IBM PC

IBM Personal Computer with IBM CGA 5153 monitor, IBM PC keyboard, IBM 5152 printer and paper support. (1988)

IBM PC 5150 PC with IBM 5151 monitor

The IBM Personal Computer (in Spanish, IBM personal computer or IBM personal computer), commonly known as the IBM PC, was the original version and progenitor of the IBM PC compatible hardware platform. It is the IBM model 5150, and was introduced on August 12, 1981 as part of the fifth generation of computers. It was created by a team of engineers and designers under the direction of Don Estridge and William C. Lowe of the IBM Entry Systems Division in Boca Raton, Florida. Next to the "microcomputer" and to "home computer", the term "personal computer" it was already in use before 1981. It began to be used in 1972 to characterize the Xerox PARC Alto. However, due to the success of the IBM Personal Computer, the term PC came to mean more specifically a microcomputer compatible with IBM PC products. The IBM PC is the predecessor of today's personal computers and progenitor of the IBM PC compatible platform.

Origin

Background

2 years before the IBM PC, IBM had released its first desktop microcomputer, the IBM 5100, introduced in 1975. It was a complete system, incorporating a monitor, keyboard, and data storage into a single case. It was also very expensive - up to $20,000. It was designed specifically for professional and scientific problem solvers, not for business users or hobbyists. It was never a personal computer.

In 1975 the Altair 8800 was introduced in an article in the January issue of Popular Electronics magazine, sold as a kit. The Altair surprised its creators when it generated thousands of orders in its first month of sale. The introduction of the Altair spawned an entire industry based on basic layout and internal design.

New companies like Cromemco began offering add-on kits, while Microsoft was founded to supply a BASIC interpreter for the systems.

Shortly thereafter, several full clone designs appeared on the market, typified by the IMSAI 8080. This spawned the appearance of a wide variety of systems based on the S-100 bus introduced with the Altair. The Altair is considered the spark that led to the personal computer revolution.

In 1977, three microcomputers appeared and ignited an explosion in the home computer market: Apple Computer's Apple II, Tandy's TRS 80 model I, and Commodore's Commodore PET. They were easy-to-use computers and were the first used by the general population. Other computers soon followed, such as the first of the Atari 8-bit family, CP/M machines, different models made by Tandy such as the TRS 80 Models II and III and the TRS-80 Color Computer, the Texas Instruments TI-99/ 4A, the Commodore VIC 20 and others.

More and more companies were added, both established ones and newly created ones that produced computers or all kinds of peripherals, components, and software for microcomputers.

In 1978 WordStar appeared, originally developed for the CP/M. It was the most feature-rich and easy-to-use word processor available for this operating system, and it became a de facto standard.

In 1979, VisiCalc, the first spreadsheet, was seen as the application that turned the microcomputer from a hobby for computer enthusiasts into a serious business tool.

Seeing the rise of these programs and the lightning success of Apple, with its Apple II, IBM decided to enter the PC market, which they had ignored until then.

The first video games for personal computers also appeared, the most popular being Microchess, SARGON, Adventureland, Mystery House and Zork, among others. The microcomputer market was growing very rapidly, but IBM, the world's largest computer company and then offering everything from minicomputers to mainframes, was not yet a player in this segment.

The development of the IBM PC

The original line of PCs was part of a strategy by IBM to enter the home computer market, which until then had been ignored and dominated by others. The original model of the IBM PC was designated the 5150, putting it in the series of the "5100" that it had released in 1975, although its architecture was not a direct descendant of the IBM 5100.

Instead of using IBM's usual design process, a special team was assembled with permission to bypass the company's normal constraints and quickly get something to market. This project was codenamed Project Chess at the IBM Entry Systems Division in Boca Raton, Florida. The team consisted of twelve people led by Don Estridge with Lead Designer Lewis Eggebrecht.

External Components

Previously IBM had always developed its own components, but they didn't in this case.

They developed the PC in about a year. To achieve this, they first decided to build the machine with parts available (off-the-shelf) from a variety of different original equipment manufacturers (OEMs) and from different countries, second, for reasons of time and cost, rather than develop unique designs for the IBM PC monitor and printer, project management decided to use an existing available IBM monitor, previously developed at IBM Japan as well as a very popular existing printer model, the Epson MX 80. Consequently, the only industrial design elements of the IBM PC were relegated to the system unit and keyboard.

• The processor was an Intel 8088.
• The support chips of the base plate were a 8284A clock generator, a 8259A interruption controller, a 8288 bus driver, a DMA 8237A controller, a timer 8253, a programmable peripheral interface 8255A, all of Intel, plus additional discrete logic implemented with TTL circuits used as glue logic.
• RAM chips were manufactured by third parties.
• The video controller, both for the monochromatic card (MDA), and for the color graphics card (CGA), was Motorola's 6845.
• The disk drives were of the Tandon brand and the driver of the disk drives was a NEC μPD765.
• The keyboard had an Intel's 8048 microprocessor.
• The BIOS of the base plate occupied 8 KiB of ROM memory and was written by IBM.
• There was a BASIC interpreter written by Microsoft on 32 KiB of ROM from the base plate.

Open architecture

They also decided to use an open architecture, so that other manufacturers could produce and sell peripheral components and compatible software without purchasing licenses.

IBM also sold a manual, the IBM PC Technical Reference Manual which included complete schematic diagrams of the circuits, a listing of the ROM BIOS source code, and much detailed engineering and programming information on each of the components of the IBM PC and its design in general.

Six weeks after IBM announced the IBM PC, on August 12, 1981, at COMDEX, Tecmar had 20 PC products available for sale. These products included memory expansion, IEEE-488, data acquisition, and expansion chassis.

At that time, Don Estridge and his team were considering 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 was more powerful than the Intel 8088, and the operating system was much more advanced than Microsoft's DOS 1.0 operating system which was ultimately selected.

