website/content/museum/ibm-pc-at.md

---
title: IBM PC AT
categories:
- Computers
types:
- Professional Computer
origins:
- U.S.A.
years:
- '1984'
cpus:
- Intel 80286
params:
  name: PC AT
  company: /companies/ibm
  company_name: IBM
  specs:
  - label: NAME
    value: PC AT
  - label: MANUFACTURER
    value: IBM
  - label: TYPE
    value: Professional Computer
  - label: ORIGIN
    value: U.S.A.
  - label: YEAR
    value: '1984'
  - label: BUILT IN LANGUAGE
    value: Microsoft Basic interpreter
  - label: KEYBOARD
    value: Full stroke keyboard with numeric keypad and function keys
  - label: CPU
    value: Intel 80286
  - label: SPEED
    value: 6 MHz
  - label: RAM
    value: 512 KB (up to 3 MB)
  - label: ROM
    value: 64 KB
  - label: TEXT MODES
    value: 80 x 24 / 40 x 24
  - label: GRAPHIC MODES
    value: 'EGA mode: 640 x 350'
  - label: COLORS
    value: '64'
  - label: SOUND
    value: Beeper
  - label: I/O PORTS
    value: 8 x internal slots (six 16 bit ISA and two 8 bit ISA), RS232c, Centronics
  - label: BUILT IN MEDIA
    value: one 5.25'' disk-drive, various Hard disk drives, from 20 MB
  - label: OS
    value: MS-DOS
  - label: POWER SUPPLY
    value: Built-in switching power supply unit
  - label: PRICE
    value: Unknown
  tabs:
  - photos
  - adverts
  - links
  photos:
  - title: INSIDE
    subtitle: ''
    text: |-
      Top view of an opened PC-AT. The new 20Mb hard-drive can be easily spotted (centre, yellow). On the left is the disk-drive.

      On the right we can see the motherboard with the memory chips and the expansion slots.

      The big yellow box at the bottom left is the power unit.
    image: null
  adverts:
  - title: US ADVERT, JULY 1985
    subtitle: ''
    text: Byte magazine
    image: null
  - title: IBM T-SHIRT
    subtitle: ''
    text: ''
    image: null
  links:
  - title: Web Wiz CAPTCHA
    url: http://www.webwizcaptcha.com
    text: ''
  comments:
  - author: SERGIO CUTIÉ (España)
    date: Wednesday 27th April 2022
    text: Tengo un ordenador IBM PC AT TYPE 2 20 MB por si alguien desea adquirirlo.
  - author: chenzf (China)
    date: Tuesday 18th October 2016
    text: |-
      Where there is sell IBM PC/XT?\
      Thanks!
  - author: Christoph (Australia)
    date: Sunday 16th October 2016
    text: |-
      I have a IBM System 1981 ni monitor no key board, but I do have boot disks and some chips as well, I would like to sell to a collector who will value this ?\
      Can send Photos
  - author: Xcursion666 (Usa)
    date: Friday 26th September 2014
    text: |-
      i recently found one of these beasts in a dumpster with a zenith data systems display it didnt have the keyboard though :(\
      i got a laugh when i booted it up and it said keyboard error press f1 to resume lol i think the hd in it still works it has a sticker on the back that says its a reference drive
  - author: Nick Y (UK)
    date: Monday 27th September 2010
    text: This was one of my first pc, eventually I upgraded this computer with 1152K (512+640) and then installed a 1.44MB floppy drive however the bios only allowed me to for 720K drives, I managed to install a version of Windows 3.0 my splitting and zipping the setup files onto floppys and unpacked on the 30MB drive I had.. I remember when I first get it and switched it on, my father went straight to electric cupboard ... had plenty of experiemce with this computer, finished up with just the mainboard in cupboard now as I installed a faster motherboard in the IBM case afterwards..
  - author: vintageNerd (Australia)
    date: Friday 9th July 2010
    text: A total of 5 models were released - see http://wiki.vintage-computer.com/index.php/IBM_PC_AT\_$285170$29
  - author: Russ Blakeman (Kentucky USA)
    date: Sunday 24th August 2008
    text: Same here - I have gobs of this old iron in dry storage and I have an AT-1 and AT-2 both in working order that I use from time to time. I also have some PC and XT items and lots of PS/2 stuff.
  - author: Andrew J (UK)
    date: Sunday 17th August 2008
    text: This machine was a beast! 19.5kgs I think.... Made in Scotland from girders, Greenock to be precise! I love my AT, got three XT's too - they're ace! You can't beat the clunkiness of these machines.
  - author: insektenfang (Earth)
    date: Saturday 15th December 2007
    text: There was also a further and later model named the AT-X. I used one for several years, but can't recall the advantages over the standard AT. It was along time ago...
  - author: Jayme (Tyler,TX)
    date: Tuesday 14th May 2002
    text: |-
      I have lots of computer parts for sale like: mother boards,hard drives,processors,keyboards,mouses, joysticks,cases,chips,memmory expansion slots,cd-rom drives.You name a price!\
      My phone number:(903)534-2883 my email address:Txbillfrog111@aol.com
  related:
  - /museum/amstrad-pc-1512
  - /museum/amstrad-pc-1640
  - /museum/amstrad-ppc-512-640
  - /museum/ibm-pc-model-5150
  - /museum/ibm-pc-portable-model-5155
  - /museum/ibm-pc-xt-286
  - /museum/ibm-pc-xt-model-5160
  - /museum/ibm-ps-1
  - /museum/micronique-hector-xt
  oc_id: 185
  source:
    url: https://web.archive.org/web/20230607114003/http://www.old-computers.com/museum/computer.asp?c=185
    date: '2023-06-07'
---

