IBM Personal Computer / AT
The IBM Personal Computer / AT (Type 5170) or just IBM AT (for A dvanced T echnology) or PC AT or PC / AT is the third generation of computers made by IBM . It was the successor to the IBM PC XT and IBM PC . The system was introduced to the public on August 14, 1984 as the 5170 model with an integrated hard disk and 6 MHz CPU. In contrast to its two predecessor series, the IBM PC and IBM PC XT , IBM was the first to use the 80286 architecture from Intel and PC DOS 3.0 - which was specially developed for the AT. Since the PC AT had numerous innovations - it had a main processor with protected mode support, a new 16-bit system bus and, for the first time, a non-volatile CMOS RAM - the expression "Advanced Technology" was very well justified at the time.
The PC AT form factor has long been the standard for mainboards and cases, regardless of whether it is an IBM PC or a compatible replica. The AT format was only replaced at the turn of the millennium by the ATX format licensed by Intel in 1996 or adapted to today's requirements.
technology
The AT used the Intel 80286 as its CPU with initially 6 MHz and later 8 MHz, which enabled memory expansion up to 16 MB , for the first time with an IBM PC, via the new protected mode of the CPU . In the so-called real mode , this CPU was downwardly compatible like an original 8086 CPU.
Most of the other motherboard components remained the same as in the previous IBM PC XT. However, two interrupt and DMA controllers were used instead of just one as in the XT. This increased the number of plug-in cards that could be operated simultaneously without conflict.
The keyboard of AT offered three main innovations: a clearly separated from the rest of the keyboard cursor keys / numeric keypad , three light-emitting diodes (LED) that the current state of the three shift keys Caps Lock , Num Lock and Scroll Lock anzeigten and the new system request (internationally labeled "SysRq", German "S-Abf"). This AT keyboard layout was then replaced at IBM in 1987 by the MF-II keyboard with cursor keys separated from the numeric keypad, other manufacturers followed a little later. The AT keyboard had the same 5-pin. DIN plug , but was not electrically compatible with the XT keyboard, etc. a. because it used completely different scan codes and also now worked bidirectionally in order to be able to control the status LEDs. PC-compatible keyboards from other manufacturers from 1984 to approx. 1990 often had an XT / AT mode switch on the underside.
Another innovation was the 16-bit wide system bus. It used the then newly defined 16-bit ISA bus and thus stood out from the previous PC architecture . Since IBM simply extended the slot by a few contacts, the new bus remained logically, electrically and mechanically downward compatible and thus also enabled the previous 8-bit cards to be operated.
Also new was the revised, flatter (only half the height, i.e. 1¾ inch ) and initially very unreliable 5¼ ″ floppy disk drive , for the first time with a high capacity (HD 1.2 MB), but at the same time backwards compatible with the old 160/320 or 180/16 360 kB standard.
New were also the battery-backed clock and the CMOS - RAM , for the first time could configurations directly on the motherboard battery backup are stored. This was mainly used for hard drives. They could now save their geometry data permanently in the hardware, which made the hard disk controller simpler and cheaper. If, however, the backup battery failed, the consequence was not only the missing time (as before), but also hard disk access. This access information was absolutely necessary because the hard disks of that time could not give any information about their size and geometry, but the control was carried out directly, analogous to the disk mechanics, via tracks, heads and sectors. These tables still exist for reasons of compatibility, but with today's disks the sectors are simply addressed linearly via Logical Block Addressing (LBA), the product of tracks, heads and sectors provides information about the disk size used when the system is started.
The AT also laid the foundation for almost all subsequent PC-AT clones from other manufacturers that were designated as AT-compatible or even just as IBM-compatible . Even today's computers known as PCs are still AT-compatible, at least in terms of software.
history
Like all IBM PCs, the AT was originally sold at very high prices. Since IBM did not have a monopoly on the components used (with the exception of the BIOS), Compaq , in cooperation with Microsoft , who supplied the Compaq-DOS operating system , was able to bring the first computer compatible with the IBM PC to the market as early as 1983. Since Microsoft had not developed the PC-DOS operating system exclusively for IBM and was not further contractually bound, the almost identical MS-DOS was freely licensed to any customer or manufacturer. Numerous manufacturers worldwide, and especially in Asia, seized this opportunity. With the AT, IBM wanted to take countermeasures and tried to protect the term AT after the market launch of the IBM PC AT . Since this did not succeed, all manufacturers soon used the term AT as the standard for PCs with 80286 or faster processors, which were delivered with an operating system compatible with IBM-PC-DOS (mainly MS-DOS). Intel also published in 1985 (own AT motherboard Engl. Motherboard ) together with case and power supply, in addition to the original IBM PC AT served for many copier, who up to the last little screw, as a template. Companies created numerous replicas, especially in Southeast Asia. The market that developed in this way led through competition to falling prices and increased innovation.
In 1987 IBM said goodbye to the AT standard and tried in a second attempt to regain market leadership with the PS / 2 systems and the OS / 2 operating system that were technically highly developed for the time . To this end, IBM tried to use proprietary concepts (including the Micro Channel Architecture and OS / 2) to better seal off the market from its competitors, but at the expense of its own compatibility. The competition, led by Compaq, which already had a system based on the i386 an AT-386 - which at that time was still provided with the old ISA slots - rebelled. Manufacturers such as Compaq, HP, Intel and Microsoft jointly developed competing concepts and cross-manufacturer hardware and software standards (e.g. EISA or Windows ) and, in contrast to IBM, were able to enforce these as industry standards with their market power. IBM was only able to establish its PS / 2 models with moderate success. As a result, the overpriced and incompatible Micro Channel disappeared completely from the market in 1995 with the last PS / 2 systems.