My First Computer

The following reading will give you an idea of how computers have been a integral part of my life since my late teenage years.  They probably are the driving force in saving my life.

The "IBM Personal Computer XT", IBM's model 5160, was an enhanced machine that was designed for diskette and hard drive storage, introduced two years after the introduction of the "IBM Personal Computer". It had eight expansion slots and a 10 MB hard disk (later versions 20 MB). Unlike the model 5150 PC, the model 5160 XT no longer had a cassette jack, but still contained the Cassette Basic interpreter in ROMs. The XT could take 256 kB of memory on the main board (using 64 kbit DRAM); later models were expandable to 640 kB. (The BIOS ROM and adapter ROM and RAM space, including video RAM space [since the video hardware was always an adapter] filled the remaining 384 kB of the one megabyte address space of the 8088 CPU.) It was usually sold with a Monochrome Display Adapter (MDA) video card.[citation needed] The processor was a 4.77 MHz Intel 8088 and the expansion bus 8-bit XT bus architecture (later called 8-bit Industry Standard Architecture (ISA) by IBM's competitors). The XT's expansion slots were placed closer together[20] than with the original PC;[21] this rendered the XT's case and mainboard incompatible with the model 5150's case and mainboard. The slots themselves and the peripheral cards however were compatible, unless a rare card designed for the PC happened to use the extra width of the 5150's slots, in which case the card might require two slots in the XT. The XT's expansion slot mechanical design, including the slot spacing and the design of the case openings and expansion card retaining screws, was identical to the design that was later used in the IBM PC AT and is still used as of 2011, though (since the phase-out of ISA slots) with different actual slot connectors and bus standards.

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. It was released as IBM product number 5160 on March 8, 1983, and came standard with a hard drive. It was based on essentially the same architecture 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, and a corresponding 3270 PC featuring 3270 terminal emulation was released later in October 1983. XT stands for X-tended Technology.


The standard XT originally came with 128kB of memory, a 360kB double-sided 5.25" full-height floppy disk drive, a 10MB Seagate ST-412 hard drive with Xebec 1210 MFM controller, an Asynchronous Adapter (serial card with 8250 UART) and a 130W PSU. 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 adapter, the hard drive adapter, and the Async card. The basic specification was soon upgraded to have 256kB of memory as standard. Slot 8 on the XT motherboard was wired slightly different than the other slots, making it incompatible with some cards. This was done for cards designed to allow the XT to be connected to IBM mainframes. Video cards initially comprised the MDA and CGA, with EGA and PGC becoming available in 1984.


There were two widely used configurations of the XT motherboard. The first could support up to 256kB on the motherboard itself (four banks of 64kB chips), with a maximum of 640kB achieved by using expansion cards. This was the configuration the XT originally shipped in. The second configuration - introduced in stock units in 1986 - could support the whole 640kB on the motherboard (two banks of 256kB chips, two banks of 64kB), had the later revision AT-compatible BIOS with a faster booting time, as well as support for 101-key keyboards and 3.5" floppy drives. The earlier configuration could be adapted to 'late' configuration after a couple of minor modifications.


There were also two or three revisions of the motherboard, however there are only minor differences between them. Most notable is that the first revision is missing U90 and has some parts located at another place on the motherboard.


Beginning in 1985, the XT was offered in floppy-only models without a hard disk (submodel 068 and 078). XTs with the 256kB-640kB motherboard configuration came standard with half-height floppy drives in place of the full-height drives, as well as the option for a 20MB ST-225 half-height hard disk and 'enhanced' keyboard (essentially a Model M without the LED panel, and cross-compability between the AT and XT keyboard transfer protocol). The XT was discontinued in the spring of 1987, replaced by the PS/2 Model 30.


In 1986, the XT/286 (IBM 5162) with a 6 MHz Intel 80286 processor was introduced. This system actually turned out to be faster than the ATs of the time using 8 MHz 286 processors due to the fact that it had zero wait state RAM that could move data more quickly.


Like the original PC, the XT came with BASIC in its ROM. Despite the lack of a cassette port on XTs, IBM's licensing agreement with Microsoft forced them to include BASIC on all their machines.


PC and XT keyboards are not compatible with those on more modern PCs (IBM AT or newer) even with DIN to PS/2 mini-DIN plug adapters, because PC/XT keyboards have a different transfer protocol from PC/AT keyboards and also use different keyboard scan codes. Some keyboards are switchable between the two interfaces for compatibility with either computer. Also, the 'parkbd' driver can be used under Linux in order to support either sort of keyboard through the parallel port via a simple adapter.

The IBM XT came with documentation giving the schematic diagrams, BIOS listing, character set, specifications and other detailed technical information.

The newer 101-key keyboard based on the Model M will work with modern PCs however.