Removing an internal solution made the team's job much easier and may have prevented a schedule delay, but the ultimate consequences of this decision for IBM were far-reaching.

IBM had recently developed the System/23 Datamaster business microcomputer that 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 processor choice for the new product.

Even the 62 pin expansion slots on the expansion bus were designed to be similar to the System/23 Datamaster expansion slots. Delays due to internal development of the Datamaster software also swayed the design team toward an accelerated development process for the PC, with publicly available technical information to encourage third-party developers.

Pricing for the IBM PC started at $1,565 for a bare-bones configuration with 16 KiB of RAM and no floppy drives. However, most systems were sold with one or two floppy drives and the PC operating system. DOS, and to work with the operating system you needed a system with at least 32 KiB of RAM. All of this pushed the base price of the IBM PC to over $2,000.

Consequences

IBM hoped to maintain its position in the market by having the licensing rights to the BIOS, and staying ahead of the competition. Unfortunately for IBM, other manufacturers quickly reverse-engineered the BIOS and produced their own versions without paying IBM royalties. Columbia Data Products introduced the first IBM PC compatible computer in June 1982, (Compaq Computer Corporation announced the Compaq Portable, the first IBM compatible portable PC. The first models were shipped in March 1983.

Once the IBM PC became a commercial success, the product returned to IBM's usual tighter traditional management control.^{[citation needed]} The IBM tradition of "streamlining" their product lines, deliberately restricting the performance of lower-priced models to prevent them from "cannibalizing" the benefits of more expensive models, ^{[citation needed]} made the job easier for its competitors who had no problem taking IBM's lead over its own product.

The IBM PC as standard

The success of the IBM PC led other companies to develop IBM-compatible systems, which in turn led to the marketing of things like floppy disks advertised as having the "IBM Format". Due to the open architecture and standard external components that were readily available on the market, an IBM PC clone could be built with off-the-shelf parts, but the BIOS required reverse engineering.

Companies like Phoenix Software Associates, American Megatrends, Award, and others achieved working versions of the BIOS, allowing companies like Dell, Compaq, and HP, and others, to manufacture PCs that functioned like IBM products.

The IBM PC became the industry standard.

Third-party distribution

ComputerLand and Sears Roebuck partnered with IBM early in development. IBM sales and marketing chief H.L. ('Sparky') Sparks, trusted these retail partners with important market knowledge.

Computerland and Sears became the primary outlets for the new product.

More than 190 Computerland stores already existed, while Sears was in the process of creating a handful of computer centers, within the stores, for the sale of the new product. This ensured the wide distribution of IBM throughout the US.

Aiming the new PC at the home market, Sears Roebuck's sales failed to meet expectations.

This unfavorable result revealed that the strategy was to target the office market, where sales were highest.

Commercial success

The first IBM PC was released on August 12, 1981. While not cheap, with a base price of $U1,565 (approximately $4,700 as of August 2021), it was inexpensive for businesses.

However, it wasn't corporate IT departments that bought it, but a series of mid-level managers who saw the potential of the PC when the VisiCalc spreadsheet was ported.

Relying on the prestige of the IBM name, they began to buy machines with their own budgets to do the calculations they had learned in business schools.

In a few years the IBM PC and its successors, both IBM and clones, displaced almost all microcomputers with other architectures, thus emerging the standard of the x86 architecture, and the MS DOS disk operating system at first, and then the Windows operating system, thus becoming Wintel computers.

Technology

System drive

Rear view of the IBM PC system drive housing, without the cover and without the components it hosted.

The IBM PC system unit contained the motherboard, the power supply (which powered all components), had space for up to two full-height 5.25-inch floppy disk drives, and to accommodate up to 5 memory cards. expansion plugs attached to the motherboard. On the back there were 5 long openings covered with screw-on metal plates that were removed to allow the expansion cards to be inserted and screwed. There were also two 5-pin DIN connectors, one to connect the keyboard, and one to connect cables to control a cassette recorder intended as a storage device. There was also a small speaker with which tones could be generated. A sliding cover closed the system unit.

The typical IBM PC shipped with one or two floppy drives, a controller card for the floppy drives, and a video card, so at least two of the five expansion slots were occupied by the IBM PC. computer.

Motherboard

The original IBM Personal Computer base plate, IBM 5150. It had a 16-bit Intel 8088 microprocessor and a socket to be able to plug in an Intel 8087 numeric coprocessor (up on the left), and on the left of them two DIN connectors, one for the keyboard and one for the cassette interface; five 8-bit ISA expansion slots for the insertion of expansion cards (down to the left), four RAM banks where it could be

The IBM PC motherboard (called planar in IBM terminology or system board). It mainly contained the CPU, RAM, and had a bus with expansion slots for cards. On the motherboard there is also the ROM subsystem, DMA, IRQ, and BUS controllers, a socket for a coprocessor, four banks of RAM of 16 KB each, the sound circuit (a tone-generating PC speaker), a keyboard interface and a cassette interface.

