Editing IBM Personal Computer (section)

== Hardware ==
[[File:Ibm 5150 inthebox edit.jpg|thumb|Internal view of the IBM PC (from the back), showing components and layout. This PC has been outfitted with aftermarket floppy and hard disk drives, but the motherboard and most of the expansion cards are stock.|alt=]]
For low cost and a quick design turnaround time, the hardware design of the IBM PC used entirely "off-the-shelf" parts from third party manufacturers, rather than unique hardware designed by IBM.<ref>{{Cite book|last=Henderson|first=Harry|url=https://books.google.com/books?id=3Tla6d153uwC&q=ibm%20pc%20open%20standards&pg=PA236|title=Encyclopedia of Computer Science and Technology|date=2009|publisher=Infobase Publishing|isbn=978-1-4381-1003-5|pages=236|language=en}}</ref>

The PC is housed in a wide, short steel chassis intended to support the weight of a CRT monitor. The front panel is made of plastic, with an opening where one or two disk drives can be installed. The back panel houses a power inlet and switch, a keyboard connector, a cassette connector and a series of tall vertical slots with blank metal panels which can be removed in order to install expansion cards.

Internally, the chassis is dominated by a motherboard which houses the CPU, built-in RAM, expansion RAM sockets, and slots for expansion cards.

The IBM PC was highly expandable and upgradeable, but the base factory configuration included:
{| class="wikitable"
! {{rh}} |CPU
|[[Intel 8088]] @ 4.77&nbsp;MHz
|-
! {{rh}} |RAM
|16 KB or 64&nbsp;KB minimum (expandable to 640&nbsp;KB)
|-
! {{rh}} |Video
|[[IBM Monochrome Display Adapter]] or<br>[[Color Graphics Adapter|IBM Color Graphics Adapter]]
|-
! {{rh}} |Display
|[[IBM 5151|IBM 5151 monochrome display]]<br>IBM 5153 color display<br>[[Composite video|Composite]]-input television
|-
! {{rh}} |Input
|[[Model F keyboard|IBM Model F]] 83-key keyboard with five-pin connector
|-
! {{rh}} |Sound
|Single programmable-frequency square wave with built-in [[PC speaker|speaker]]
|-
! {{rh}} |Storage
|Up to two internal 5.25-inch single- or double-sided full-height [[Floppy disk|floppy disk drives]]<ref>Drive capacities varied: Early drives were single-side/double-density (SSDD) and stored 160&nbsp;KB per floppy, but were soon replaced by double-side/double-density (DSDD) drives which stored 320&nbsp;KB. The release of DOS 2.0 increased support for storing 9 rather than 8 sectors/track, increasing capacities to 180&nbsp;KB (SSDD) and 360&nbsp;KB (DSDD) per floppy. More drives could be attached externally, with certain caveats.</ref><br>Port for attaching to [[magnetic-tape data storage|cassette tape recorder]]<br>Initially no hard drive option, but see text
|-
! {{rh}} |Expansion
|Five 62-pin [[Industry Standard Architecture|expansion slots]] attached to 8-bit CPU I/O bus<br>IBM 5161 Expansion Chassis with eight (seven usable) extra I/O slots
|-
! {{rh}} |Communication
|Optional [[serial port|serial]] and [[parallel port|parallel]] ports
|}

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

The peripheral chips included an [[Intel 8259]] [[Programmable interrupt controller|PIC]], an [[Intel 8237]] [[Direct memory access|DMA]] controller, and an [[Intel 8253]] [[Programmable interval timer|PIT]]. The PIT provides {{nowrap|18.2 Hz}} clock "ticks" and dynamic memory refresh timing.