The IBM PC AT was the successor of the [PC](/museum/ibm-pc-model-5150/) and the [XT](/museum/ibm-pc-xt-model-5160/). IBM added a lot of new features: they abandoned the old Intel 8086 to the Intel 80286, so the PC AT used new 16 bit expansion slots.

The PC AT had a new version of the Microsoft OS: MS-DOS 3.0 which could manage the new 5.25" floppy disk format (1.2 MB), the new hard disk capacities (20 MB and more) and allowed file sharing. It had a new keyboard too (the same we use now, more or less) with cursor keys and a key that could lock it.

Two models were launched: the PC-AT model 1 (256 KB RAM, two floppy disk units and a color screen) and the PC-AT Model 2 (512 KB RAM, one floppy disk unit, one hard disk and a color screen).

This computer was revolutionary, but it was the last time IBM imposed a standard to the PC clone industry. The next year, the first PC based on a 80386 was made by Compaq and IBM failed to impose the PS/2 standard in 1986•

---

**"Doorbasher"** specifies:\
There was a second revision of the IBM AT. From the outside, it was impossible to tell them apart, but inside it was a new machine. It had an 8 MHz 80286, and a new motherboard to handle the higher speed. The new revision of motherboard (referred to as the "type two", the older 6 MHz model being the type one) used industry-standard 256k DRAMs, rather than the weird 128k piggybacked chips in the original model.\
The new motherboard was also smaller than the original, but the case was still large enough for a full-size board. Also, if there was any 512k RAM limit in the type one, it was fixed in the type two.\
Also, the RAM could be taken beyond 3MB. The real limit was 12.6 MB. (I believe this was true for both types of AT.) It was possible to install more RAM than that, but due to a bug in the BIOS/motherboard implementation, a manimum of 12.6MB would be recognized.

More technical information from **Dennis Stanton**:\
The first AT used 128 k chips, which appeared to be two 64k chips stacked. It used two DMA chips, which tended to fail in tandem. It also used a second IRQ controller. If the AT had more than 640 k of RAM, the CMOS would only allocate the first 512 as Convential, the rest as Extended.\
Only 17 hard drive type were supported in the CMOS, causing no end of headaches when Seagate realsed their 40 meg half height. The 1.2 meg floppy drive could read and write 360's, but if you formatted one, it couldn't be read by a regular double density drive.