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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 System Unit 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. == 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 (and XT). The doubling of the IRQs was achieved by adding another 8259A. IRQs 8–15 (from the second 8259A) are cascaded through IRQ 2 of the first 8259A, which leaves 15 available instead of 16. Similarly, the number of DMA channels was increased by adding another 8237A, also in master-slave configuration. 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 (RTC) 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.) Additionally the AT RTC had a 1024-Hz timer (on IRQ 8), which was a much finer resolution compared to the 18-Hz RTC used by IBM PC XT (IRQ 0). The AT timer was accessible via INT 70h.〔 The RTC was implemented using a Motorola MC146818 integrated circuit.* 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 * 1.2 MB 135mm (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. 90mm (3.5") floppy drives became available in later ATs. * A 20 MB hard disk drive, although the early drives manufactured by Computer Memories were very unreliable. 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. * ATs could be equipped with CGA, MDA, EGA, or PGA video cards. * The 8250 UART from the XT was upgraded to the 16450, although this chip still had only a one byte buffer, so high-speed serial communication was just as problematic as with the XT. * 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「IBM Personal Computer/AT」の詳細全文を読む スポンサード リンク
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