Chips

The motherboard consisted of several special purpose chips plus glue logic implemented with a TTL technology IC suite:

• An Intel 8088 to 4.77 MHz microprocessor (which was an Intel 8086 version but with an 8-bit data bus instead of 16). Along with the 8086, the 8088 is the first x86 architecture processor used in most of today's computers.
• An Intel 8284 watch generator, with a crystal at a frequency of 14,318 MHz that was divided between 3 to generate the 4,7727267 MHz of the 8088 microprocessor, and divided between 4 to generate the signal of 3,579545 MHz used by the CGA video card to generate the burst for color in the NTSC television standard. The frequency of 4,7727267 in turn was divided into 4 to generate a clock signal of 1,1931817 MHz for the three timer 8253 counters, one of which divided that frequency into 65536 and generated the signal of 18,2065 ticks (by second) that triggered a disruption that served to keep the time of the system
• An Intel 8288 bus driver, who received the clock signal of 4.77 MHz, the CPU S0, S1 and S2 and the INTA signals from the interruption controller, among others, and processed them to generate several control bus signals.
• A ROM memory with 8 KiB of the BIOS (the IBM PC ROM BIOS). On the ROM of the BIOS was the implementation of the BIOS functions. The first thing that was executed was the Power On Self Test (POST), which performed a diagnosis of the components of the computer and started the chips of the system and the BIOS itself; then the Boot Strap Loader was executed, which read the boot sector of a floppy and passed the control to start the load of the operating system, or in case of problems started the IBM PC ROM BASIC
• The BASIC player on ROM (the IBM PC ROM BASIC), who resided in 4 modules of 8 KiB of ROM each
• There were four RAM banks with capacity for 16 KiB each, which allowed a memory expansion from 16 KiB to 64 KiB on the base plate. Each bank was composed of 9 16 Kibit chips (4116 DRAM). It was 8 chips for the 8 bits of the byte plus 1 chip for parity checkup. The first bank had the circuits of the RAM soldiers to the base plate. The other three had sockets to insert extra memory circuits for enlargement up to 64 KB in total. A later version of the IBM PC allowed to expand up to 256 KiB RAM in 4 banks of 64 KiB each
• The Intel 8259 programmable interruption controller (PIC). It handled IBM PC hardware interruptions. It was able to process up to 8 interruptions (IRQ 0 to IRQ 7). IRQ 0 received a signal, 18.2 times a second, from timer 8253, used by the BIOS interruption handler to maintain a counter. IRQ 1 received an interruption issued by the keyboard controller every time a key was pressed or dropped. IRQ 6 received a signal from the floppy controller. The IRQ 7 was reserved for the printer, other IRQs were reserved or were not used on the original IBM PC, but then added devices that fired those interruptions.
• The programmable interval timer (PIT) Intel 8253. He had three timers (counters) (0, 1, 2). The timer 0 was used to generate a signal 18.2 times per second to shoot the IRQ 0 interruption. Timer 1 generated a signal periodically for the DMA controller to refresh the RAM. Timer 2 could be used to generate tones of different frequencies for the speaker of the base plate or also to generate tones that represented some and zeros, and were used to record data and programs in the cassette recorder that could be connected to the system
• The DMA Intel 8237 controller was used for transfers between E/S devices and RAM memroria and vice versa without CPU intervention. It had four channels (0, 1, 2, 3). Channel 0 was used to refresh RAM, which was done regularly thanks to a signal issued by the timer 1 of 8253. Channel 2 served for data transfer from and to disk drive.
• The programmable peripheral interface (PPI) 8255 was used to implement the ports of E/S 60h, 61h and 62h of the base plate. The 60h port was used for keyboard input or to read the status of 8 DIP switches, the 61h port was output and each of its 8 bits served to control system components, and the 62h port was used for reading additional DIP switches and also bits of input signals
• On the base plate there was a socket to insert an Intel 8087 numerical coprocessor that added the ability to perform computations in a floating coma by hardware. It was not common to place a numerical coprocessor to the PC, so programs that needed to make numerical calculations in floating point used libraries in software to perform calculations with the 8088 processor.

Expansion bus

IBM XT Bus Scheme. The PC and XT bus were almost identical, with some addition for the XT. Then, with the appearance of the IBM AT in 1984, this bus became longer to be able to add more functions, creating the ISA standard, which was subsequently expanded to produce the EISA standard.

On the motherboard, the IBM PC had 5 62-contact expansion slots, called by IBM as the I/O Channel. They were used to insert expansion cards that added functionality to the computer. Adjacent to each expansion slot was, on the back of the computer case, an opening through which expansion cards could expose connectors. When no expansion cards were installed, a metal cover covered the opening to prevent the intrusion of dust, dirt, and control airflow.

The expansion slots were an extension of the address, data, and control buses of the 8088 processor. It had the 8 data bus lines and the 20 address bus lines (demultiplexed), along with the control bus lines from the CPU, and to these lines were added IRQ lines for interrupts, DMA lines, clock lines, +5, -5, +12, and -12 volt power lines (in addition to ground), and others.

It was different from the S-100 bus, which was an industry standard at the time of the IBM PC. The bus used in the original PC became very popular and became a new standard. Many third-party manufacturers made all kinds of expansion cards for the PC and the later IBM XT that also used this bus, thus creating an industry around the PC and XT architecture. It was known as the PC bus or XT bus, and later as the 8-bit ISA bus.

With the release of the IBM AT computer, IBM expanded the original IBM PC bus to accommodate the new AT architecture that had an Intel 80286 processor and handled 16 bits of data instead of 8 and could address up to 16 MB of data. memory. The original PC bus was then expanded by adding 8 additional data lines to work with 16 bits, additional address lines to access the 16 MB, new lines for IRQ, DMA, and other functions.

Later, when IBM released the PS/2 series of computers, with a new architecture and a new bus, (the MCA), IBM intended to license the new bus to third-party computer manufacturers, but the rest of the industry would not agree. went down that path and chose to continue manufacturing machines based on the open architecture and not needing a license from the IBM AT. The AT bus then began to be called the Industry Standard Architecture (ISA bus), becoming the new industry standard and making IBM's MCA a failure. So, the original PC bus was called 8-bit ISA bus or XT ISA bus, while the term ISA (alone) was it was referring to the 16-bit AT bus (as defined in the ISA specifications). Later, in 1988, the Extended Industry Standard Architecture (EISA) standard was developed, which extended the existing ISA bus by making it 32-bit and adding additional functionality equivalent to that of the IBM MCA bus while maintaining backwards compatibility with the ISA bus.

