Editing IBM Personal Computer AT (section)

== Features ==
The AT is [[IBM PC compatible]], with the most significant difference being a move to the 80286 processor from the 8088 processor of prior models. Like the IBM PC, the AT supported an optional math co-processor chip, the [[x87#80287|Intel 80287]], for faster execution of [[Floating-point arithmetic|floating point operations]].

In addition, it introduced the [[AT bus]], later known as the ISA bus, a 16-bit bus with backward compatibility with 8-bit PC-compatible expansion cards. The bus also offered fifteen [[Interrupt request (PC architecture)|IRQs]] and seven [[Direct memory access|DMA]] channels, expanded from eight IRQs and four DMA channels for the PC, achieved by adding another [[8259A]] IRQ controller and another [[8237A]] DMA controller.<ref>{{cite magazine |last=Derfler |first=Frank J. Jr. |url=https://books.google.com/books?id=-Xr7Ic-ivyMC&pg=PT53|title=Tutor |magazine = PC Magazine |date=December 26, 1989|publisher=Ziff Davis |page=53 |issn=0888-8507}}</ref><ref>{{cite book|first=N. |last=Mathivanan |title=PC-Based Instrumentation: Concepts and Practice |url=https://books.google.com/books?id=OB65rMNQDo8C&pg=PA227 |year=2007 |publisher=PHI Learning |isbn=978-81-203-3076-4 |pages=227–229}}</ref> Some IRQ and DMA channels are used by the motherboard and not exposed on the expansion bus.  Both dual IRQ and DMA chipsets are cascading which shares the primary pair. In addition to these chipsets, Intel 82284 Clock Driver and Ready Interface and Intel 82288 Bus Controller are to support the microprocessor.

The 24-bit address bus of the 286 expands RAM capacity to 16&nbsp;[[Megabyte|MB]].

[[PC DOS]] 3.0 was included with support for the new AT features, including preliminary kernel support for networking (which was fully supported in a later version 3.x release).

The motherboard includes a battery-backed [[real-time clock]] (RTC) using the Motorola MC146818.<ref name="Horan2013">{{cite book|author=Brendan Horan|url=https://books.google.com/books?id=PSonZP4Nj5sC&pg=PA146|title=Practical Raspberry Pi|date=March 26, 2013|publisher=Apress|isbn=978-1-4302-4972-6|page=146}}</ref><ref name="ShanleyAnderson1995">{{cite book|author-last1=Shanley|author-first1=Tom|url=https://archive.org/details/ISA_System_Architecture|title=ISA System Architecture|author-last2=Anderson|author-first2=Don|date=1995|publisher=Mindshare, Inc. / [[Addison-Wesley Publishing Company]]|isbn=0-201-40996-8|editor-last=Swindle|editor-first=John|edition=3|pages=[https://archive.org/details/ISA_System_Architecture/page/n451 441]–444}}</ref> This was an improvement from the PC, which required setting the clock manually or installing an RTC expansion card. The RTC also included a 1024&nbsp;Hz timer (on IRQ 8), a much finer resolution than the 18&nbsp;Hz timer on the PC.<ref name="Austerlitz2002">{{cite book|author=Howard Austerlitz|url=https://books.google.com/books?id=gK4DxMaqmYYC&pg=PA90|title=Data Acquisition Techniques Using PCs|publisher=Academic Press|year=2002|isbn=978-0-08-053025-3|pages=90–91}}</ref>

In addition to keeping the time, the RTC includes 50 bytes of [[CMOS memory]] which is used to store software-adjustable [[BIOS]] parameters. A disk-based BIOS setup program which saved to this memory took the place of the [[DIP switch]]es used to set system settings on PCs. Most AT clones have the setup program in [[Read-only memory|ROM]] rather than on disk.

