IBM PC (model 5150)

IBM Personal Computer model 5150 with IBM CGA monitor (model number 5153), IBM PC keyboard, IBM 5152 printer and paper stand.
Type	Personal computer
Release date	August 12, 1981; 30 years ago  (1981-08-12)
Discontinued	April 2, 1987; 25 years ago  (1987-04-02)
Operating system	IBM BASIC / PC-DOS 1.0 CP/M-86 UCSD p-System
CPU	Intel 8088 @ 4.77 MHz
Memory	16 kB ~ 256 kB
Sound	1-channel PWM

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 PC came to mean more specifically a microcomputer compatible with IBM's PC products.

Origins[link]

Desktop sized programmable calculators by Hewlett Packard had evolved into the HP 9830 BASIC language computer by 1972, with IBM's releasing its own IBM 5100 in 1975. It 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 $20,000 USD. It was specifically designed for professional and scientific problem-solvers, not business users or hobbyists.^[1] When the PC was introduced in 1981, it was originally designated as the IBM 5150, putting it in the "5100" series, though its architecture was not directly descended from the IBM 5100.

The original line of PCs were part of an IBM strategy to get into the small personal computer market then dominated by the Commodore PET, Atari 8-bit family, Apple II, Tandy Corporation's TRS-80s, and various CP/M machines.^[2]

New products at IBM typically required about four years for development. The company recognized that to compete with other personal computers it needed to develop its offering much more quickly.^[3] Rather than going through the usual IBM design process, a special team was assembled with authorization to bypass normal company restrictions and get something to market rapidly. This project was given the code name Project Chess at the IBM Entry Systems Division in Boca Raton, Florida. The team consisted of twelve people directed by Don Estridge with Chief Designer Lewis Eggebrecht.^[4] 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 its own components. Secondly for scheduling and cost reasons, rather than developing unique IBM PC monitor and printer designs, project management decided to utilize an existing "off-the-shelf" IBM monitor developed earlier in IBM Japan as well as an existing Epson printer model. Consequently, the unique IBM PC industrial design elements were relegated to the system unit and keyboard.^[5] 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 that included complete circuit schematics, a listing of the ROM BIOS source code, and other engineering and programming information.^[6] IBM announced the PC on August 12, 1981. Six weeks later at COMDEX Fall, Tecmar had 20 PC products available for sale. These products included memory expansion, IEEE-488, data acquisition, and PC Expansion chassis^{[7][8][9][10]} .^[11] Pricing for the IBM PC started at $1,565 for a bare-bones configuration without disk drives.^[12]

At the time, Don Estridge and his team considered using the IBM 801 processor (an early RISC CPU) and its operating system that had been developed at the Thomas J. Watson Research Center in Yorktown Heights, New York. The 801 processor was more than an order of magnitude more powerful than the Intel 8088, and the operating system more advanced than the DOS 1.0 operating system from Microsoft, which 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.^[13]

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.^{[citation needed]} IBM's tradition of "rationalizing" 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.^{[citation needed]}

IBM PC as standard[link]

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 Phoenix Software Associates, American Megatrends, Award, and others achieved workable versions of the BIOS, allowing companies like DELL, Compaq, and HP to manufacture PCs that worked like IBM's product. The IBM PC became the industry standard.

Third-party distribution[link]

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. 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 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.

Models[link]

IBM Personal Computer

IBM 5150 PC

All IBM personal computers are software backwards-compatible with each other in general, but not every program will work in every machine. Some programs are time sensitive to a particular speed class. Older programs will not take advantage of newer higher-resolution 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 IBM PC BIOS ROM.^{[citation needed]} Most notably, BASICA which was dependent on the BIOS ROM had a sister program called GW-BASIC which supported more functions and was 100% backwards compatible and could run independent from the BIOS ROM.

PC[link]

The CGA video card, with a suitable modulator, could use an NTSC television set or an RGB monitor for display; IBM's RGB monitor was their display model 5153. The other option that was offered by IBM was 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,^[16] if supported by the application program. For example, AutoCAD, 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 combo card.

