IBM Personal Computer

The design process was kept under a policy of strict secrecy, with all other IBM divisions kept in the dark about the project.[16]

Several CPUs were considered, including the Texas Instruments TMS9900, Motorola 68000 and Intel 8088. The 68000 was considered the best choice,[17] but was not production-ready like the others.[18] The IBM 801 RISC processor was also considered, since it was considerably more powerful than the other options, but rejected due to the design constraint to use off-the-shelf parts.

IBM chose the 8088 over the similar but superior 8086 because Intel offered a better price for the former and could provide more units,[19] and the 8088's 8-bit bus reduced the cost of the rest of the computer. The 8088 had the advantage that IBM already had familiarity with it from designing the IBM System/23 Datamaster. The 62-pin expansion bus slots were also designed to be similar to the Datamaster slots,[20] and its keyboard design and layout became the Model F keyboard shipped with the PC,[21] but otherwise the PC design differed in many ways.

The 8088 motherboard was designed in 40 days,[22] with a working prototype created in four months,[23] demonstrated in January 1981. The design was essentially complete by April 1981, when it was handed off to the manufacturing team.[24] PCs were assembled in an IBM plant in Boca Raton, with components made at various IBM and third party factories. The monitor was an existing design from IBM Japan; the printer was manufactured by Epson.[25] Because none of the functional components were designed by IBM, they obtained no patents on the PC.[26]

Many of the designers were computer hobbyists who owned their own computers,[8] including many Apple II owners, which influenced the decisions to design the computer with an open architecture[27] and publish technical information so others could create compatible software and expansion slot peripherals.[28]

During the design process IBM avoided vertical integration as much as possible, for example choosing to license Microsoft BASIC rather than utilizing the in-house version of BASIC used for mainframes due to the better existing public familiarity with the Microsoft version.[29]

The IBM PC debuted on August 12, 1981 after a twelve-month development. Pricing started at $1,565 for a configuration with 16 KB RAM, Color Graphics Adapter, and no disk drives. The price was designed to compete with comparable machines in the market.[30] For comparison, the Datamaster, announced two weeks earlier as IBM's least expensive computer, cost $10,000.[31]

IBM's marketing campaign licensed the likeness of Charlie Chaplin's character "The Little Tramp" for a series of advertisements based on Chaplin's movies, played by Billy Scudder.[32]

The PC was IBM's first attempt to sell a computer through retail channels rather than directly to customers. Because IBM did not have retail experience, they partnered with the retail chains ComputerLand and Sears, who provided important knowledge of the marketplace[33][34][35][36] and became the main outlets for the PC. 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.

Reception was overwhelmingly positive, with analysts estimating sales volume in the billions of dollars in the first few years after release.[37] After release, IBM's PC immediately became the talk of the entire computing industry.[38] Dealers were overwhelmed with orders, including customers offering pre-payment for machines with no guaranteed delivery date.[30] By the time the machine began shipping, the term "PC" was becoming a household name.[39]

Sales exceeded IBM's expectations by as much as 800%, with the company at one point shipping as many as 40,000 PCs per month.[40] IBM estimated that home users made up 50 to 70% of purchases from retail stores.[41] In 1983, IBM sold more than 750,000 machines,[42] while Digital Equipment Corporation, one of the companies whose success had spurred IBM to enter the market, sold only 69,000.[43]

Software support from the industry grew rapidly, with the IBM nearly instantly becoming the primary target for most microcomputer software development.[31] One publication counted 753 software packages available a year after the PC's release, four times as many as were available for the Macintosh a year after its launch.[44] Hardware support also grew rapidly, with 30–40 companies competing to sell memory expansion cards within a year.[45]

By 1984, IBM's revenue from the PC market was $4 billion, more than twice that of Apple.[46] A 1983 study of corporate customers found that two thirds of large customers standardizing on one computer chose the PC, while only 9% chose Apple.[47] A 1985 Fortune survey found that 56% of American companies with personal computers used PCs while 16% used Apple.[48]

Almost as soon as the PC reached the market, rumors of clones began,[49] and the first PC compatible clone was released in June 1982, less than a year after the PC's debut.

The PC is built around a single large circuit board called a motherboard which carries the processor, built-in RAM, expansion slots, keyboard and cassette ports, and the various peripheral integrated circuits that connected and controlled the components of the machine.

The peripheral chips included an Intel 8259 PIC, an Intel 8237 DMA controller, and an Intel 8253 PIT. The PIT provides 18.2 Hz clock "ticks" and dynamic memory refresh timing.

Original IBM Personal Computer motherboard

Second Generation IBM Personal Computer System Board

The CPU is an Intel 8088, a cost-reduced form of the Intel 8086 which largely retains the 8086's internal 16-bit logic, but exposes only an 8-bit bus.[51] The CPU is clocked at 4.77 MHz, which would eventually become an issue when clones and later PC models offered higher CPU speeds that broke compatibility with software developed for the original PC.[52] The single base clock frequency for the system was 14.31818 MHz, which when divided by 3, yielded the 4.77 MHz for the CPU (which was considered close enough to the then 5 MHz limit of the 8088), and when divided by 4, yielded the required 3.579545 MHz for the NTSC color carrier frequency.

The PC motherboard included a second, empty socket, described by IBM simply as an "auxiliary processor socket", although the most obvious use was the addition of an Intel 8087 math coprocessor, which improved floating-point math performance.[53]

From the factory the PC was equipped with either 16 KB or 64 KB of RAM. RAM upgrades were provided both by IBM and third parties as expansion cards, and could upgrade the machine to a maximum of 256 KB on the motherboard, and 640 KB total.[51]

The BIOS is the firmware of the IBM PC, occupying one 8 KB chip on the motherboard. It provides bootstrap code and a library of common functions that all software can use for many purposes, such as video output, keyboard input, disk access, interrupt handling, testing memory, and other functions. IBM shipped three versions of the BIOS throughout the PC's lifespan.