=== CPU and RAM ===
[[File:IBM PC Motherboard (1981).jpg|thumb|Original IBM PC motherboard with 16&nbsp;KB RAM soldered and 48&nbsp;KB socketed, for a total of 64&nbsp;KB onboard|alt=]]
[[File:Ibm5150mobo1984black.jpg|thumb|Later IBM PC motherboard with 64&nbsp;KB RAM soldered and 192&nbsp;KB socketed, for a total of 256&nbsp;KB onboard|alt=]]
The CPU is an [[Intel 8088]], a cost-reduced form of the [[Intel 8086]] which largely retains the 8086's internal 16-bit logic, but exposes only an 8-bit bus.<ref name=":0">{{Cite book|url=https://archive.org/stream/byte-magazine-1981-10/1981_10_BYTE_06-10_Local_Networks#page/n31/mode/2up|title=Byte Magazine Volume 06 Number 10 - Local Networks|date=October 1981|pages=28–34}}</ref> The CPU is clocked at 4.77&nbsp;MHz; clones and later PC models have higher CPU speeds that break compatibility with software developed for the original PC.<ref>{{Cite book|last=Graves|first=Michael W.|url=https://books.google.com/books?id=GZFv0Vv8CEcC&q=%22turbo%20button%22%20crash&pg=PA66|title=A+ Guide to PC Hardware Maintenance and Repair|date=September 17, 2004|publisher=Cengage Learning|isbn=978-1-4018-5230-6|language=en}}</ref> The single base clock frequency for the system is 14.31818&nbsp;MHz, which when divided by 3, yielded the 4.77&nbsp;MHz for the CPU (which was considered close enough to the then 5&nbsp;MHz limit of the 8088), and when divided by 4, yields the required 3.579545&nbsp;MHz for the [[NTSC]] color carrier frequency.

The PC motherboard includes a second, empty socket, described by IBM simply as an "auxiliary processor socket", although the most obvious use was the addition of an [[Intel 8087]] math coprocessor, which improves [[Floating-point arithmetic|floating-point math]] performance.<ref>{{Cite book|url=https://archive.org/stream/byte-magazine-1982-01/1982_01_BYTE_07-01_The_IBM_Personal_Computer#page/n41/mode/2up|title=Byte Magazine Volume 07 Number 01 - The IBM Personal Computer|date=January 1982}}</ref>

PC mainboards were manufactured with the first [[memory bank]] of initially [[Mostek]] 4116-compatible,<ref>[https://minuszerodegrees.net/memory/4116.htm 4116 class RAM] One bank used nine of these chips (eight 16-[[kilobit]] chips = 16 kilobytes, plus one chip for parity).</ref> or later 4164-compatible<ref>[https://minuszerodegrees.net/memory/4164.htm 4164 class RAM] – One bank used nine of these chips (eight 64-kilobit chips = 64 kilobytes, plus one chip for parity).</ref> [[Dual_in-line_package|DIP]] [[DRAM]]s soldered to the board,<ref>{{cite web | url=https://minuszerodegrees.net/5150/ram/5150_ram.htm | title=Minuszerodegrees.net }}</ref> for a minimum configuration of first just 16 KB, or later 64 KB of RAM. Memory upgrades from IBM and third parties provide socketed installation in three further onboard banks, and as [[Industry Standard Architecture|ISA]] expansion cards.

The first 400,000 16&nbsp;KB mainboards ("16KB-64KB" ID) sold until March 1983 can be upgraded to a maximum of 64&nbsp;KB onboard without using slots, and the later 64&nbsp;KB revision ("64KB-256KB" ID) to a maximum of 256&nbsp;KB on the motherboard.<ref name="feldmann19851001">{{Cite magazine |last=Feldmann |first=Peter |date=1985-10-01 |title=Upgrading Your 64K PC |url=https://archive.org/details/PC-Mag-1985-10-01/page/n146/mode/1up?view=theater |access-date=2025-04-13 |magazine=PC |pages=145-149}}</ref> [[RAM card]]s can upgrade either variant further,<ref name=":0" /> for a total of 640 KB [[conventional memory]], and possibly several megabytes of [[expanded memory]] beyond that, though on PC/XT-class machines, the latter was a very expensive [[Expanded memory#Expansion boards|third-party hardware option]] only available later in the IBM 5150's [[product lifecycle|lifecycle]] and only usable with dedicated software support (i.e. only accessible via a RAM window in the [[Upper Memory Area]]<ref>{{cite web|url=https://minuszerodegrees.net/5150/misc/5150%20-%20Memory%20Map%20of%20the%20640%20KB%20to%201%20MB%20Area.jpg|title=IBM 5150 - Memory Map of the 640 KB to 1 MB Area (the 'Upper Memory Area')|website=minuszerodegrees.net|archive-url=https://web.archive.org/web/20250409172823/https://minuszerodegrees.net/5150/misc/5150%20-%20Memory%20Map%20of%20the%20640%20KB%20to%201%20MB%20Area.jpg|archive-date=9 April 2025|url-status=live}}</ref>); this was relatively rarely equipped and utilized on the original IBM PC, much less fully so, thus the machine's maximum RAM configuration as commonly understood is 640&nbsp;KB.