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, or without any drives or storage medium. In the latter case IBM intended a user to connect his own cassette recorder via the 5150's cassette socket. The cassette tape socket was the same as the keyboard socket and next to it. A hard disk could not be installed into the 5150's system unit without changine to a higher-rated power supply. The "IBM 5161 Expansion Chassis" came with one 10 MB hard disk and allowed the installation of a second hard disk.^[17] 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 in to the 5150's motherboard; the only motherboard external connectors were the keyboard and cassette ports. The simple PC speaker sound hardware was also on-board. The original PC's maximum memory using IBM parts was 256 kB, 64 kB on the motherboard and three 64 kB expansion cards. The processor was an Intel 8088 running at 4.77 MHz (4/3 the standard NTSC color burst frequency of 3.579545 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 80286 instructions. An Intel 8087 co-processor 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-kbit DRAM chips. (The ninth bit was used for parity checking of memory.) After the IBM XT shipped, the IBM PC motherboard was configured more like the XTs motherboard with 8 narrower slots^{[dubious – discuss]}, as well as 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,^[18] 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 $3,005 ($ 7,682 in today's dollars), while the cheapest configuration (1565 US$) 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).^[19][20] While the 5150 did not become a top selling home computer, its floppy-based configuration became an unexpectedly large success with businesses.

XT[link]

The "IBM Personal Computer XT", IBM's model 5160, was an enhanced machine that was designed for diskette and hard drive storage, introduced two years after the introduction of the "IBM Personal Computer". It had eight expansion slots and a 10 MB hard disk (later versions 20 MB). Unlike the model 5150 PC, the model 5160 XT no longer had a cassette jack, but still contained the Cassette Basic interpreter in ROMs. The XT could take 256 kB of memory on the main board (using 64 kbit DRAM); later models were expandable to 640 kB. (The BIOS ROM and adapter ROM and RAM space, including video RAM space [since the video hardware was always an adapter] filled the remaining 384 kB of the one megabyte address space of the 8088 CPU.) It was usually sold with a Monochrome Display Adapter (MDA) video card.^{[citation needed]} The processor was a 4.77 MHz Intel 8088 and the expansion bus 8-bit XT bus architecture (later called 8-bit Industry Standard Architecture (ISA) by IBM's competitors). The XT's expansion slots were placed closer together^[21] than with the original PC;^[22] this rendered the XT's case and mainboard incompatible with the model 5150's case and mainboard. The slots themselves and the peripheral cards however were compatible, unless a rare card designed for the PC happened to use the extra width of the 5150's slots, in which case the card might require two slots in the XT. The XT's expansion slot mechanical design, including the slot spacing and the design of the case openings and expansion card retaining screws, was identical to the design that was later used in the IBM PC AT and is still used as of 2011, though (since the phase-out of ISA slots) with different actual slot connectors and bus standards.

XT/370[link]

The 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 processors. The computer booted into DOS, then ran the VM/PC Control Program.^[23][24]

PCjr[link]

The IBM PCjr was IBM's first attempt to enter the market for relatively inexpensive educational and home-use personal computers. The PCjr, 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 the PCjr to be a commercial failure.

Portable[link]

The 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 IBM Convertible.

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" 360K 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 television.

AT[link]

The "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 ^[25] was introduced in 1986.^[26]

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, IBM PC/ATs 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 Drive^[27]

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.

AT/370[link]

The "IBM Personal Computer AT/370 was an AT with two custom 16-bit cards, running almost the exact same setup as the XT/370.

Convertible[link]

The 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 Intel 8088) 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 (5.8 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.

Next Generation IBM PS/2[link]

The 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" drive bays, it came with a 3.5" floppy drive and optionally a 3.5" 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.

Technology[link]

Electronics[link]

Original IBM Personal Computer motherboard, IBM 5150. It has five 8-bit Industry Standard Architecture slots, and two DIN connectors for keyboard and cassette interface.

The main circuit board in an IBM PC is called the motherboard (IBM terminology calls it a planar). This mainly carries the CPU and RAM, and it has a bus with slots for expansion cards. On the motherboard are also the ROM subsystem, DMA and IRQ controllers, coprocessor socket, sound (PC speaker, tone generation) circuitry, and keyboard interface. The original PC also adds to this the 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 IBM PC AT architecture rather than license the PS/2 architecture and its MCA bus from IBM. The XT-bus was then retroactively named 8-bit ISA or XT ISA, while the unqualified term ISA usually refers to the 16-bit AT-bus (as better defined in the ISA specifications.) The AT-bus is an extension of the PC-/XT-bus and is in use to this day in computers for industrial use, 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 reliability).

