IBM Personal Computer/AT

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IBM PC AT (System Unit 5170)
Type Personal Computer
Release date 1984[1]
Discontinued April 2, 1987[1]
Operating system PC DOS 3.0 and later, OS/2 1.x, PC/IX 1.1, IBM & SCO Xenix
CPU Intel 80286 @ 6 and 8 MHz
Memory 256 KB ~ 16 MB

The IBM Personal Computer AT, more commonly known as the IBM AT and also sometimes called the PC AT or PC/AT, was IBM's second-generation PC, designed around the 6 MHz Intel 80286 microprocessor and released in 1984 as System Unit 5170. The name AT stood for "Advanced Technology", and was chosen because the AT offered various technologies that were then new in personal computers; one such advancement was that the 80286 processor supported protected mode. IBM later released an 8 MHz version of the AT.

AT features[edit]

  • AT bus: The AT motherboard had a 16-bit data bus and 24-bit address bus (16 MB) that was backward compatible with PC-style expansion cards (which were 8-bit data, 20-bit address).
  • Fifteen IRQs and seven DMA channels, expanded from eight IRQs and four DMA channels for the PC (and XT). The doubling of the IRQs was achieved by adding another 8259A. IRQs 8–15 (from the second 8259A) are cascaded through IRQ 2 of the first 8259A, which leaves 15 available instead of 16.[2] Similarly, the number of DMA channels was increased by adding another 8237A, also in master-slave configuration. DMA channel 4 is reserved for cascading 0–3 leaving seven channels active.[3] Some IRQ and some DMA channels are used on the motherboard and not exposed on the expansion bus.
  • 16 MB maximum memory (because of the 24-bit address bus of the 286), compared to the PC's 640 KB maximum.
  • Battery backed real-time clock (RTC) on motherboard with 50 bytes CMOS memory available for power-off storage of BIOS parameters. (The basic PC had required either manual setting of its software clock using Time and Date commands, or the addition of an accessory expansion card with real-time clock, to avoid the default 01-01-80 file date.) Additionally the AT RTC had a 1024-Hz timer (on IRQ 8), which was a much finer resolution compared to the 18-Hz RTC used by IBM PC XT (IRQ 0). The AT timer was accessible via INT 70h.[4][2] The RTC was implemented using a Motorola MC146818 integrated circuit.[5][6]
  • A disk-based BIOS setup program took the place of the DIP switches on PCs and PC XTs. Most AT clones would have the setup program in ROM rather than on a disk.
  • 84-key AT keyboard layout: the 84th key being <SysRq> i.e. System Request; numerical keypad now clearly separated from main key group; also added indicator LEDs for Caps lock/Scroll lock/Num lock. The AT keyboard uses the same 5-pin DIN connector as the PC keyboard, but it uses a different, bidirectional interface (the PC and PC XT keyboard interface is unidirectional) and generates different keyboard scan codes. The bidirectional interface allows the computer to set the LED indicators on the keyboard, reset the keyboard, set the typematic rate, etc. Later ATs had 101-key keyboards which featured integrated numeric keypad with Num Lock key.
  • 1.2 MB 135mm (5-1/4 inch) floppy disk drive (15 sectors of 512 bytes, 80 tracks, two sides) stored over three times as much data as the 360 KB PC floppy disk (nine sectors of 512 bytes, 40 tracks, two sides). However, they had compatibility problems with 360k disks. 90mm (3.5") floppy drives became available in later ATs.
  • A 20 MB hard disk drive, although the early drives manufactured by Computer Memories were very unreliable.[7] This was attributed partly to failure to automatically retract the read/write heads when the computer was powered off, and partly to a bug in the DOS 3.0 FAT algorithm.[citation needed]
  • Early ATs (1984–1986) could be equipped with CGA, MDA, EGA, or PGA video cards.
  • The 8250 UART from the XT was upgraded to the 16450, although this chip still had only a one byte buffer, so high-speed serial communication was just as problematic as with the XT.[8]
  • PC DOS 3.0 was released to support the new AT features, including preliminary kernel support for networking (which was fully supported in a later version 3.x release.)
  • The AT was equipped with a physical lock that could be used to prevent access to the computer by disabling the keyboard.
  • Just like its IBM PC predecessor, the PC AT supported an optional math co-processor chip, the Intel 80287, for faster execution of floating point operations.

Power supply[edit]

The IBM PC AT came with a 192-watt switching 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 +5V line and a minimum of 2.5 amperes was on its +12V line. In practice, the AT power supply would randomly fail to start unless these minimum load requirements were met. Because the AT motherboard didn't provide much load on the +12V line, entry-level IBM AT models that didn't have a hard drive were shipped with a 5-ohm, 50-watt (maximum power) sandbar resistor connected on the +12V line of the hard disk power connector. In normal operation this resistor drew 2.4 amperes (28.8 watts), getting fairly hot.[9]


In addition to the unreliable hard disk drive,[10] the high-density floppy disk drives turned out to be problematic. Some ATs came with one high-density (HD) disk drive and one double-density (DD) 360 kB drive. High-density floppy diskette media were compatible only with high-density drives. There was no way for the disk drive to detect what kind of floppy disk was inserted, and the only clue the user had was the disk label and an asterisk molded into the 360 kB disk drive faceplate. If the user accidentally used a high-density diskette in the 360 kB drive, it would sometimes work, for a while, but the high-coercivity oxide would take a very weak magnetization from the 360 kB write heads, so reading the diskette would be problematic.

