IBM Personal Computer/AT - at Features

AT Features

• AT bus: The AT motherboard had a 16-bit data bus and 24-bit address bus (16 MB) 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 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 which featured integrated numeric keypad with Num Lock key.
• 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 (seek times of 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.
• Early ATs (1984–1986) could be equipped with CGA, MDA, EGA, or PGA video cards.
• 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 physical lock that could be used to prevent access to the computer by disabling the keyboard.
• Just like its IBM PC predecessor, the PC/AT supported an optional math co-processor chip, the Intel 80287, for faster execution of floating point operations.