=== Storage ===
The standard floppy drive was upgraded to a 1.2&nbsp;MB {{frac|5|1|4}}&nbsp;inch [[floppy disk drive]] (15 [[Disk sector|sectors]] of 512 bytes, 80 tracks, two sides), which stored over three times as much data as the 360 KB PC floppy disk, but had compatibility problems with 360k disks (see [[#Problems|Problems]] below). {{frac|3|1|2}}&nbsp;inch floppy drives became available in later ATs.{{fact|date=January 2023}}

A 20&nbsp;MB [[hard disk drive]] was included as standard. Early drives were manufactured by [[Computer Memories Inc.|Computer Memories]] and were found to be very unreliable.<ref name="dickinson19850625">{{cite news|author=Dickinson, John|date=June 25, 1985|title=The AT's Slipped Disk|pages=55|work=PC Magazine|url=https://books.google.com/books?id=uqInkwnBdacC&pg=PA55|access-date=October 28, 2013}}</ref>

=== Peripherals ===
The AT included the [[AT keyboard]], initially a new 84-key layout (the 84th key being [[System request|SysRq]]). The numerical keypad was now clearly separated from the main key group, and indicator [[light-emitting diode|LEDs]] were added for Caps Lock, Scroll Lock and Num Lock. The AT keyboard uses the same 5-pin [[DIN connector]] as the PC keyboard, but a different, bidirectional electrical interface with different keyboard [[scan code]]s. The bidirectional interface allows the computer to set the LED indicators on the keyboard, reset the keyboard, set the typematic rate, and other features. Later ATs included 101-key keyboards, e.g. the [[Model M keyboard]].

The AT is also equipped with a [[computer lock|physical lock]] that prevents access to the computer by disabling the keyboard and holding the system unit's cover in place.

ATs could be equipped with [[Color Graphics Adapter|CGA]], [[IBM Monochrome Display Adapter|MDA]], [[Enhanced Graphics Adapter|EGA]], or [[Professional Graphics Controller|PGA]] video cards.

The [[8250 UART]] from the PC was upgraded to the [[Universal asynchronous receiver/transmitter#UART models|16450]], but since both chips had single-byte buffers, high-speed serial communication was problematic as with the XT.<ref name="NickallsRamasubramanian1995">{{cite book|first1=Richard W. D. |last1=Nickalls |first2=R. |last2=Ramasubramanian |title=Interfacing the IBM-PC to Medical Equipment: The Art of Serial Communication |url=https://archive.org/details/interfacingibmpc0000nick |url-access=registration |year=1995 |publisher=Cambridge University Press |isbn=978-0-521-46280-8|page=[https://archive.org/details/interfacingibmpc0000nick/page/25 25]}}</ref>

