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".