IBM Personal Computer
From Wikipedia, the free encyclopedia
- This article discusses to the original IBM PC. For IBM-like PCs in general ("clones"), see IBM PC compatible. For the generic meaning of "personal computer", see personal computer. For the second generation of microcomputers released from the late-1970s to the mid-1980s, see home computer.
| IBM PC Series | ||
|---|---|---|
| IBM 5120 | IBM Personal Computer XT • IBM Portable Personal Computer • IBM PCjr | |
| ← | → | |
 | |
| IBM PC (model 5150) | |
|---|---|
| Type | Personal computer |
| Released | August 12, 1981 |
| Discontinued | April 2, 1987 |
| Processor | Intel 8088 @ 4.77 MHz |
| Memory | 16 kB ~ 640 kB |
| OS | IBM BASIC / PC-DOS 1.0 CP/M-86 UCSD p-System |
The IBM Personal Computer, commonly known as the IBM PC, was the original version and progenitor of the IBM PC compatible hardware platform. It was 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 term "personal computer" was common currency before 1981, and was used as early as 1972 to characterize Xerox PARC's Alto. However, because of the success of the IBM PC, what had been a generic term came to mean specifically a microcomputer compatible with IBM's specification.
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[edit] The IBM PC concept
The original PC was an IBM attempt 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.[1]
Rather than going through the usual IBM design process, which had already failed to design an affordable microcomputer (the unsuccessful IBM 5100), 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.
The team consisted of twelve people headed by Don Estridge. 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 developed their own components. Second, they decided on an open architecture so that other manufacturers could produce and sell peripheral components and compatible software. IBM also sold an IBM PC Technical Reference Manual which included a listing of the ROM BIOS source code.[2][3]
At the time, Don Estridge and his team considered using the IBM 801 processor and its operating system that had been developed at the IBM research laboratory in Yorktown Heights, New York (The 801 was 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.
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 PC came back under the usual IBM management control, with the result that competitors had little trouble taking the lead from them. (In this regard, 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).
[edit] Third-party distribution channels
Sears Roebuck and Computerland executives were involved with the IBM team from the start. The IBMers - especially H.L. ('Sparky') Sparks, who was in charge of sales and marketing - relied on them for much of their knowledge of the marketplace. In turn, almost by default, they were to become the main outlets for the new product. Sears Roebuck would set up a handful of computer centers. Most important, more than 190 Computerland stores already existed. From IBM's point of view, this meant that there would be immediate widespread distribution across the US. In the event, Sears Roebuck failed to live up to expectations, when the new PC turned out to be selling to the office market rather than the home - where it had originally been targeted.
The use of outside organizations ('third parties' in IBM terminology) to sell IBM's products did not stop with the PC; for the whole of IBM's business had gradually evolved to the state where it was selling ever larger numbers of ever cheaper 'boxes'. The only way that IBM felt - at that time - it could, in general, handle the numbers of these new customers was by handing over the lower end of its business to 'retailers'; an approach that many other companies had successfully adopted in the past - from the producers of groceries (whose experience may not all have been totally relevant, though their experience of advertising to large numbers of end users might have been) to the manufacturers of cars (most of whose experiences, from handling their dealers to advertising on the large scale, might have been very relevant indeed). Certainly, by the end of the 1990s, more PCs were being sold worldwide than cars or even TVs.
[edit] IBM PC models
The models of IBM's first-generation Personal Computer (PC) series have names:
The original PC had a version of Microsoft BASIC —IBM Cassette BASIC— in ROM. The CGA (Color Graphics Adapter) video card could use a standard television for display; the other option offered by IBM was a MDA (Monochrome Display Adapter) and a monochrome display model 5151. The standard storage device was cassette tape. A floppy disk drive was an optional extra; no factory-installed hard disk was available. It had only five expansion slots; maximum memory using IBM parts was 256 kilobytes, 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 enhanced mathematical processing power. 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.) The IBM 5161 Expansion Chassis was eventually released that allowed for more expansion boards to be installed as well as additional hard drives.
The original PC proved too expensive for the home market, but was an unexpectedly large success with businesses. The "IBM Personal Computer XT" was an enhanced machine designed for business use. It had 8 expansion slots and a 10 megabyte hard disk. It could take 256 kB of memory on the main board (when 64 Kbit DRAM was introduced); later models were expandable to 640 kB. (The 384 kB of BIOS ROM + video RAM space filled the rest of the one megabyte address space of the 8088 CPU.) It was usually sold with a Monochrome Display Adapter (MDA) video card. The processor was still a 4.77 MHz Intel 8088 and the expansion bus still 8-bit Industry Standard Architecture (ISA) with XT bus architecture.
The "IBM Personal Computer/AT", announced August 1984, used an Intel 80286 processor, originally at 6 MHz. It had a 16-bit ISA bus and 20 MB hard drive. A faster model, running at 8 MHz, was introduced in 1986. IBM made some attempt at marketing it as a multi-user machine, but it sold mainly as a faster PC for power users. Early PC/ATs were plagued with reliability problems, in part because of some software and hardware incompatibilities, but mostly related to the internal 20 MB hard disk. While some people blamed IBM's hard disk controller card and others blamed the hard disk manufacturer Computer Memories Inc. (CMI), the IBM controller card worked fine with other drives, including CMI's 33-megabyte model. The problems introduced doubt about the computer and, for a while, even about the 286 architecture in general, but after IBM replaced the 20 MB CMI drives, the PC/AT proved reliable and became a lasting industry standard.
