The 74HC688 is a comparator, it's output (/P=Q) goes active=low when the P inputs are exactly the same as the Q inputs. You set the Q inputs by the dipswitches and when the same address occurs at the A4-A9 lines on ISA, the /P=Q output goes low. This enables the 74LS138 decoders, so when this address occured simultaneously with /IOR or /IOW active(=low) on the ISA bus, one of the outputs of 74LS138 (according to the state of A0-A3 address) goes low.

So the state of DIPSW determines the bits 4-9 of I/O address space where the card is active (=> "base address"). This is an 8-bit card, the 8-bit in and out instructions issue only 10-bit address on x86.

Jan Waclawek