IBM PC with MDA monitor

While most home computers had built-in video output hardware, IBM took the unusual approach of offering two different graphics options, the MDA and CGA cards. The former provided high-resolution monochrome text, but could not display anything except text, while the latter provided medium- and low-resolution color graphics and text.

CGA used the same scan rate as NTSC television, allowing it to provide a composite video output which could be used with any compatible television or composite monitor, as well as a direct-drive TTL output suitable for use with any RGBI monitor using an NTSC scan rate. IBM also sold the 5153 color monitor for this purpose, but it was not available at release[54] and was not released until March 1983.[55]

MDA scanned at a higher frequency and required a proprietary monitor, the IBM 5151. The card also included a built-in printer port.[56]

Both cards could also be installed simultaneously for mixed graphics and text applications.[57] For instance, AutoCAD, Lotus 1-2-3 and other software allowed use of a CGA Monitor for graphics and a separate monochrome monitor for text menus. Third parties went on to provide an enormous variety of aftermarket graphics adapters, such as the Hercules Graphics Card.

The software and hardware of the PC, at release, was designed around a single 8-bit adaptation of the ASCII character set, now known as code page 437.

The two bays in the front of the machine could be populated with one or two 5.25″ floppy disk drives, storing 160 KB per disk side for a total of 320 KB of storage on one disk.[56] The floppy drives require a controller card inserted in an expansion slot, and connect with a single ribbon cable with two edge connectors. The IBM floppy controller card provides an external 37-pin D-sub connector for attachment of an external disk drive, although IBM did not offer one for purchase until 1986.

As was common for home computers of the era, the IBM PC offered a port for connecting a cassette data recorder. Unlike the typical home computer however, this was never a major avenue for software distribution,[58] probably because very few PCs were sold without floppy drives. The port was removed on the very next PC model, the XT.[59]

At release, IBM did not offer any hard disk drive option[51] and adding one was difficult - the PC's stock power supply had inadequate power to run a hard drive, the motherboard did not support BIOS expansion ROMs which was needed to support a hard drive controller, and both PC DOS and the BIOS had no support for hard disks. After the XT was released, IBM altered the design of the 5150 to add most of these capabilities, except for the upgraded power supply. At this point adding a hard drive was possible, but required the purchase of the IBM 5161 Expansion Unit, which contained a dedicated power supply and included a hard drive.[60]

Although official hard drive support did not exist, the third party market did provide early hard drives that connected to the floppy disk controller, but required a patched version of PC DOS to support the larger disk sizes.

Model F keyboard

The only option for human interface provided in the base PC was the built-in keyboard port, meant to connect to the included Model F keyboard. The Model F was initially developed for the IBM Datamaster, and was substantially better than the keyboards provided with virtually all home computers on the market at that time in many regards - number of keys, reliability and ergonomics. While some home computers of the time utilized chiclet keyboards or inexpensive mechanical designs, the IBM keyboard provided good ergonomics, reliable and positive tactile key mechanisms and flip-up feet to adjust its angle. Public reception of the keyboard was extremely positive, with some sources describing it as a major selling point of the PC and even as "the best keyboard available on any microcomputer."[56]

IBM PC with CGA monitor (model 5153), original keyboard, 5152 printer and paper stand (1988)

At release, IBM provided a Game Control Adapter which offered a 15-pin port intended for the connection of up to two joysticks, each having two analog axes and two buttons.

Connectivity to other computers and peripherals was initially provided through serial and parallel ports.

IBM provided a serial card based on an 8250 UART. The BIOS supports up to two serial ports.

IBM provided two different options for connecting Centronics-compatible parallel printers. One was the IBM Printer Adapter, and the other was integrated into the MDA as the IBM Monochrome Display and Printer Adapter.

The back of a PC, showing the five expansion slots

The expansion capability of the IBM PC was very significant to its success in the market. Some publications highlighted IBM's uncharacteristic decision to publish complete, thorough specifications of the system bus and memory map immediately on release, with the intention of fostering a market of compatible third-party hardware and software.[61]

The motherboard includes five 62-pin card edge connectors which are connected to the CPU's I/O lines. IBM referred to these as "I/O slots," but after the expansion of the PC clone industry they became retroactively known as the ISA bus. At the back of the machine is a metal panel, integrated into the steel chassis of the system unit, with a series of vertical slots lined up with each card slot.

Most expansion cards have a matching metal bracket which slots into one of these openings, serving two purposes. First, a screw inserted through a tab on the bracket into the chassis fastens the card securely in place, preventing the card from wiggling out of place. Second, any ports the card provides for external attachment are bolted to the bracket, keeping them secured in place as well.

The PC expansion slots can accept an enormous variety of expansion hardware, adding capabilities such as:

• Graphics
• Sound
• Mouse support
• Expanded memory
• Additional serial or parallel ports
• Networking
• Connection to proprietary industrial or scientific equipment

The market reacted as IBM had intended, and within a year or two of the PC's release the available options for expansion hardware were immense.

The expandability of the PC was important, but had significant limitations.

One major limitation was the inability to install a hard drive, as described above. Another was that there were only five expansion slots, which tended to get filled up by essential hardware - a PC with a graphics card, memory expansion, parallel card and serial card was left with only one open slot, for instance.

IBM rectified these problems in the later XT, which included more slots and support for an internal hard drive, but at the same time released the 5161 Expansion Unit, which could be used with either the XT or the original PC. The 5161 connected to the PC system unit using a cable and a card plugged into an expansion slot, and provided a second system chassis with more expansion slots and a hard drive.