A different problem occurred when using a double-density diskette in the 1.2 MB drive; the high-density drive's heads had a track width half that of the 360 kB drive, so they were incapable of fully erasing and overwriting tracks written by a 360 kB drive. Therefore, overwriting a DD disk that had been written to in a DD drive with an HD drive would result in a disk perfectly readable on an HD drive, but producing many read errors in a DD drive. Whereas a HD read head would only pick up the half track that drive had written, the wider DD read head would pick up the half-track written by the HD drive mixed with the unerased half-track remnant of the track written earlier by a DD drive. So, the DD drive would end up reading both new and old information together, causing it to "see" garbled data.

The combination of the faster clock rate, fewer clock cycles per instruction, and the 16-bit bus led to a computer that was in the marketing sense too fast. IBM was protective of their lucrative mainframe and minicomputer businesses and consequently ran the original PC AT (139 version) at a very conservative 6 MHz with one wait state. They also used a three-to-one interleave on the hard disk, even though the controller supported two to one. Many customers replaced the 12 MHz crystal (which ran the processor at 6 MHz) with a 16 MHz crystal, so IBM introduced the PC AT 239 which would not boot the computer at any speed faster than 6 MHz, by adding a speed loop in the ROM. This also introduced the Baby AT motherboard form factor. The final PC AT, the 339, ran the processor at 8 MHz with one wait state, and was built as IBM's flagship microcomputer until the 1987 introduction of the PS/2 line.


IBM's efforts to trademark the name AT largely failed, and most 286-based PCs were modeled after it. The label also became a standard term in reference to PCs that used the same type of power supply, case, and motherboard layout as the 5170. Even further, "AT-class" became a term describing any machine which supported the BIOS functions, 16-bit expansion slots, keyboard interface, and other defining technical features of the IBM PC AT; in the case of the expansion slots, the term is largely synonymous with "ISA" (when the latter is not applied as a retronym to XT-class machines, as in the phrase "8-bit ISA slot".) As such, most systems with 486 and Pentium CPUs, and at least some with Pentium Pro and Pentium II processors, were describable as AT-class.

As of 2011, modern PCs still maintain nearly complete backwards compatibility with the PC AT from a software perspective, but AT mechanical and electrical compatibility is extremely rare. The AT power supply pins and its connectors, the AT motherboard form factor, and the physical ISA bus slots are no longer present on modern PCs outside of specialized embedded designs. The ATX standard from Intel has completely replaced the original AT power supply and motherboard design. Modern motherboards do not have ISA expansion bus connectors any more, but a functionally equivalent bus lives on as the modern LPC bus for software compatibility. Nearly all PC BIOS ROMs, even modern UEFI based ROMs, include code which is backwards compatible with the original AT BIOS interrupt calls. Even the 0xaa55 signature in the master boot record is still required by many BIOSes to be present on an attached hard disk for it to be recognized as a valid boot device. The PS/2 successor to the AT keyboard interface still survives in the modern market, though it is increasingly being replaced by USB in new systems. The PS/2 keyboard interface is identical to the AT keyboard interface except for the connector: the AT uses a 5-pin DIN connector, while the PS/2 uses a 6-pin mini-DIN.

See also[edit]


  1. ^ a b IBM PC AT at Vintage Computer
  2. ^ a b Ziff Davis, Inc. (26 December 1989). PC Mag. Ziff Davis, Inc. pp. 53–. ISSN 08888507. 
  3. ^ N. MATHIVANAN (2007). PC-BASED INSTRUMENTATION: CONCEPTS AND PRACTICE. PHI Learning Pvt. Ltd. pp. 227–229. ISBN 978-81-203-3076-4. 
  4. ^ Howard Austerlitz (2002). Data Acquisition Techniques Using PCs. Academic Press. pp. 90–91. ISBN 978-0-08-053025-3. 
  5. ^ Brendan Horan (26 March 2013). Practical Raspberry Pi. Apress. p. 146. ISBN 978-1-4302-4972-6. 
  6. ^ Tom Shanley; Don Anderson (1995). ISA System Architecture. Addison-Wesley Professional. pp. 441–444. ISBN 978-0-201-40996-3. 
  7. ^ Dickinson, John (1985-06-25). "The AT's Slipped Disk". PC Magazine. p. 55. Retrieved 28 October 2013. 
  8. ^ Richard W. D. Nickalls; R. Ramasubramanian (1995). Interfacing the IBM-PC to Medical Equipment: The Art of Serial Communication. Cambridge University Press. p. 25. ISBN 978-0-521-46280-8. 
  9. ^ Scott M. Mueller (2011). Upgrading and Repairing PCs (20 ed.). Que Publishing. p. 882. ISBN 978-0-13-268218-3. 
  10. ^ IBM's official 1986 response to "What percentage of the 20 MB drives in PC ATs have failed?" was "We consider that information to be confidential. However, based on the several customer surveys on the AT that we have conducted for IBM, an overwhelming percentage of AT owners tell us they're satisfied with the system." (questions on page 110, answers on page 111, PC Magazine, April 29, 1986). The article's opening sentence, which reads "If you own an IBM PC AT and your hard disk hasn't crashed yet, don't worry -- it probably will." was described as "a rarity in computer journalism" by the Chicago Sun-Times and the Sun-Times called it a "badly flawed 20-megabyte" disk drive.
  • IBM (1986). Personal Computer Hardware Reference Library: Guide to Operations, Personal Computer XT Model 286. IBM Part Number 68X2523.
  • PC AT entry at

External links[edit]

Preceded by
IBM Personal Computer XT
IBM Personal Computers Succeeded by
IBM Personal System/2