The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981. It was created by a team of engineers and designers under the direction of Don Estridge of the IBM Entry Systems Division in Boca Raton, Florida.
Alongside "microcomputer" and "home computer," the term "personal computer" was already in use before 1981. It was used as early as 1972 to characterize Xerox PARC's Alto. However, because of the success of the IBM Personal Computer, the term came to mean more specifically a microcomputer compatible with IBM's PC products.
The original line of PCs were part of an IBM strategy to get into the small computer market then dominated by the Commodore PET, Atari 8-bit family, Apple II and Tandy Corporation's TRS-80s, and various CP/M machines. IBM's first desktop microcomputer was the IBM 5100, introduced in 1975. It was a complete system - with a built-in monitor, keyboard, and data storage. It was also very expensive - up to US$20,000. It was specifically designed for professional and scientific problem-solvers, not business users or hobbyists. When the PC was introduced in 1981, it was originally designated as the IBM 5150, putting it in the "5100" series, though its architecture wasn't directly descended from the IBM 5100.
Rather than going through the usual IBM design process, a special team was assembled with authorization to bypass normal company restrictions and get something to market rapidly. This project was given the code name Project Chess at the IBM Entry Systems Division in Boca Raton, Florida. The team consisted of twelve people headed by Don Estridge and Chief Scientist Larry Potter. They developed the PC in about a year. To achieve this they first decided to build the machine with "off-the-shelf" parts from a variety of different original equipment manufacturers (OEMs) and countries. Previously IBM had always developed their own components. Secondly for scheduling and cost reasons, rather than developing unique IBM PC monitor and printer designs, project management decided to utilize an existing "off-the-shelf" IBM monitor developed earlier in IBM Japan as well as an existing Epson printer model. Consequently, the unique IBM PC industrial design elements were the system unit and keyboard. They also decided on an open architecture, so that other manufacturers could produce and sell peripheral components and compatible software without purchasing licenses. IBM also sold an IBM PC Technical Reference Manual which included a listing of the ROM BIOS source code.
At the time, Don Estridge and his team considered using the IBM 801 processor and its operating system that had been developed at the Thomas J. Watson Research Center in Yorktown Heights, New York (The 801 is an early RISC microprocessor designed by John Cocke and his team at Yorktown Heights.) The 801 was at least an order of magnitude more powerful than the Intel 8088, and the operating system many years more advanced than the DOS operating system from Microsoft, that was finally selected. Ruling out an in-house solution made the team’s job much easier and may have avoided a delay in the schedule, but the ultimate consequences of this decision for IBM were far-reaching. IBM had recently developed the Datamaster business microcomputer which used an Intel processor and peripheral ICs; familiarity with these chips and the availability of the Intel 8088 processor was a deciding factor in the choice of processor for the new product. Even the 62-pin expansion bus slots were designed to be similar to the Datamaster slots. Delays due to in-house development of the Datamaster software also influenced the design team to a fast track development process for the PC, with publicly-available technical information to encourage third-party developers.
Other manufacturers soon reverse engineered the BIOS to produce their own non-infringing functional copies. Columbia Data Products introduced the first IBM-PC compatible computer in June 1982. In November 1982, Compaq Computer Corporation announced the Compaq Portable, the first portable IBM PC compatible. The first models were shipped in March 1983.
Once the IBM PC became a commercial success, the product came back under the more usual tight IBM management control. IBM's tradition of "rationalizing" their product lines, deliberately restricting the performance of lower-priced models in order to prevent them from "cannibalizing" profits from higher-priced models, worked against them.
ComputerLand and Sears Roebuck partnered with IBM from the beginning of development. IBM's head of sales and marketing, H.L. ('Sparky') Sparks, relied on these retail partners for important knowledge of the marketplace.
As a natural progression, Computerland and Sears became the main outlets for the new product. More than 190 Computerland stores already existed, while Sears was in the process of creating a handful of in-store computer centers for sale of the new product. This guaranteed IBM widespread distribution across the United States.