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.
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[edit] Technology
[edit] Electronics
The main circuit board in an IBM PC is called the motherboard. This carries the CPU and memory, and has a bus with slots for expansion cards.
The bus used in the original PC became very popular, and was subsequently named ISA. It is in use to this day in computers for industrial use. Later, requirements for higher speed and more capacity forced the development of new versions. IBM introduced the MCA bus with the PS/2 line. The VESA Local Bus allowed for up to three, much faster 32-bit cards, and the EISA architecture was developed as a backward compatible standard including 32-bit card slots, but it only sold well in high-end server systems. The lower-cost and more general PCI bus was introduced in 1994 and has now become ubiquitous.
The motherboard is connected by cables to internal storage devices such as hard disks, floppy disks and CD-ROM drives. These tend to be made in standard sizes, such as 3.5" (90 mm) and 5.25" (133.4 mm) widths, with standard fixing holes. The case also contains a standard power supply unit (PSU) which is either an AT or ATX standard size.
Intel 8086 and 8088-based PCs require EMS (expanded memory) 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.
[edit] Keyboard
The original 1981 IBM PC's keyboard at the time was an extremely reliable and high quality keyboard originally developed in North Carolina for another $10,000 IBM computer system that had been canceled. Each key was rated to be reliable to over 100 million keystrokes. Compared to 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. Byte magazine in the fall of 1981 went so far as to state that the keyboard was 50 percent of the reason to buy an IBM PC. The importance of the keyboard was definitely established when the 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 but lower quality keyboard. This mistake was narrowly avoided by the advice of one of the original development engineers.
However, the original 1981 IBM PC's keyboard was severely criticized by typists for its non-standard placement of the return and left shift keys. In 1984, IBM corrected this on its AT keyboard, but shortened the backspace key, making it harder to reach. In 1987, it introduced the enhanced keyboard, which relocated all the function keys and the Ctrl keys. The Esc key was also relocated to the opposite side of the keyboard.
Another criticism of the original keyboard was the relatively loud "clack" sound each key made when pressed. Since typewriter users were accustomed to keeping their eyes on the hardcopy they were typing from and had come to rely on the 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 "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.
See also: Keyboard layout
[edit] Serial port addresses and interrupts
| COM Port | IRQ | Base Port Address |
|---|---|---|
| COM1 | IRQ4 | 3F8H |
| COM2 | IRQ3 | 2F8H |
| COM3 | IRQ4 | 3E8H |
| COM4 | IRQ3 | 2E8H |
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.
[edit] Character set
The original IBM PC used the 7-bit ASCII alphabet as its basis, but extended it to 8 bits with nonstandard character codes. This character set was not suitable for some international applications, and soon a veritable cottage industry emerged providing variants of the original character set in various national variants. In IBM tradition, these variants were called code pages. These codings are now obsolete, having been replaced by more systematic and standardized forms of character coding, such as ISO 8859-1, Windows-1251 and Unicode. The original character set is known as Code page 437.
[edit] Storage media
Originally, the only storage medium for the original IBM PC model 5150 was a port for connection to a cassette drive. This was not the sort of unit expected in such an expensive computer and it was seldom used; few (if any) IBM PCs left the factory without a floppy disk drive installed. The 1981 PC had one or two 160 kilobyte 5¼ inch single-sided double-density floppy disk drives (180 kilobyte in later MS-DOS versions).[4] XTs generally had one double-sided 360 kB drive (next to the hard disk).
The first IBM PC that included a fixed, non-removable, hard disk was the XT. Hard disks for IBM compatibles soon became available with very large storage capacities. If a hard disk was added that was not compatible with the existing disk controller, a new controller board had to be plugged in; some disks were integrated with their controller in a single expansion board, commonly called a "Hard Card".
In 1984, IBM introduced the 1.2 megabyte dual sided floppy disk along with its AT model. Although often used as backup storage, the high density floppy was not often used by software manufacturers for interchangeability. In 1986, IBM began to use the 720 kB double density 3.5" microfloppy disk on its Convertible laptop computer. It introduced the 1.44 MB high density version with the PS/2 line. These disk drives could be added to existing older model PCs. In 1988 IBM introduced a drive for 2.88 MB "DSED" diskettes in its top-of-the-line PS/2 models; it was an instant failure and is all but forgotten today (but survives as a possible "size" choice in disk-formatting utilities).
[edit] Original software
All IBM PCs include a relatively small piece of software stored in ROM. The original IBM PC 40 KiB ROM included 8 KiB for power-on self-test (POST) and basic input/output system (BIOS) functions plus 32 KiB BASIC in ROM (Cassette BASIC). The ROM BASIC interpreter was the default user interface if no DOS boot disk was present. BASICA was distributed on floppy disk and provided a way to run the ROM BASIC under PC-DOS control.
In addition to PC-DOS, buyers could choose either CP/M-86 or UCSD p-System as operating systems. Due to their higher prices, they never became very popular and PC-DOS came to be the dominant operating system.
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