Quadram Quadboard.

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.) The original IBM PC AT used an Intel 80286 processor which can access up to 16 MiB of memory (though standard DOS applications cannot use more than one megabyte without using additional APIs.) Intel 80286-based computers running under OS/2 can work with the maximum memory.

The original 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 a Intel 8253 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;^[28] one DMA channel is used to perform the memory refresh. Math co-processor 8087 at 0xF0. The 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),^[29] 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).^[30] On expansion cards, the Intel 8255 programmable peripheral interface (PPI) (at I/O addresses 0x378 is used for parallel I/O controls the printer,^[31] and the 8250 universal asynchronous receiver/transmitter (UART) (at I/O address 0x3F8 or 0x3E8) controls the serial communication at the (pseudo-)^[32]RS-232 port.

Keyboard[link]

The original keyboard for the IBM 5150

The original 1981 IBM PC's keyboard at the time was an extremely reliable and high quality electronic keyboard originally developed in North Carolina for the Datamaster system.^[33] Each key was rated to be reliable to over 100 million keystrokes. For the IBM PC, a separate keyboard housing was designed with a novel usability feature that allowed users to adjust the keyboard angle for personal comfort. Compared with the keyboards of other small computers at the time, the IBM PC keyboard was far superior and played a significant role in establishing a high quality impression. For example, the industrial design of the keyboard, together with the system unit, was recognized with a major design award.^[5] 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 the 1983 IBM PCjr flopped, in very large part for having a much different and mediocre Chiclet keyboard that made a poor impression on customers. Oddly enough, the same thing almost happened to the original IBM PC when in early 1981 management seriously considered substituting a cheaper and lower quality keyboard. This mistake was narrowly avoided on the advice of one of the original development engineers.

However, the original 1981 IBM PC 84-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. In 1982, Key Tronic introduced the now standard 101-key PC keyboard. In 1984, IBM corrected the Return and left Shift keys on its AT keyboard, but shortened the 'backspace' key, making it harder to reach. In 1986, 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 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.

The 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.

An "IBM PC compatible" may have a keyboard that does not recognize every key combination a true IBM PC does, such as shifted cursor keys. In addition, the "compatible" vendors sometimes used proprietary keyboard interfaces, preventing the keyboard from being replaced.

Although the PC/XT and AT used the same style of keyboard connector, the low-level protocol for reading the keyboard was different between these two series. The AT keyboard uses a bidirectional interface which allows the computer to send commands to the keyboard. An AT keyboard could not be used in an XT, nor the reverse. Third-party keyboard manufacturers provided a switch on some of their keyboards to select either the AT-style or XT-style protocol for the keyboard.

Character set[link]

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.

Storage media[link]

Cassette tape[link]

As mentioned above, IBM equipped the model 5150 with a cassette port for connecting a cassette drive, and originally intended compact cassettes to become the 5150's most common storage medium. However, adoption of the floppy- and monitor-less configuration was low; few (if any) IBM PCs left the factory without a floppy disk drive installed. Also, DOS was not available on cassette tape, only on floppy disks (hence "Disk Operating System"). 5150s with just external cassette recorders for storage could only use the built-in ROM BASIC as their operating system. As DOS saw increasing adoption, the incompatibility of DOS programs with PCs that used only cassettes for storage made this configuration even less attractive. The ROM BIOS supported cassette operations.

Interestingly, the IBM PC cassette interface encodes data using a 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.

Floppy diskettes[link]

Tandon 5.25-inch Diskette Drive with a partially inserted double-density diskette containing DOS 1.1.

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 formated for double-sided use could not be read on a single-sided drive.

The disks were Modified Frequency Modulation (MFM) coded in 512-byte sectors, and were soft-sectored.^[34] They contained 40 tracks per side at the 48 track per inch (TPI) density,^[35] and initially were formatted to contain eight sectors per track. This meant that SSDD disks initially had a formatted capacity of 160 kB,^[36] 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 while DSDD disks had a capacity of 320 kB.^[37] However, the DOS /drives,^[38] and 360 kB with DSDD disks/drives.^[39] 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.