Targeting the new PC at the home market, Sears Roebuck sales failed to live up to expectations. This unfavourable outcome revealed that the original strategy - targeting the office market - was the key to higher sales.
|Model name||Model #||Introduced||CPU||Features|
|PC||5150||Aug 1981||8088||Floppy disk or cassette system (external hard drive an optional extra)|
|XT||5160||Mar 1983||8088||First IBM PC to come with an internal hard drive as standard.|
|XT/370||5160/588||Oct 1983||8088||System/370 mainframe emulation|
|3270 PC||5271||Oct 1983||8088||With 3270 terminal emulation|
|PCjr||4860||Nov 1983||8088||Floppy-based home computer|
|PC Portable||5155||Feb 1984||8088||Floppy-based portable|
|AT||5170||Aug 1984||80286||Medium-speed hard disk|
|Convertible||5140||Apr 1986||8088||Microfloppy laptop portable|
|XT 286||5162||Sep 1986||80286||Slow hard disk, but zero wait state memory on the motherboard. This 6 MHz machine was actually faster than the 8 MHz ATs (when using planar memory) because of the zero wait states|
All IBM personal computers are software compatible with each other in general, but not every program will work in every machine. Some programs are time sensitive to a particular speed class. Older programs will not take advantage of newer higher-resolution display standards.
The most commonly used storage medium was the floppy disk, though cassette tape was originally envisoned by IBM as a low-budget alternative. Accordingly, the IBM 5150 PC was available with one or two floppy drives or without any drives or storage medium; in the latter case IBM intended that users connect their own existing cassette recorders via the 5150's cassette jack. A hard disk could not be installed into the 5150's system unit without retrofitting a stronger power supply, but an "Expansion Unit", aka the "IBM 5161 Expansion Chassis" was available, which came with one 10MB hard disk and also allowed the installation of a second hard disk. The system unit had five expansion slots; the expansion unit had eight; however, one of the system unit's slots and one of the expansion unit's slots had to be occupied by the Extender Card and Receiver Card, respectively, which were needed to connect the expansion unit to the system unit and make the expansion unit's other slots available, for a total of 11 slots, some of which however had to already be occupied by display, disk, and I/O adapters, etc. as none of these were available on-board with the 5150; the only on-board connectors were the keyboard and cassette ports. The original PC's maximum memory using IBM parts was 256 kB, 64 kB on the main board and three 64 kB expansion cards. The processor was an Intel 8088 (early 1978 version, later were 1978/81/2 versions of intel chip; second-sourced AMDs were used after 1983) running at 4.77 MHz (4/3 the standard NTSC color burst frequency of 3.579545 MHz), which could be replaced with a NEC V20 for a slight increase in processing speed. An Intel 8087 co-processor could also be added for hardware floating-point arithmetic. IBM sold it in configurations with 16 kB or 64 kB of RAM preinstalled using either nine or thirty-six 16-kbit DRAM chips. (As was common at the time, an extra bit was used for parity checking of memory.)
Although the TV-compatible video board, cassette port and FCC Class B certification were all aimed at making it a home computer the original PC proved too expensive for the home market. At introduction a PC with 64 kB of RAM and a single 5 1/4 inch floppy drive and monitor sold for US $3,005, while the cheapest configuration ($1,565) that had no floppy drives, only 16KB RAM, and no monitor (again, the expectation was that users would connect their existing TV sets and cassette recorders) proved too unattractive and low-spec, even for its time (cf. footnotes to the above IBM PC range table). While the 5150 did not become a top selling home computer, its floppy-based configuration became an unexpectedly large success with businesses.
The bus used in the original PC became very popular, and was subsequently named ISA. It is in use to this day in computers for industrial use. Later, requirements for higher speed and more capacity forced the development of new versions. IBM introduced the MCA bus with the PS/2 line. The VESA Local Bus allowed for up to three, much faster 32-bit cards, and the EISA architecture was developed as a backward compatible standard including 32-bit card slots, but it only sold well in high-end server systems. The lower-cost and more general PCI bus was introduced in 1994 and has now become ubiquitous.
The motherboard is connected by cables to internal storage devices such as hard disks, floppy disks and CD-ROM drives. These tend to be made in standard sizes, such as 3.5" (90 mm) and 5.25" (133.4 mm) widths, with standard fixing holes. The case also contains a standard power supply unit (PSU) which is either an AT or ATX standard size.
Intel 8086 and 8088-based PCs require expanded memory (EMS) boards to work with more than one megabyte of memory. The original IBM PC AT used an Intel 80286 processor which can access up to 16 megabytes of memory (though standard DOS applications cannot use more than one megabyte without using additional APIs.) Intel 80286-based computers running under OS/2 can work with the maximum memory.
