Motherboard DIP Switches: The IBM PC and XT had DIP switches on their motherboard to configure certain aspects of operation. The XT had only one block of 8 switches, while the PC had that one plus a second 8-switch block for selecting how much memory was installed. Switch Block 1 (PC and XT): Switch 1 (PC): When on, do not boot from floppy drives. When off, boot from floppy drives. Switch 1 (XT): When on, perform continuous looping POST. When off, perform normal POST. Switch 2: Turn off if math coprocessor installed. Turn on if math coprocessor not installed. Switches 3 and 4: Used to indicate which banks have memory installed. If both switches are on, only bank 0 has memory (so there is 64K of memory). If 3 is off but 4 is on, banks 0 and 1 have memory (128K). If 3 is on but 4 is off, banks 0, 1, and 2 have memory (192K). If both 3 and 4 are off, all 4 banks have memory (256K). Switches 5 and 6: Used to indicate video adapter type. If both switches are off, a monochrome card is assumed to be installed. If 5 is off but 6 is on, a CGA card running in 40x25 character mode is installed. If 5 is on but 6 is off, a CGA card in 80x25 character mode is installed. If both switches are on, the video card has an on-board BIOS (for EGA/VGA cards and basically anything else above CGA). Switches 7 and 8: Used to indicate number of floppy drives. If both are on, 1 floppy drive is installed. If 7 is off but 8 is on, 2 floppy drives are installed. If 7 is on but 8 is off, 3 floppy drives are installed. If both are off, 4 floppy drives are installed. Switch Block 2 (PC only): This switch block indicates installed RAM memory, with various settings from 16K to 640K. In the list below, the 1s and 0s indicate the DIP switch settings. A 1 indicates a DIP switch which is on, and 0 indicates a switch which is off. 16K: 11111000 32K: 11111000 48K: 11111000 64K: 11111000 96K: 01111000 128K: 10111000 160K: 00111000 192K: 11011000 224K: 01011000 256K: 10011000 288K: 00011000 320K: 11101000 352K: 01101000 384K: 10101000 416K: 00101000 448K: 11001000 480K: 01001000 512K: 10001000 544K: 00001000 576K: 11110000 608K: 01110000 640K: 10110000