The AT ISA bus, standardized in this way, is used to this day in computers for industrial use, where its relatively low speed, 5-volt signals, and relatively simple and straightforward design (by 2011 standards) give it technical advantages (eg noise immunity for reliability).

Expansion Cards

IBM PC Game Control Adapter (Game Control Adapter)

Functionality was added to the computer by means of expansion cards, called by IBM as adapters. For the IBM PC, IBM provided two video cards, the IBM Monochrome Display and Printer Adapter (MDA) (which also had a built-in port for connecting a printer) and the IBM Color Graphics Adapter (CGA), a floppy drive controller. (to which two internal and two external floppy drives could be connected), a printer adapter (parallel port), an adapter for asynchronous communications (RS232 serial port), a game controller adapter (for up to 2 joysticks or 4 paddles) and two RAM memory expansion options, one with 32 KiB of RAM and the other with 64 KiB (with which the computer's memory could be expanded up to 256 KB).

The cards were inserted into any free expansion slot of the 5 that the computer had and, except for memory expansion, they had connectors that were exposed to the outside through slots on the back of the computer. The devices were connected by means of cables to the respective connector. IBM sold a high-resolution monochrome monitor, the IBM 5151, that plugged into the MDA video card, floppy drives that plugged into the controller's internal connector, and a printer that could plug into either the MDA's parallel port. as in the separate parallel adapter.

Quadboard Quadram

Since IBM's original expansion cards generally had only one functionality, (serial port, parallel port, floppy drive controller, game controller, etc.), it was easy to fill the PC's five expansion slots, or even the eight slots of the XT, even without the installation of any special hardware. In the PC, at least two expansion slots were occupied, one for a video card and another for the floppy disk controller, leaving only three free. In the XT a third was used for the hard drive controller, leaving five free. It was then that companies like Quadram and AST began manufacturing very popular multi-I/O cards, which combined various peripherals on a single adapter card that used a single expansion slot; Quadram offered the QuadBoard and AST the SixPak, both offering RAM memory, a serial port, a parallel port, a game port, a clock. Later this became the standard way of adding basic functionality to a computer. Today, much of the functionality is built into the motherboard itself.

On parallel cards, the Intel 8255 Programmable Peripheral Interface (PPI) (at I/O address 0x378) was used for printer I/O, and the serial card used the Intel 8250 (UART) (at I/O address 0x3F8 or 0x3E8) which controlled the serial communication on the (pseudo) RS-232 port.

Keyboard

IBM 5150 Keyboard

The original 1981 IBM PC keyboard was extremely reliable and of high quality, originally developed in North Carolina for the System/23 Datamaster. Each key was rated to be reliable over 100 million keystrokes. For the IBM PC, a separate keyboard case was designed with a new usability feature that allowed users to adjust the tilt angle for personal comfort. 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 image. For example, the industrial design of the keyboard, along with the system unit, was recognized with a major design award. In the fall of 1981, Byte magazine went as far as it could to say that the keyboard was 50 percent industrial. the reason for buying an IBM PC. The importance of the keyboard was definitely established when the 1983 IBM PCjr flopped, largely because it had a very different and mediocre chiclet keyboard that gave a poor image to customers. Oddly enough, the same thing almost happened to the original IBM PC when, in early 1981, management seriously considered replacing the keyboard with a cheaper and inferior one. This bug was narrowly avoided by advice from one of the original development engineers.

However, the 84-key keyboard of the original 1981 IBM PC was criticized by typists for the non-standard placement of the ENTER and left SHIFT keys, and for not having a separate numeric and cursor pad, which 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 ENTER and LEFT SHIFT keys on its AT keyboard, but shortened the backspace key, making it harder to reach. In 1986, IBM switched to the improved 101-key keyboard, which added a separate cursor and numeric keypad, relocated all the function keys and the control (CTRL) key, and the escape (ESC) key was also located on the right side. opposite of the keyboard.

Another criticism of the keyboard was the relatively loud 'click' sound of the keyboard. What each key did when pressed. Since typewriter users were accustomed to keeping their eyes on the paper they were typing and had to rely on the mechanical sound that was made each time a character was typed on the paper to ensure that the key had been pressed Strong enough (and only once), the PC's electronic keyboard 'click' feature was intended to provide the same security. However, it was very noisy and annoying, especially if many PCs were in use in the same room. 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 one signal when it is pressed and another signal when it is released. A microcontroller built into the keyboard, the Intel 8048, scans the keyboard and encodes a scan code and release code for each key to as it is depressed and released. Any key can be used as a shift key, and a large number of keys can be pressed simultaneously and detected separately. The controller on the keyboard handles the typematic (repeat) operation, issuing repeated scan codes for a key pressed after a while, 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 as a real PC does, for example, uppercase cursor keys. In addition, "compatible" sometimes they used proprietary keyboard interfaces, preventing the keyboard from being substituted.

Although the PC/XT and AT use 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 that allows the computer to send commands to the keyboard. An AT keyboard could not be used on an XT, nor the other way around. Third-party keyboard manufacturers have provided a switch on some of their keyboards to select between the AT-style and XT-style protocol.

Video Cards

When the IBM PC was released, IBM offered two graphics cards for it, the Monochrome Display Adapter (MDA), and the Color Graphics Adapter (CGA). Both cards used the Motorola 6845 CTR controller.

Monochrome Display Adapter

IBM Monochrome Display Adapter (MDA)

The MDA card had 4 KB of RAM, could not display graphics, and had only a very high quality, 720x350 (720x350) character text mode. Characters could individually have certain attributes: normal, invisible, underlined, bright (bold), reverse video, and flicker. Some of those attributes could be combined. The MDA came with a connector to connect a monochrome monitor and one for a printer. IBM optionally sold the IBM 5151 monitor and a printer that connected to the MDA card. Some IBM MDA cards had circuitry that allowed them to display text in 16 colors with a background of 8 plus flicker, if connected to an RGBI color monitor.

