The cover simply slides off the front once you remove the screws in the back. Except for the fact that everything is modular the internal layout is very similar to modern computers. In these days of this machine there was an expansion card for everything. Nothing was built on to the motherboard except for the processor, ram, bios, and keyboard controller. All other devices had to be controlled by a expansion card. In fact there wasn't even a real time clock. Every time you booted the machine you had to set the time and date. However, you could add a real time clock expansion card.

A note about removing the drives... Two of the screws that hold the drives in place are accessible from the bottom of the case in recessed hole.

Another note... The ISA slot closest to the power supply was considered the "Bad Slot" as most cards will not work in it.

    The heart of the IBM XT was the Intel 8088 microprocessor. The 8088 is a 16 bit processor with an 8 bit data bus. The clock ran at 4.77Mhz. An 8087 numeric co-processor could be optionally installed in the socket adjacent to the processor. It's not installed in this machine. The numeric co-processor was basically a second microprocessor designed to perform floating point calculations. It could take some of the load off the main processor enabling some task to be done faster. The benefits of the numeric co-processor was only seen while running software that supported it.

The IBM XT's memory could contain up to four banks of nine DRAM (Dynamic Random Access Memory) chips. DRAM chips came in in 16 pin IC packages and were available in numerous sizes and speeds. This particular machine has 64K chips in all 4 banks for a total of 256k (64k * 4) bytes of memory. The first two banks are filled with Mostek chips, the rest are NEC. Evidently this machine got upgraded at some point. Identifying memory chip sizes is usually easy. The Mostek's say MK4564N -20 and the NEC's say D4164C -3. The "64" in both part numbers indicate 64k chips. The -20 means the Mostek's have a 200ns access time and the NEC's are a bit slower at 300ns. When installing memory an entire bank must be filled (9 DRAM Chips). I explain why below. I remember it being recommend not to mix brands or speeds within a bank.

It should be noted that a 64K chip can hold 64,000 BITS! not bytes. To the XT a word is 8 bits wide. Therefore, it takes eight 64k Bit chips to equal 64K Bytes of memory (An entire bank). As shown in the photo this machine has four banks of nine 64K chips. So whats the 9th chip for? The 9th chip is called a parity chip and is used for error checking. When 8 bits of data are written to an address a Parity Bit (The 9th bit) is set and stored. The parity bit is based on whether number of bits that were set in the data word are odd or even. Whenever the data is read from the address a comparator test the parity bit against the data bits. If the test fails something has changed. The computer halts and gives a nasty parity check memory error.

Most memory errors I have encountered on PC/XT machines were fixed by "Re seating the ram". I would not recommend removing the chips entirely from their sockets and reinserting them. Even when using an IC extractor pens can get bent. A word of caution... Pins that have been bent break really easily. I always took a small slotted screwdriver and raised the chip a little on each end, then pressed the chip back into place. Hopefully reestablishing a good electrical connection.

    The IBM XT is configured by a set of 8 dip switches. There isn't a built in or external setup program. The XT didn't even have a backup battery. There wasn't anything to back up. The dip switches are for configuring the basic hardware and is pretty straight forward. Sometimes things get real interesting when installing something new. The XT came with a hard drive. But, the BIOS didn't support it. It was entirely up to the drive controller to support, operate, and recognize the drive. Which meant that buying a new hard drive often included buying a compatible controller card. Plug and Configure! Not plug and play. I remember using DOS debug to invoke the low level format software embedded in some drive controllers. ...And let's not forget those terminating resistors on those hard drives and the floppy drives too.

For anyone working on one of these fine machines I have included the dip switch settings for the IBM XT. Note... The Original IBM PC had 2 banks of 8 dip switches. The configuration for the PC differs a bit from the XT. The dip switch settings for the IBM PC can be viewed here

SW 1	SW 2	SW 3	SW 4	SW 5	SW 6	SW 7	SW 8	Function
OFF								Always OFF on an XT
	ON							Co-processor NOT Installed
	OFF							Co-processor Installed
		ON	ON					64K Ram
		OFF	ON					128K Ram
		ON	OFF					192K Ram
		OFF	OFF					256K or more Ram
				ON	ON			No Video
				OFF	ON			CGA 40 Column
				ON	OFF			CGA 80 Column
				OFF	OFF			Monochrome
						ON	ON	No Floppy Drives
						ON	ON	One Floppy Drive
						OFF	ON	Two Floppy Drives
						ON	OFF	Three Floppy Drives
						OFF	OFF	Four Floppy Drives

    The video installed in this XT is an original full length text only card. This thing is over 13" long, about 4" tall, has 67 integrated circuits (I counted), and will only do text. No graphics!

    A simple loop in BASIC saying hello to the world.