keyboard
and mouse connectors.
IBM model 5150 Personal Computer has become a collectable among
vintage computer collectors, due to the system being the first true
“PC” as we know them today. As of 2007[update], the system had a
market value of $50–$500. The
IBM model 5150 has proven to be
reliable; despite their age of 30 years or more, some still function
as they did when new.
Information technology portal
IBM token ring networks
Input/Output Base Address
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IBM did not offer own brand cassette recorders, but the 5150 had a
cassette player jack, and
IBM anticipated that entry level home users
would connect their own cassette recorders for data storage instead of
using the more expensive floppy drives (and use their existing TV sets
as monitors); to this end,
IBM initially offered the 5150 in a basic
configuration without any floppy drives or monitor at the price of
$1,565, whereas they offered a system with a monitor and single floppy
drive for an initial $3,005. Few if any users however bought
PCs without floppy drives.
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PC Magazine article reads
"If you own an
IBM PC AT and your hard disk hasn't crashed yet, don't
worry – it probably will." highbeam.com & encyclopedia.com (the
latter a Chicago Sun-Times article citing the
PC Magazine story). IBM
recovered, although with mixed comments, as noted in the Sept. 30,
PC Magazine article, "The Two Faces of IBM's 8-MHz AT," pp. 179 -
^ wustl.edu - ECE306 Lecture 16 Archived 2011-07-24 at the Wayback
^ The DMA address register extends the 16-bit transfer memory address
capacity of the 8237 to 24 bits
^ illinois.edu - Real time clock plus RAM
^ ctv.se - PC KITS-tutorial page (parallel port, joystick port)
Archived 2011-07-28 at the Wayback Machine.
IBM PC serial port is not strictly RS-232, since it uses TTL
signal levels, whereas
RS-232 requires signals of +/- 3 to 15 volts;
some signal levels that are valid for a TTL high state, and all signal
levels that represent a TTL low state, fall within the forbidden range
of -3 to +3 volts for standard RS-232. (However, it is not difficult
to design and construct a level converter that will convert between
IBM serial port and standard
IBM (July 1982). Technical Reference: Personal Computer Hardware
Reference Library (Revised ed.).
IBM Corp. pp. 2–93.
^ Sometimes the tracks were also referred as cylinders, which is
technically correct and analogous to hard drive cylinders. One floppy
disk track equaled one cylinder, however with double-sided floppies,
only the first side's cylinder numbers were identical to the track
numbers; on the second side, the cylinders 1-40 corresponded to tracks
41-80 of the formatted floppy.
^ 163,840 bytes, i.e. 512 bytes × 8 sectors × 40 tracks on the one
^ 327,680 bytes, i.e. 512 bytes × 8 sectors × 40 tracks × 2 sides
^ 184,320 bytes, i.e. 512 bytes × 9 sectors × 40 tracks on the one
^ 368,640 bytes, i.e. 512 bytes × 9 sectors × 40 tracks × 2 sides
^ Edlin, Jim (June–July 1982). "
CP/M Arrives". PC Magazine.
p. 43. Retrieved 21 October 2013.
^ "PCommuniques". PC Magazine. February 1983. p. 53. Retrieved 21
^ McCracken, Harry (27 August 2007). "The Most Collectible PCs of All
Time". PCWorld. Retrieved 25 September 2017.
^ "Can You Do Real Work With the 30-Year-Old
Norton, Peter (1986). Inside the
IBM PC. Revised and enlarged. New
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August 12, 1981 press release announcing the
IBM PC (
Mueller, Scott (1992). Upgrading and Repairing PCs, Second Edition,
Que Books, ISBN 0-88022-856-3
Chposky, James; Ted Leonsis (1988). Blue Magic - The People, Power and
Politics Behind the
IBM Personal Computer. Facts On File.
IBM (1983). Personal Computer Hardware Reference Library: Guide to
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IBM Part Number 6936831.
IBM (1984). Personal Computer Hardware Reference Library: Guide to
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This article is based on material taken from the Free On-line
Dictionary of Computing prior to 1 November 2008 and incorporated
under the "relicensing" terms of the GFDL, version 1.3 or later.
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