Color Graphics Adapter

The CGA, in addition to text modes, could display graphics in resolutions of 320x200 with four colors, and 640x200 with 2 colors. You could select between several color palettes to choose the 4 or 2 colors of the graphic modes. It could display text in low resolution at 40x25, and high resolution at 80x25. In both resolutions the text could have 16 colors with a background of 8 colors and it could also blink. In low-res text mode you had 8 pages of text and in high-res text mode 4. One of those pages would display at any given time, but you could switch to another page instantly. The card had a connector for an RGBI monitor, an internal connector for a light pen input, and an RCA composite video output, along with an internal connector for connecting an RF modulator to view the displayed image on a normal television. of the time, which usually did not have a direct input for video. With the radio frequency modulator, the image produced by the CGA could be seen on channel 3 or 4 of the television.

Third-party adapters

Soon other third-party graphics cards appeared that exceeded the capabilities of those IBM sold. One of the most popular was the Hercules Graphics Card (HGC), which significantly enhanced the capabilities of IBM's MDA, adding to the MDA's text mode, a high-resolution 720x348 monochrome graphics mode. The HGA became the industry standard for monochrome monitors. Cards such as the Plantronics Colorplus also appeared, which had double the RAM memory and improved the features of the original CGA by adding two additional graphics modes, 320x200 at 16 colors, and 640x200 at 4 colors.

Character set

When the IBM PC came out, ASCII code was an industry standard, which had 128 characters defined in 7 bits. IBM created Extended ASCII for the IBM PC by using 8 bits to add another 128 characters to the 128 ASCII. This new character set was used by IBM's personal microcomputers and by the clone machine industry, thus becoming a standard.

Extended ASCII had, between code 128 and 255 (above traditional ASCII), characters to support some languages other than English, such as accented vowels and the Spanish eñe, and also had some Greek characters, mathematical symbols, graphic characters, and others. The standard ASCII characters 0 through 31 were nonprintable control characters, but were exploited by IBM to display additional printable characters, such as faces, sex signs, the suits of the French deck of cards, arrows in various directions, and others.

These extended ASCII characters could be displayed by video cards in text modes, but in graphics modes the BIOS only supported the first 128 (including the 32 printable special characters between 0 and 31).

This Extended ASCII character set did not have all the necessary ones for its use in certain languages, which led to the creation of variants of the original set in which the extra 128 characters were replaced to adapt to the corresponding language. These variants were called code pages. Today these codes are obsolete, having been replaced by much better schemes, such as ISO-8859-1 or Unicode.

Storage media

Cassette tape

As already mentioned, IBM equipped the 5150 model with a port for connecting a cassette drive, originally intended for compact cassettes to become the most common storage medium for the 5150, supported by the BIOS and ROM BASIC of the IBM PC. However, adoption of floppyless configurations was low. Few (if any) IBM PCs left the factory without a floppy drive installed. Also, DOS was not available on cassette tapes, only on floppy disks (hence the "Disk Operating System (DOS)"). 5150s with only an external cassette recorder for storage could only use the computer's 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.

It is interesting that the cassette interface of the IBM PC encodes the data using frequency modulation with a variable rate. Both a one and a zero are represented by a simple cycle of a square wave, but the frequencies of the waves differ by a factor of two, with the ones having a lower frequency. Thus, the bit periods for 0s and 1s also differed by a factor of two, with the unusual effect that the data stream with more 0s than 1s will use less tape (and time) than a data stream of equal length (in bits) containing more ones than zeros, or with an equal number of each.

Floppy Disks

IBM PC 5 1/4" disk drive adapter

5.25 inches Tandon disk drive with a partially inserted dual density diskette containing PC DOS 1.1

Most 5150 PCs had one or two 5.25-inch floppy drives. These floppy drives were either single-sided, double-density (SS/DD, also known as SSDD), or double-sided, double-density (DS/DD or DSDD). The IBM PC never used single-density floppy drives. Drives and disks were commonly referred to by their capacity, eg "160 KB floppy disk", or "360 KB floppy disk.".Dual-sided drives were backwards compatible; they could read and write single-sided floppies. The same type of physical floppy disk could be used in both drives, however, to convert a single-sided 5.25-inch disk to a double-sided disk, reformatting was necessary, and single-sided drives side could not read it with that format.

Disks were encoded in Modified Frequency Modulation (MFM) in 512-byte sectors, and the sectors were created by software (soft-sectors). They contained 40 tracks per side at a density of 48 tracks per inch (TPI), while double-sided drives had a capacity of 320 KB. However, the DOS operating system was later updated to allow formatting drives with nine sectors per track. This yielded a formatted capacity of 180 KB with single-sided drives/disks, and 360 KB with double-sided ones. The unformatted capacity of floppy disks was advertised as "250 KB" for single side and "500 KB" for double-sided, however, these "250/500 KB" raw was not the same thing as usable formatted capacity, under DOS, the maximum capacity for single-sided and double-sided drives was 180KB and 360KB respectively. Regardless of type, the file system of all floppy disks (under DOS) was FAT12.

Although single-sided drives were initially the only ones available for the 5150 model, IBM soon switched to dual-sided drives, and most 5150 PCs eventually shipped with one or two dual-sided drives. The successor to the 5150, the IBM XT never shipped with single-sided drives; and usually had a double-sided 360KB drive attached to its internal hard drive. Although it is technically possible to upgrade the original IBM PC with more advanced floppy drives such as high-density floppy drives (released in 1984), this was not an option offered by IBM for the 5150 model, and the move to high-density 5.25-inch floppy disks was notoriously loaded with disk compatibility issues.

