The IBM PC XT was configured to work by a set of dip switches on the main logic board. The 286, 386, 486 and Pentium PC s normally do not have dip switches, they save their configuration in CMOS memory chip backed up by a Lithium battery.
Upon boot up of the PC XT a POST (Power On Self Test) reads the dip switch information, then checks the hardware to the configuration. If there is an error the PC XT will give a POST error code and will not boot up until the configuration and the physical hardware matches EXACTLY!

Device	Switch s On / Off	Function
POST	1 On	POST loop test
POST	1 Off	Normal position
Coprocessor	2 Off	Coprocessor installed
Coprocessor	2 On	Coprocessor NOT installed
Memory	3 On / 4 On	256 KB RAM
Memory	3 Off / 4 On	512 KB RAM
Memory	3 Off / 4 Off	640 KB RAM
Video	5 On / 6 On	EGA / VGA
Video	5 Off / 6 Off	Monochrome
Video	5 On / 6 Off	CGA 80x25 mode
Video	5 Off / 6 On	CGA 40x25 mode
Floppy Drives	7 On / 8 On	1 Floppy Drive present
Floppy Drives	7 Off / 8 On	2 Floppy Drives present
Floppy Drives	7 On / 8 Off	3 Floppy Drives present
Floppy Drives	7 Off / 8 Off	4 Floppy Drives present

Note: If you have an original IBM PC. NOT a IBM PC XT. There will be a extra set of dip switches, Don t get the two confused, or your PC will get POST error codes.