=== ROM BIOS ===
The BIOS is the firmware of the IBM PC, occupying one 8&nbsp;KB chip on the motherboard. It provides bootstrap code and a library of common functions that all software can use for many purposes, such as video output, keyboard input, disk access, interrupt handling, testing memory, and other functions.

IBM shipped three versions of the BIOS throughout the PC's lifespan, with the dates 04/24/81, 10/19/81, and 10/27/82 (the first to boot from hard disk). The company offered an upgrade kit.{{r|feldmann19851001}}

=== Display ===
[[File:IBM PC XT color.jpg|thumb|IBM PC with MDA monitor]]
While most home computers had built-in video output hardware, IBM took the unusual approach of offering two different graphics options, the [[IBM Monochrome Display Adapter|MDA]] and [[Color Graphics Adapter|CGA]] cards. The former provided high-resolution monochrome text, but could not display anything except text, while the latter provided medium- and low-resolution color graphics and text.

CGA used the same scan rate as [[NTSC|NTSC television]], allowing it to provide a composite video output which could be used with any compatible television or [[composite monitor]], as well as a direct-drive TTL output suitable for use with any [[RGBI interface|RGBI]] monitor using an NTSC scan rate. IBM also sold the 5153 color monitor for this purpose, but it was not available at release<ref name="williams1982012">{{cite news|author=Williams, Gregg|date=January 1982|title=A Closer Look at the IBM Personal Computer|page=36|work=[[Byte (magazine)|Byte]]|url=https://archive.org/stream/byte-magazine-1982-01/1982_01_BYTE_07-01_The_IBM_Personal_Computer#page/n37/mode/2up|access-date=October 19, 2013}}</ref> and was not released until March 1983.<ref>{{cite web |publisher=IBM |type=Announcement letter |id=183-002 |title=IBM Color Display, 5153 Model 1 For IBM Personal Computer |date=February 4, 1983 |url=https://www.ibm.com/docs/en/announcements/archive/ENUS183-002}}</ref>

MDA scanned at a higher frequency and required a proprietary monitor, the [[IBM 5151]]. The card also included a built-in printer port.<ref name=":1">{{Cite book|url=https://archive.org/stream/byte-magazine-1982-01/1982_01_BYTE_07-01_The_IBM_Personal_Computer#page/n37/mode/2up|title=Byte Magazine Volume 07 Number 01 - The IBM Personal Computer|date=January 1982}}</ref>

Both cards could also be installed simultaneously for mixed graphics and text applications.<ref>{{Cite web|title=Dual-Head operation on vintage PCs|url=https://www.seasip.info/VintagePC/dualhead.html|access-date=August 16, 2020|website=www.seasip.info}}</ref> For instance, [[AutoCAD]], [[Lotus 1-2-3]] and other software allowed use of a CGA Monitor for graphics and a separate [[monochrome monitor]] for text menus. Third parties went on to provide an enormous variety of aftermarket graphics adapters, such as the [[Hercules Graphics Card]].

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

=== Storage ===
The two bays in the front of the machine could be populated with one or two 5.25″ floppy disk drives, storing 160&nbsp;KB per disk side for a total of 320&nbsp;KB of storage on one disk.<ref name=":1" /> The floppy drives require a controller card inserted in an expansion slot, and connect with a single ribbon cable with two edge connectors. The IBM floppy controller card provides an external 37-pin D-sub connector for attachment of an external disk drive, although IBM did not offer one for purchase until 1986.

