---
title: IBM PC AT
categories:
- Computers
types:
- Professional Computer
origins:
- U.S.A.
years:
- '1984'
cpus:
- Intel 80286
params:
  name: PC AT
  image: /media/museum/photos/ibm_at_system_s1.jpg
  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
  forum_thread: 195
  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: /media/museum/photos/ibm_pc-at_inside.jpg
  adverts:
  - title: US ADVERT, JULY 1985
    subtitle: ''
    text: Byte magazine
    image: /media/museum/pubs/ibm_at_pub_s1.jpg
  - title: IBM T-SHIRT
    subtitle: ''
    text: ''
    image: /media/museum/pubs/ibm_at_t-shirt_1.jpg
  links:
  - title: Web Wiz CAPTCHA
    url: http://www.webwizcaptcha.com
    text: ''
  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.