=== Models ===
{{Main|List of IBM Personal Computer models}}
{| class="wikitable sortable sticky-header sort-under" 
|- 
|+ IBM Personal Computer models
! Type
! IBM {{abbr|P/N|part number}}
! Date announced
! Date withdrawn
! Bus
! {{abbr|No.|Number}} of<br />slots
! {{abbr|No.|Number}} of<br />bays
! Processor          
! Clock<br />speed<br />{{small|(MHz)}}
! Stock onboard RAM<br />{{small|(KB)}}
! Maximum onboard RAM<br />{{small|(KB)}}
! {{abbr|FDD|Floppy disk drive}}
! {{abbr|HDD|Hard disk drive}}
! Notes
! Ref(s).
|-
| AT
| 5170-068
| {{date table sorting|1984|08|14|format=my}}
| {{date table sorting|1987|06|30|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 6
| 256&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| none
|  
| <ref name=ibmref40>{{cite book | date=April 6, 1987 | url=https://bitsavers.org/pdf/ibm/pc/PS2_and_IBM_PC_Product_Reference_Version_4.0_Apr87.pdf#page=244 | title=IBM Personal System/2 and IBM Personal Computer Product Reference | edition=4.0 | publisher=International Business Machines Corporation | via=[[Bitsavers]]}}</ref>{{rp|49}}<ref name=pcweekatannounced>{{cite journal | last=Whitmore | first=Sam | date=August 14, 1984 | url=https://link.gale.com/apps/doc/A573108/GPS?sid=wikipedia | title=IBM's New Generation: The PC AT: AT Hikes CPU Speed Sixfold, Memory Fivefold | journal=PC Week | publisher=Ziff-Davis | volume=1 | issue=32 | page=1 | via=Gale | quote=The IBM PC AT Base Model 68, with one 1.2-megabyte floppy-disk drive, will sell for $3,995, according to a source close to IBM. Model 99, with a 1.2-megabyte floppy-disk drive and a 20-megabyte hard disk, will cost $5,795. Both models will use a 6-MHz 80286 microprocessor.}}</ref><ref name=atdiscontinued>{{cite journal | last=Abruzzese | first=George | date=June 29, 1987 | url=https://books.google.com/books?id=xjAEAAAAMBAJ&pg=PA1 | title=IBM Plans to Terminate Remainder of PC Line | journal=InfoWorld | publisher=IDG Publications | volume=9 | issue=26 | pages=1, 69 | via=Google Books}}</ref>
|-
| AT
| 5170-099
| {{date table sorting|1984|08|14|format=my}}
| {{date table sorting|1987|06|30|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 6
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| 20&nbsp;MB
|  
| <ref name=ibmref40 />{{rp|236}}<ref name=pcweekatannounced /><ref name=atdiscontinued />
|-
| AT
| 5170-239
| {{date table sorting|1985|10|01|format=my}}
| {{date table sorting|1986|09|02|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 6
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| 30&nbsp;MB
|  
| <ref name=ibmref40 />{{rp|236}}<ref name=at30mb>{{cite journal | last=Kuzbyt | first=Bob | date=October 7, 1985 | url=https://link.gale.com/apps/doc/A3972202/GPS?sid=wikipedia | title=IBM introduces 30-MB 5.25-inch Winchester disk drive for AT | journal=Electronic News | publisher= Sage Publications | volume=31 | page=26 | via=Gale}}</ref><ref name=Ristelhueber>{{cite journal | last=Ristelhueber | first=Robert | author2=Ira Sager | date=September 8, 1986 | url=https://link.gale.com/apps/doc/A4398861/GPS?sid=wikipedia | title=4 PC suppliers add 286 units in battle with compatibles | journal=Electronic News | publisher=Sage Publications | volume=32 | pages=1 ''et seq'' | via=Gale}}</ref>
|-
| AT
| 5170-839
| {{date table sorting|1986|01|08|format=my}}
| {{date table sorting|1987|06|30|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 6
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| two&nbsp;30&nbsp;MB
| Includes controllers for the [[IBM 4680|IBM 4680 Store System]]
| <ref name=ibmref40 />{{rp|236}}<ref name=at4680>{{cite journal | last=Whitmore | first=Sam | date=January 14, 1986 | url=https://link.gale.com/apps/doc/A656864/GPS?sid=wikipedia | title=IBM Debuts PC AT-Based Point-of-Sale Retail System | journal=PC Week | publisher=Ziff-Davis | volume=3 | issue=2 | page=8 | via=Gale}}</ref><ref name=atdiscontinued />
|-
| AT
| 5170-899
| {{date table sorting|1986|01|08|format=my}}
| {{date table sorting|1987|06|30|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 6
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| two&nbsp;20&nbsp;MB
| Includes controllers for the [[IBM 4680|IBM 4680 Store System]]
| <ref name=ibmref40 />{{rp|236}}<ref name=at4680 /><ref name=atdiscontinued />
|-
| AT
| 5170-319
| {{date table sorting|1986|04|02|format=my}}
| {{date table sorting|1987|06|30|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 8
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| 30&nbsp;MB
|  
| <ref name=ibmref40 />{{rp|236}}<ref name=at8mhz>{{cite journal | last=Whitmore | first=Sam | date=April 1, 1986 | url=https://link.gale.com/apps/doc/A666974/GPS?sid=wikipedia | title=IBM meetings said to key on new products | journal=PC Week | publisher=Ziff-Davis | volume=3 | issue=13 | page=1 | via=Gale}}</ref><ref name=atdiscontinued />