As was common for home computers of the era, the IBM PC offered a port for connecting a [[IBM cassette tape|cassette data recorder]]. Unlike the typical home computer however, this was never a major avenue for software distribution,<ref>{{Cite book|title=The Peter Norton Programmer's Guide to the IBM PC|publisher=Microsoft Corporation|year=1985|isbn=0914845462|quote=I have never encountered a PC program on tape for sale. In fact, about the only use of the cassette port that I am aware of is the homespun and jerry-rigged use of this port as a poor-man's serial port.}}</ref> probably because very few PCs were sold without floppy drives. The port was removed on the very next PC model, the XT.<ref>{{Cite book|last=Robert|first=Brenner|title=IBM Personal Computer: Troubleshooting & Repair for the IBM PC, PC/XT, and PC AT|publisher=Sams|year=1989|isbn=0672226626|quote=Next to the keyboard connector is a 5-pin circular connector for cassette data input/output. This connection is not available on the XT or AT.}}</ref>

At release, IBM did not offer any hard disk drive option<ref name=":0" /> and adding one was difficult - the PC's stock power supply had inadequate power to run a hard drive, the motherboard did not support BIOS expansion ROMs which was needed to support a hard drive controller, and both PC DOS and the BIOS had no support for hard disks. After the XT was released, IBM altered the design of the 5150 to add most of these capabilities, except for the upgraded power supply. At this point adding a hard drive was possible, but required the purchase of the IBM 5161 Expansion Unit, which contained a dedicated power supply and included a hard drive.<ref>{{Cite web|title=minuszerodegrees.net|url=http://www.minuszerodegrees.net/5150/hdd/5150_hard_drive_support.htm|access-date=October 3, 2020|website=www.minuszerodegrees.net}}</ref>

Although official hard drive support did not exist, the third party market did provide early hard drives that connected to the floppy disk controller{{Citation needed|date=December 2022}}, but required a patched version of PC DOS to support the larger disk sizes.

=== Human interface <span class="anchor" id="Keyboard"></span> ===
[[File:IBM Model F XT.png|left|thumb|[[Model F keyboard|Model F]] keyboard]]
The only option for [[Human interface device|human interface]] provided in the base PC was the built-in keyboard port, meant to connect to the included [[Model F keyboard|Model F]] keyboard. The Model F was initially developed for the [[IBM System/23 Datamaster|IBM Datamaster]], and was substantially better than the keyboards provided with virtually all home computers on the market at that time in many regards - number of keys, reliability and ergonomics. While some home computers of the time utilized [[chiclet keyboard]]s or inexpensive mechanical designs, the IBM keyboard provided good ergonomics, reliable and positive tactile key mechanisms and flip-up feet to adjust its angle. Public reception of the keyboard was extremely positive, with some sources describing it as a major selling point of the PC and even as "the best keyboard available on any microcomputer."<ref name=":1" />
[[File:Bundesarchiv B 145 Bild-F077948-0006, Jugend-Computerschule mit IBM-PC.jpg|thumb|IBM PC with [[Color Graphics Adapter|CGA]] monitor (model 5153), [[Model F keyboard|original keyboard]], [[IBM 5152|5152]] printer and paper stand (1988)]]
At release, IBM provided a [[Game port|Game Control Adapter]] which offered a 15-pin port intended for the connection of up to two joysticks, each having two analog axes and two buttons. (The early PCs predated the advent of the "[[WIMP (computing)|Windows, Icons, Mouse, Pointer" concept]] and so did not have a mouse.)

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

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

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

=== Expansion ===
{{refimprove|section|date=August 2021}}
[[File:Personal_computer_-_Museo_scienza_tecnologia_Milano_14617_02.jpg|alt=|thumb|The back of a PC, showing the five expansion slots]]
The expansion capability of the IBM PC was very significant to its success in the market. Some publications highlighted IBM's uncharacteristic decision to publish complete, thorough specifications of the system bus and memory map immediately on release, with the intention of fostering a market of compatible third-party hardware and software.<ref>{{Cite book|url=https://archive.org/stream/byte-magazine-1982-01/1982_01_BYTE_07-01_The_IBM_Personal_Computer#page/n57/mode/2up|title=Byte Magazine Volume 07 Number 01 - The IBM Personal Computer|date=January 1982}}</ref>

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

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

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

* Graphics
* Sound
* [[Computer mouse|Mouse]] support
* Expanded memory
* [[Joystick]] port
* Additional serial or parallel ports
* Networking
* Connection to proprietary industrial or scientific equipment

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

==== 5161 Expansion Unit ====
The expandability of the PC was important, but had significant limitations.

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

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