DOS Days

IBM PC and XT DIP Switch Settings

Both the IBM PC and IBM PC-XT had DIP switches that were used to tell the system what was installed or connected. On the original IBM PC, there were two blocks of DIP switches. On the XT there was one block. Set these wrong and worst-case is the machine won't boot and won't display anything on the screen.

IBM PC

DIP switch block 1 (in centre of motherboard):

Switch Setting Meaning
1 Off Boot from floppy disk
1 On Don't boot from floppy (go straight to Cassette BASIC in ROM)
2 Off 8087 math coprocessor installed
2 On 8087 math coprocessor NOT installed
3,4 On,On One bank of memory
3,4 Off,On Two banks of memory
3,4 On,Off Three banks of memory
3,4 Off,Off Four banks of memory
5,6 Off,Off MDA or Hercules video
5,6 Off,On 40-column CGA video
5,6 On,Off 80-column CGA video
5,6 On,On No video or special (like EGA or VGA)*
7,8 On,On 1 floppy drive
7,8 Off,On 2 floppy drives
7,8 On,Off 3 floppy drives
7,8 Off,Off 4 floppy drives

DIP switch block 2 (near the power supply):

Switch Setting Meaning
1,2,3,4,5 On,On,On,On,On 16K of memory installed
1,2,3,4,5 On,On,On,On,On 32K of memory installed
1,2,3,4,5 On,On,On,On,On 48K of memory installed
1,2,3,4,5 On,On,On,On,On 64K of memory installed
1,2,3,4,5 Off,On,On,On,On 96K of memory installed
1,2,3,4,5 On,Off,On,On,On 128K of memory installed
1,2,3,4,5 Off,Off,On,On,On 160K of memory installed
1,2,3,4,5 On,On,Off,On,On 192K of memory installed
1,2,3,4,5 Off,On,Off,On,On 224K of memory installed
1,2,3,4,5 On,Off,Off,On,On 256K of memory installed
1,2,3,4,5 Off,Off,Off,On,On 288K of memory installed
1,2,3,4,5 On,On,On,Off,On 320K of memory installed
1,2,3,4,5 Off,On,On,Off,On 352K of memory installed
1,2,3,4,5 On,Off,On,Off,On 384K of memory installed
1,2,3,4,5 Off,Off,On,Off,On 416K of memory installed
1,2,3,4,5 On,On,Off,Off,On 448K of memory installed
1,2,3,4,5 Off,On,Off,Off,On 480K of memory installed
1,2,3,4,5 On,Off,Off,Off,On 512K of memory installed
1,2,3,4,5 Off,Off,Off,Off,On 544K of memory installed
1,2,3,4,5 On,On,On,On,Off 576K of memory installed
1,2,3,4,5 Off,On,On,On,Off 608K of memory installed
1,2,3,4,5 On,Off,On,On,Off 640K of memory installed
6,7,8 Off,Off,Off Never changed - all always set to Off

IBM XT (and the majority of XT compatible motherboards)

DIP switch block:

Switch Setting Meaning
1 Off Normal POST then boot
1 On Continuous power-on self-test (POST) - used for burn-in testing only
2 Off 8087 math coprocessor installed
2 On 8087 math coprocessor NOT installed
3,4 On,On One bank of memory
3,4 Off,On Two banks of memory
3,4 On,Off Three banks of memory
3,4 Off,Off Four banks of memory
5,6 Off,Off MDA or Hercules video
5,6 Off,On 40-column CGA video
5,6 On,Off 80-column CGA video
5,6 On,On No video or special (like EGA or VGA)*
7,8 On,On 1 floppy drive
7,8 Off,On 2 floppy drives
7,8 On,Off 3 floppy drives
7,8 Off,Off 4 floppy drives

* The reason why this is specified as 'No video' is because with MDA, Hercules or CGA graphics, the system's ROM BIOS handles all video communications, whereas in the more advanced graphics standards (EGA and VGA), these cards have their own dedicated ROM BIOS which software uses to communicate with the video card. So in such a scenario, the system's BIOS needs to have its built-in video communications "switched off".