The original IBM floppy controller card included an external 37-pin D-shell connector. This allowed users to connect additional external floppy drives from third party vendors. IBM itself did not offer external floppy drives.

Fixed disks

The 5150 couldn't have hard drives on its own since its 63.5-watt power supply didn't generate enough power. Later, IBM offered the 5161 Expansion Unit (5161 Expansion Unit) which not only provided more expansion slots, but also included a 10 MB (later 20 MB) hard drive powered by its own 130 watt power supply. The IBM 5161 Expansion Unit was released in early 1983, two years after the release of the IBM PC.

A floppy drive was a rare and expensive feature on early IBM PCs. A floppy drive (drive A) was standard, and a second floppy drive, if present, was designated B. Floppy drives came to known as the "C drive", because on IBM PCs that had them, it was the third drive.

The first IBM personal computer to ship with an internal, fixed hard disk drive was the IBM model 5160, the IBM XT. As other IBM PC compatible computers began to appear, hard drives with larger storage capacities became available. They could be installed on the IBM PC Expansion Unit, on PCs upgraded with new power supplies, or on the XT. Adding a third drive sometimes required plugging in a new controller card, because some of those drives were not compatible with the existing drive controller. Some third-party drives for the IBM PC were sold as kits, including the controller card and a replacement power supply. Eventually, some disk drives were integrated with their controllers on a single expansion card, commonly called a "HardCard".

After floppy drives became obsolete in the early 2000s, the letters A and B fell out of use. But for 25 years, virtually all DOS-based software assumed the drive for program installation to be C, so the primary drive is still "the C drive" even today. Other families of operating systems (eg Unix) are not restricted by these designations.

Printer

IBM Personal Computer Printer (Model 5152)

With the release of the IBM PC, IBM offered a 9-pin dot matrix printer, the IBM 80 CPS Matrix Printer. This printer was actually the extremely popular Epson MX 80, re-labeled 'IBM'. It accepted 8 1/2-inch continuous forms and printed at 80 characters per second (cps). It was connected to the computer through a parallel port (Centronics interface) and had a set of characters that could be printed in various sizes and styles: normal, condensed, enlarged (double width). and enlarged condensate. It could be printed in emphasized mode, and most modes could be printed in double strike, which made the text bold.

IBM PC Memory Map

Below is the memory map of the IBM PC. The first 640 KiB, from 00000h to 9FFFFh, contained the interrupt vectors, the BIOS data area, and the rest for PC DOS and programs. The area between A0000h and FFFFFh was reserved for system ROMs and adapter card ROMs and adapter card RAM. By the time the IBM PC came out, much of this area was unallocated. This space was then used for memory from new video cards, the EMS expanded memory specification, etc. As an example, the very popular Hercules Graphics Card, which soon replaced IBM's MDA as standard, took up 32 KiB of memory space between B0000h and B7FFFh.

Expanded memory

The IBM PC's 8088 microprocessor could address up to 1 MiB of memory (20 address bits). In the IBM PC architecture, the top 384 KiB was reserved as space for the IBM BIOS, the BASIC ROM, other BIOS ROMs that might have expansion cards, RAM for video cards, and other devices. This left usable space for programs of up to 640 KiB of RAM, which was much higher than the 48 KiB of program RAM, and up to 16 KiB of ROM for the BASIC interpreter, that were used in many of the personal computers., (with 8-bit microprocessors), at the time the IBM PC came out.

Later, the IBM AT computer, with its Intel 80286 processor, could address up to 16 MiB of memory, but the MS DOS and PC DOS programs could not address RAM memory above one megabyte, even though the computer had it, except in a very inefficient way. Other operating systems, such as OS/2, did have the ability to access memory in excess of one megabyte.

The Expanded Memory Specification (EMS) was then created, which allowed viewing, from DOS, large portions of memory accessed in pages or blocks of 64 KiB of memory, one at a time, in an efficient manner. So, the expanded memory could be accessed from DOS, both with the 8088 processor of the IBM PC and the XT, and by the 80286 processor of the AT, which under DOS worked in real mode and therefore had no access to memory. above the megabyte (except almost 64 KiB of the high memory area).

IBM PC I/O Addresses

The 8088 processor used 16 bits of the address bus to address input/output, thus leaving the I/O address space at 2¹⁶ = 64K = 65,536 I/O addresses different, and can be accessed as 8-bit or 16-bit I/O ports.

The following is a list of input/output (I/O) port addresses used on the IBM PC. The addresses are in hexadecimal.

Motherboard chips

• 00-0F - Intel 8237 DMA Controller
• 20-21 - Intel 8259 Interruption Controller
• 40-43 - Intel 8253 Programmable Timer
• 60-63 - Intel 8255 programmable peripheral interface
• 80-83 - Page records for the DMA
• A0-A? - Mask for non-maskable interruption
• C0-C?
• E0-E?

Expansion Cards

• 200-20F - Game Controller Card
• 3B0-3BF - MDA monochromatic card and LPT1 parallel port incorporated into the card
• 3D0-3DF - CGA color graphics card
• 3F0-3F7 - Disk controller card
• 378-37F - LPT2 parallel port
• 278-27F - Reserved. LPT3 parallel port
• 3F8-3FF - COM1 serial port
• 2F8-2FF - Reserved. COM2 serial port

Chips added to the IBM AT motherboard

Later, more components were added to IBM personal computers that continued on to the IBM PC. The IBM PC AT added the following:

• 70-71 - A real-time watch (RTC) Motorola MC146818 with non-volatile memory (NVRAM), used for system configuration (replacement of DIP switches and jumpers used for this purpose in PC and PC-XT models prior to AT.
• 80-9F - A DMA address registry (implemented with an IC 74LS612)
• A0-BF - Second slave interruption controller PIC 8259 slave
• C0-DF - A second DMA 8237 controller for 16-bit DMA

Today's computers generally have the same functionality inherited from the IBM PC, XT, and AT, while maintaining backward compatibility to the IBM PC. But instead of individual chips and add-on cards, today this functionality is found on the motherboard's southbridge chip for almost every component.