However, the original 1981 IBM PC's keyboard was severely criticized by typists for its non-standard placement of the Return and left Shift keys. In 1984, IBM corrected this on its AT keyboard, but shortened the 'backspace' key, making it harder to reach. In 1987, it introduced the enhanced keyboard, which relocated all the function keys and the Ctrl keys. The Esc key was also relocated to the opposite side of the keyboard.
Another criticism of the original keyboard was the relatively loud "clack" sound each key made when pressed. Since typewriter users were accustomed to keeping their eyes on the hardcopy they were typing from and had come to rely on the mechanical sound that was made as each character was typed onto the paper to ensure that they had pressed the key hard enough (and only once), the PC keyboard electronic "clack" feature was intended to provide that same reassurance. However, it proved to be very noisy and annoying, especially if many PCs were in use in the same room, and later keyboards were significantly quieter.
An "IBM PC compatible" may have a keyboard that does not recognize every key combination a true IBM PC does, such as shifted cursor keys. In addition, the "compatible" vendors sometimes used proprietary keyboard interfaces, preventing the keyboard from being replaced.
Although the PC/XT and AT used the same style of keyboard connector, the low-level protocol for reading the keyboard was different between these two series. An AT keyboard could not be used in an XT, nor the reverse. Third-party keyboard manufacturers provided a switch to select either AT-style or XT-style protocol for the keyboard.
|COM Port||IRQ||Base Port Address|
Only COM1: and COM2: addresses were defined by the original PC. Attempts to share IRQ 3 and IRQ4 to use additional ports require special measures in hardware and software, since shared IRQs were not defined in the original PC design.
The disks were Modified Frequency Modulation (MFM) coded in 512-byte sectors, and were soft-sectored. They contained 40 tracks per side at the 48 track per inch (TPI) density, and initially were formatted to contain 8 sectors per track. This meant that SSDD disks initially had a formatted capacity of 160 KB, while DSDD disks had a capacity of 320 KB. However, the DOS operating system was later updated to allow formatting the disks with 9 sectors per track. This yielded a formatted capacity of 180 KB with SSDD disks/drives, and 360 KB with DSDD disks/drives. The unformatted capacity of the floppy disks was advertised as 250KB (SSDD) and 500KB (DSDD), however these "raw" 250/500KB were not the same thing as the usable formatted capacity; under DOS, the maximum capacity for SSDD and DSDD disks was 180KB and 360KB, respectively. Regardless of type, the file system of all floppy disks was FAT12.
While the SSDD drives initially were the only floppy drives available for the model 5150 PC, IBM later switched to DSDD drives, and the majority of 5150 PCs sold eventually shipped with one or two DSDD drives. The 5150's successor, the model 5160 IBM XT, never shipped with SSDD drives; it generally had one double-sided 360 kB drive (next to its internal hard disk). While it was technically possible to retrofit more advanced floppy drives such as the high-density drive (released in 1984) into the original IBM PC, this was not an option offered by IBM for the 5150 model, and the move to high-density 5.25" floppies in particular was notoriously fraught with compatibility problems.
IBM's original floppy disk controller card also included an external 37-pin D-shell connector. This allowed users to connect additional external floppy drives by third party vendors. IBM themselves did not offer external floppy drives.
The first IBM PC that shipped with an internal, fixed, non-removable hard disk was IBM's model 5160, the XT. However, as other IBM-compatible PCs started to appear, hard disks with larger storage capacities than the 5160's and 5161's initial 10MB (later 20MB) also became available, and could — space permitting — be installed into either the IBM PC's Expansion Unit or into PSU-upgraded model 5150 IBM PCs (or into XTs). Adding a third-party hard disk sometimes required plugging in a new controller board, because some of these hard drives were not compatible with the existing disk controller. Some third party hard disks for IBM PCs even sold as kits including a controller card and replacement power supply. Finally, some hard disks were integrated with their controller in a single expansion board, commonly called a "Hard Card".
In addition to PC-DOS, buyers could choose either CP/M-86 or UCSD p-System as operating systems. Due to their higher prices, they never became very popular and PC-DOS or MS-DOS came to be the dominant operating system.