| Release date | 1984[1] |
|---|---|
| Discontinued | April 2 ,1987[1] |
| 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. 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.
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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.In addition to the unreliable hard disk drive,[2] 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 physical ISA expansion bus connectors any more, but the bus signals live on in 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.
| Preceded by IBM Personal Computer XT |
IBM Personal Computers | Succeeded by IBM Personal System/2 |