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The IBM Personal Computer XT, often shortened to the IBM XT, PC XT, or simply XT, was IBM’s successor to the original IBM PC, factory equipped with a hard drive. It was released as IBM Machine Type number 5160 on March 8, 1983, and essentially the same as the original PC, with only incremental improvements. A new 16-bit bus architecture would follow in the AT. The XT was mainly intended as an enhanced machine for business use, though later floppy-only models would effectively replace the original model 5150 PC. A corresponding 3270 PC featuring 3270 terminal emulation was released later in October 1983. XT stands for X-tended Technology.

The XT originally came with 128 kB of RAM, a 360 kB double-sided 5.25 in (133 mm) full-height floppy disk drive, a 10 MB Seagate ST-412 hard drive with Xebec 1210 MFM controller, an Asynchronous Adapter (serial card with 8250 UART) and a 130 watt power supply. The motherboard had eight 8-bit ISA expansion slots, and an Intel 8088 microprocessor running at 4.77 MHz (with a socket for an 8087 math coprocessor); the operating system usually sold with it was PC DOS 2.0 and above. The eight expansion slots were an increase over the five in the IBM PC, although three were taken up by the floppy drive and hard drive adapters, and the Async card. (An IBM PC also typically had a floppy disk controller in one of its five slots; both the PC and XT also usually required a display adapter in another slot.) The basic specification was soon upgraded to have 256 kB of RAM as standard.

Slot 8 on the XT motherboard was wired slightly different than the other slots, making it incompatible with some cards and had additional signal CARD SLCTD (pin B8). This was done for cards designed to allow the XT to be connected to IBM mainframes (3270-PC). The modified slot is found only on the 5160 and 5155 Portable PC; it is not in the 5150, IBM AT, or any clone PCs. During this period, it was commonplace for ISA cards to include a jumper allowing them to work in Slot 8. Video cards initially comprised the MDA and CGA, with EGA and PGC becoming available in 1984.

source: wikipedia