Hardware interrupts

The motherboard hardware had an Intel 8259 interrupt controller, which could process up to 8 hardware interrupt lines (IRQ0 to IRQ7). IRQ0 was mapped to motherboard PIC 8253 counter 2 and was interrupted 18.2 times per second, and IRQ1 was interrupted by the keyboard controller each time a key was pressed or released. The rest of the interrupts, IRQ2 through IRQ7, were available to expansion cards on the I/O channel (the 8-bit ISA bus). IRQ 6 received a signal from the floppy controller card.

IRQ0 to IRQ7 of the 8259 interrupt controller were mapped on the IBM PC as follows:

• IRQ0 - Generated by programmable interval timer Intel 8253, 18.2 times per second (timer tick)
• IRQ1 - Generated by Intel 8048 keyboard controller
• IRQ2 - Not used on IBM PC or XT. Subsequently, IBM AT was assigned a line from the 8259 slave interruption controller to process IRQ8 to IRQ15
• IRQ3 - Booking for communications. Generated by serial port, 8250 UART of COM2 and COM4
• IRQ4 - Booking for communications. Generated by serial port, 8250 UART of COM1 and COM3
• IRQ5 - Not used on IBM PC, used for the hard drive controller on the XT. At IBM AT, parallel port, Intel 8255 LPT2
• IRQ6 - Generated by NEC μPD765 floppy controller
• IRQ7 - Reserved for the printer.

The 8088 processor called interrupt vectors 8, 9, A, B, C, D, E, F (in hexadecimal) in response to hardware interrupts IRQ0 to IRQ7 being received by the 8259. interrupt 8 (timer tick), 9 (keyboard), E (floppy drive) pointed to routines in the IBM PC ROM BIOS to process the corresponding hardware interrupts.

Firmware

The firmware of the IBM PC was housed in 5 chips on the motherboard, each with 8 KiB of ROM memory. The first of these, which resided in the highest part of the memory addressable by the CPU, was the IBM PC ROM BIOS, where the BIOS routines, the Power On Self Test, the Boot Strap Loader, and some configuration tables were located. data. The other four ROM chips contained the IBM PC ROM BASIC. There was a free sixth socket on the motherboard so that the user could place an 8 KiB ROM chip with custom programs.

Additionally, each card that was plugged into the system could have its own controller ROM, which were BIOS extensions.

ROM BIOS

Dinastías del norte

The BIOS had basic I/O functionality for devices such as the keyboard, display, printer, floppy drives, cassette recorder, serial interfaces, and others, and responded to interrupts generated by hardware, such as the timer tick, 18.2 times per second, the keyboard, when you press or release a key, and the floppy drive controller.

Soon the IBM ROM BIOS was cloned using reverse engineering and the clean room method, allowing other companies to make clones of the IBM PC thus giving rise to the clone market.

In later versions of personal computers, more functionality was added to the original IBM PC BIOS.

ROM BASIC

By the time the IBM PC came out (1981), home computers of the day typically had a BASIC interpreter in their ROM memory, usually written and licensed by Microsoft to microcomputer manufacturers. The BASIC interpreter screen waiting for commands was what users usually found when turning on the machine, and they generally used a cassette recorder as a storage medium for programs and data, and also to load the programs that were frequently marketed on that medium.. Then floppy drives appeared as a storage medium, and with them a rudimentary disk operating system (DOS), but it was a very expensive alternative for the typical home user.

The IBM PC had, like the other microcomputers of the time, an interpreter of the BASIC programming language, in 4 ROMs of 8 KiB, and also had a port to connect a cassette recorder as a storage medium. BASIC in ROM was activated if there was no floppy in the drive or there was a failure to read the first sector of the floppy, or there was no floppy drive.

The ROM BASIC had almost all of the computer's RAM at its disposal since the operating system had not been loaded from the floppy disk. It could read and write files and programs on the cassette recorder, but it did not have the ability to handle files on floppy disks, making it very limited. Also, it was the basis of the BASIC shells that came with PC DOS, BASIC and BASICA. These couldn't work without the BASIC in the IBM PC ROM.

The clones that soon appeared, had a cloned BIOS but did not have the BASIC in ROM, so Microsoft offered them MS DOS, which was a version of PC DOS that could work on any computer, and it had an interpreter BASIC called GW-BASIC, which was a version of BASIC that did not need the BASIC ROM that only original IBM computers had.

Software

The IBM PC came with 8 KiB of ROM for its BIOS, the IBM PC ROM BIOS, and 40 KiB of ROM (on 4 x 8 KiB chips) for its BASIC interpreter, the IBM Cassette BASIC.

Three versions of the BASIC language were available for the IBM PC (Cassette, Disk and Advanced), the disk operating system (DOS), PC DOS, a Pascal compiler, the CP/M-86 operating system, the UCSD p-System, the General Ledger, Accounts Receivable and programs Accounts Payable by Peachtree Software, Inc; EasyWriter word processor, VisiCalc spreadsheet, Microsoft Adventure (a version of the Colossal Cave Adventure program), a communications program.

The PC DOS operating system was necessary to operate floppy drives and to work with the computer. A BASIC compiler, BASCOM (BASic COMpiler), was later available to speed up interpreted BASIC, as was a macro assembler, and a Fortran compiler. Later, when the IBM PCjr was developed, another version of BASIC called Cartridge BASIC became available as an expansion cartridge, but only for that machine.

Versions for the IBM PC were made of programs that existed for other microcomputers. All kinds of programs were created for the IBM PC.