|-
| AT
| 5170-339
| {{date table sorting|1986|04|02|format=my}}
| {{date table sorting|1987|07|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 8
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| 30&nbsp;MB
| Shipped with Enhanced Keyboard
| <ref name=ibmref40 />{{rp|236}}<ref name=at8mhz /><ref name=atdiscontinued />
|-
| AT
| 5171-168
| {{date table sorting|1986|10|21|format=my}}
| {{date table sorting|1987|06|30|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 8
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| none
| Built to [[Tempest (codename)|TEMPEST]] specifications
| <ref name=ibmref40 />{{rp|64}}<ref name=atdiscontinued />
|-
| AT
| 5171-339
| {{date table sorting|1986|10|21|format=my}}
| {{date table sorting|1987|07|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 8
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| 30&nbsp;MB
| Built to [[Tempest (codename)|TEMPEST]] specifications
| <ref name=ibmref40 />{{rp|64}}<ref name=atdiscontinued />
|-
| [[IBM Personal Computer AT/370|AT/370]]
| 5170-599
| {{date table sorting|1984|10|25|format=my}}
| {{date table sorting|1987|04|02|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 6
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| 20&nbsp;MB
| 
| <ref name=ibmref40 />{{rp|202}}<ref name=givesup>{{cite journal | date=April 13, 1987 | url=https://techmonitor.ai/technology/ibm_gives_up_on_the_personal_xt_at370 | title=IBM Gives Up on the Personal XT/, AT/370 | journal=Computer Business Review | publisher=New Statesman Media Group | archiveurl=https://archive.today/20230920021426/https://techmonitor.ai/technology/ibm_gives_up_on_the_personal_xt_at370 | archivedate=September 20, 2023}}</ref>
|-
| [[IBM Personal Computer AT/370|AT/370]]
| 5170-739
| {{date table sorting|1985|10|01|format=my}}
| {{date table sorting|1987|04|02|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 6
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| 30&nbsp;MB
|  
| <ref name=ibmref40 />{{rp|202}}<ref name=sandker>{{cite journal | last=Sandker | first=C. | date=October 8, 1985 | url=https://link.gale.com/apps/doc/A649986/GPS?sid=wikipedia | title=IBM Delivers New PC AT Featuring 30M-Byte Disk: Sets a Price Tag of $5,995 | journal=PC Week | publisher=Ziff-Davis | volume=2 | issue=40 | page=1 | via=Gale}}</ref><ref name=pctjv4n13>{{cite journal | date=1987 | url=https://archive.org/details/PC_Tech_Journal_vol04_n13/page/69/ | title=Desktop Systems | journal=PC Tech Journal | publisher=Ziff-Davis | volume=4 | issue=13 | page=69 | via=the Internet Archive}}</ref><ref name=givesup />
|-
| [[IBM Personal Computer AT/370|AT/370]]
| 5170-919
| {{date table sorting|1986|04|02|format=my}}
| {{date table sorting|1987|04|02|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 6
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| 30&nbsp;MB
| 1&nbsp;MB total RAM (peripheral)
| <ref name=ibmref40 />{{rp|57}}<ref name=prnewswireat370>{{cite web | last=Kennedy | first=Don | date=April 2, 1986 | url=https://link.gale.com/apps/doc/A4196480/GPS?sid=wikipedia | title=IBM has boosted the processor speed and storage capacity of its personal computer models | publisher=PR Newswire | page=NYPR14C | via=Gale}}</ref><ref name=givesup />
|-
| [[IBM Personal Computer AT/370|AT/370]]
| 5170-939
| {{date table sorting|1986|04|02|format=my}}
| {{date table sorting|1987|04|02|format=my}}
| [[Industry Standard Architecture|ISA]], 16-bit
| 8
| 3
| [[Intel 80286]]
| 6
| 512&nbsp;KB
| 512&nbsp;KB
| 1.2&nbsp;MB
| 30&nbsp;MB
| 1&nbsp;MB total RAM (peripheral); shipped with Enhanced Keyboard
| <ref name=ibmref40 />{{rp|57}}<ref name=prnewswireat370/><ref name=givesup />
|}

=== Power supply ===
The IBM PC AT came with a 192-watt [[switching power supply]], significantly higher than the 130-watt XT power supply.

According to IBM's documentation, in order to function properly, the AT power supply needed a load of at least 7.0 amperes on the +5&nbsp;V line and a minimum of 2.5 amperes on its +12&nbsp;V line. The power supply would fail to start unless these minimum load requirements were met, but the AT motherboard did not provide much load on the +12&nbsp;V line. To solve this problem, entry-level IBM AT models that did not have a hard drive were shipped with a 5-ohm, 50-watt resistor connected on the +12&nbsp;V line of the hard disk power connector. In normal operation this resistor drew 2.4 amperes (dissipating 28.8 watts), getting fairly hot.<ref name="Mueller2011">{{cite book|author=Scott M. Mueller|title=Upgrading and Repairing PCs|year=2011|publisher=Que Publishing|isbn=978-0-13-268218-3|page=882|edition=20th}}</ref>