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-inch floppies in particular was notoriously fraught with disk compatibility problems.

IBM's original floppy disk controller card also included an external 37-pin D-shell connector. This allowed users to connect additional external floppy drives by third party vendors. IBM themselves did not offer external floppy drives.^[40]

Fixed disks[link]

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.

A hard drive was a rare and expensive feature in early IBM PCs. A floppy drive was standard and given the name "drive A:"; a second floppy drive, if present, was designated B:. The first (boot) hard disk drive was given the name C:; further drives, if present, were given the letters following.

The first IBM PC model with an internal non-removable hard disk was IBM's model 5160, the XT. As other IBM-compatible PCs started to appear, hard disks with larger storage capacities also became available. Space permitting, these could be installed into either the IBM PC's Expansion Unit, into PSU-upgraded 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 were sold as kits including a controller card and replacement power supply, and some were integrated with their controller into a single expansion card, commonly called a "Hard Card".

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.

OS support[link]

The IBM PC's ROM BASIC and BIOS supported cassette tape storage. DOS itself did not support cassette tape storage. PC-DOS version 1.00 supported only 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. In addition to PC-DOS, buyers could choose either CP/M-86 or UCSD p-System as operating systems, as well as some flavors of SysV Unix. Due to their higher prices, they never became very popular and PC-DOS (IBM-DOS) or MS-DOS came to be the dominant operating system.

Serial port addresses and interrupts[link]

The serial port is an 8250 or a derivative (such as the 16450 or 16550), mapped to eight consecutive IO addresses and one interrupt request line.

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 serial ports could be used for a modem, a printer, or a mouse or other pointing device plugged into a serial port.

Original software[link]

All IBM PCs include a relatively small ( 8 kB ) piece of software stored in ROM 8 kB for power-on self-test (POST) and basic input/output system (BIOS) functions plus 32 kB BASIC in ROM (Cassette BASIC). The IBM PC-ROM was stored on the motherboard in five 8 kB ROM DIP chip packages installed in sockets. (A sixth empty socket was provided for a customer's own custom ROM, and some vendors resold special-purpose PC units with specialized custom ROMs.) The ROM BASIC interpreter was the default user interface if no DOS boot disk was present. Microsoft's Disk Basic, BASIC.COM and Microsoft's Advanced BASICA.COM was distributed on System software floppy disks and needed the Cassette ROMs to run properly. A Compiler was available to speed up interpreted BASIC. Later when the PCjr was developed, another version of BASIC called Cartridge Basic, which came on an expansion cartridge was available, but only for that machine.

Longevity[link]

Many 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 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 high-standard IBM manufacturing, which provided for exceptional long-term reliability and durability. Many newer PCs, by contrast, use proprietary parts and PCs themselves become obsolete quickly. According to Moore's Law the power of a microprocessor doubles every 18 months and it becomes easier to simply dispose of the PC than to upgrade or repair it.

The slot specifications are still used in current PCs as well as the limitation of having 4 active partitions on a hard disk. Many systems still come with PS/2 style Keyboard and mouse connectors, and power supply connectors are based on later standards.

Collectability[link]

The 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. Today these systems can fetch anywhere from $100 to $4500, depending on cosmetic and operational condition.^{[citation needed]} The IBM model 5150 has proven to be very reliable and most systems, despite their age of 25 years or more, still function as they did when new.^{[citation needed]}

See also[link]

Information technology portal

Notes[link]

References[link]

Further reading[link]

• "Birth of the IBM PC", IBM Corporation History Archives website

External links[link]

Wikimedia Commons has media related to: IBM Personal Computer

• IBM PC 5150 System Disks and Roms
• IBM PC from IT Dictionary
• IBM PC history and technical information
• What a legacy! The IBM PC's 25 year legacy
• CNN.com - IBM PC turns 25
• IBM-5150 and collection of old digital and analog computers at oldcomputermuseum.com
• IBM PC images and information
• A brochure from November, 1982 advertising the IBM PC
• A Picture of the XT/370 cards, showing the dual 68000 processors
• The History Of The IBM Personal Computer