Manuals and documentation

The computer came with several manuals in three-ring binders distinguished by their distinctive colors:

• IBM Guide to Operationsred wine. It brought general information about how to operate and use the computer, the operating system DOS, and the hardware (expansion cards, printer, monitor, system drive, etc.). The user was explained how to assemble the computer by removing it from the box and how to connect the components. There was a small installation and configuration manual for each purchased component. The manual had information to diagnose problems and in the end there was a plastic envelope with a diagnostic diskette and the use of the diagnostic disk was detailed.
• IBM BASIC, olive green. He had technical information from the three versions of the BASIC programming language of IBM, Cassette BASIC, Disk BASIC and Advanced BASIC. There was information about how to use the BASIC interpreter, general information about BASIC language; the use of the line editor to write, edit, list the program, etc; a description of the use of the variables and of all the functions, sentences and commands of the language, error messages, etc.
• IBM DOScreamy. He came by to buy the DOS. He had information about the use of DOS, about each DOS command for users, as well as technical information of the internal operation for programmers. In a plastic envelope, at the end of the manual, there was a 5.25" diskette with the PC DOS. In later editions of the DOS, information for the user of technical programming information was separated, creating different manuals.

In addition, other manuals and software were offered in the same format, such as the cream-colored CP/M-86 operating system, the blue IBM Hardware Maintenance and Service, and the IBM PC Technical Reference Manual in dark bluish gray color, highlighting the IBM PC Technical Reference Manual for its very detailed content on the design and operation of the IBM PC and for its impact on the computer industry staff.

IBM PC Technical Reference Manual

IBM offered the IBM PC Technical Reference Manual. The manual explained in great detail how the computer works, the motherboard, its expansion cards (CGA and MDA video cards, game cards, serial and parallel, floppy disk controller), the monitor, the printer, the floppy disk drives, and each of the integrated circuits that made the system work. It included very detailed hardware specifications, diagrams of each of the computer's components, technical specifications, programming guides, and a complete listing of the IBM PC ROM BIOS.

The IBM PC technical manual was followed by corresponding technical manuals for the IBM XT, IBM PCjr, and IBM AT, as these machines appeared on the market.

These technical manuals, with such detailed and abundant engineering information, were of paramount importance in the rise of the personal computer industry: In the manufacture of expansion cards, components, and IBM PC compatible clone computers and following IBM personal computers.

Thanks to the technical information provided by IBM in this manual, just a few weeks after the launch of the IBM PC there were already compatible cards and accessories for the IBM personal computer, the following year the first clone appeared, Columbia Data Products, and in a very short time an enormous ecosystem of products was created around personal computers (compatible with IBM). Today's personal computer industry was born, and is as it is, thanks to the IBM PC and its IBM PC Technical Reference Manual.

IBM PC and its successors

The original PC had an Intel 8088 processor running at 4.77 MHz, and there was a ROM version of BASIC (the IBM Cassette BASIC). IBM sold the PC in configurations with between 16 KiB and 64 KiB of RAM pre-installed on the motherboard. the maximum memory on the motherboard was 64 KiB and three 64 KiB IBM memory cards could be added to it for a total of 256 KiB, and with third party memory cards it could go up to 576 KiB for a total of 640 KiB. A monochrome card and a color graphics card were available for video output, the color video card could use a standard TV as a display. It had a port for connecting a cassette recorder as a storage device. A floppy drive was available as an extra option that most users purchased; the hard drive was not available. It had five expansion slots. IBM offered an asynchronous communications card, a parallel port card, a game card, 32 and 64 KiB memory cards, a monitor, and a printer. The original PC failed miserably in the home market, but was widely used in businesses.

The IBM PC XT was released on March 8, 1983. The processor was a 4.77 MHz Intel 8088 and the 8-bit ISA expansion bus with XT bus architecture, which was identical to the IBM PC with some modification. It was an improved machine, designed for business use. It had 8 expansion slots and a 10 megabyte hard drive (ST-412). It could have up to 256 KiB of memory on the main card. It was sold with a monochrome MDA card.

The IBM PCjr was announced on November 1, 1983 and released in late January 1984. It was an attempt by IBM to break into the home computer market. It had a 4.77 MHz Intel 8088 CPU, 128 KiB of RAM, an infrared wireless keyboard, and slots for expansion cartridges. It was a failure due to several design and implementation decisions.

The IBM AT was released in 1984. It used an Intel 80286 processor, originally at 6 MHz and later at 8 MHz. It had a 16-bit ISA bus and a 20 MB hard drive. IBM made a few attempts at the market with a multi-user machine, but mainly sold it as a faster PC for users.

The IBM PC Portable 5155 model 68 was one of the first portable computers, developed by IBM after the success of the Compaq Portable. It was released in February 1984 and succeeded by the IBM PC Convertible.

The IBM PC Convertible (IBM PC 5140 or also known as IBM Convertible) was IBM's first portable computer, introduced on April 3, 1986. It stands out for being the first true IBM portable computer, since it had the capacity to manage its energy consumption and battery use. It was also the first IBM to use 3.5' floppy drives.

Then the second generation PC models appeared. Personal System/2 (PS/2), are known by their model numbers: Model 25, Model 30. Within each series, models are also usually referenced by their CPU clock speed.

However, IBM continued to produce computers compatible with its early PCs, this time incorporating the technological advances of its competitors that were already common in most PCs, under brands such as IBM ThinkVision, IBM ThinkPad, IBM ThinkVantage, IBM ThinkCentre, IBM Aptiva and IBM NetVista.

All IBM personal computers are generally software compatible, but not all programs will work on all machines. Some programs are time sensitive for a particular velocity class. Older programs won't take advantage of new features like high-resolution display standards or extended processing instructions.