IBM Personal Computer/AT

IBM PC/AT (model 5170)

Type	Personal Computer
Release date	1984
Discontinued	1990
Operating system	PC-DOS 3.0 and later, OS/2 1.x
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 machine type 5170. Because the AT used various technologies that were new at the time in personal computers, the name AT stood for Advanced Technology; one such advancement was that the Intel 80286 processor used in the AT supported Protected mode. IBM later released an 8 MHz version of the AT.

AT features

• AT bus: The AT motherboard had a 16-bit data bus and 24-bit address bus 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. IRQs 8–15 are cascaded through IRQ 2, which leaves 15 active instead of 16. Similarly, DMA channel 4 is reserved for cascading 0–3 leaving seven channels active. 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 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.) 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.
• 1.2 MB 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. 3.5" floppy drives became available in later ATs.
• A 20 MB hard disk drive that was twice as fast (about 40 msec) as the PC XT's 10 MB drive, although the early drives manufactured by Computer Memories (CMI) had a 25–30% failure rate after one year. 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. Later models (1987+) had only VGA, which was mostly backward compatible with the EGA and, in turn, the CGA.
• 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 lock and cylindrical key that could be used to prevent access to the computer by disabling the keyboard.

Challenges

In addition to the unreliable hard disk drive, there were problems with the floppy disk drives:

• The high-density floppy disk drives turned out to be problematic. Some AT's came with one high-density disk drive and one regular 360 KB drive. High-density floppy diskette media was 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 you accidentally used a high-density diskette in the 360 kB drive, it would sometimes work,^{[citation needed]} for a while, but the high-remanence 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 low-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 an 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 one, 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.

PC/AT clones

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 2009, modern PCs still retain many AT-class technical design features, though the AT expansion bus (ISA), the AT power supply and its connectors, and the AT motherboard form factor have disappeared, and the AT BIOS has evolved and is now facing competition, though still used in most systems. 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

• Industry Standard Architecture

References

• IBM (1986). Personal Computer Hardware Reference Library: Guide to Operations, Personal Computer XT Model 286. IBM Part Number 68X2523.
• PC AT entry at old-computers.com