23 Address linesgreenspun.com : LUSENET : ece342 : One Thread
18 address lines are required to map on to the DRAM chips. If we are not using aliasing what are the other 5 bits for ?
-- Neil Mittal (firstname.lastname@example.org), February 01, 1999
Using aliasing means that you ignore some address lines. In effect, they become don't-cares in your address decode logic.
Not using aliasing this means that you need to decode all of the address lines.
-- Steven Caranci (email@example.com), February 02, 1999.
You need the 4 upper address bits to decode one of the 10 1MB memory regions between $200000 & $BFFFFF. Since your memory is only 512KB, you have one extra address pin left over. With aliasing you wouldn't need to connect this pin and your memory would occupy the whole 1MB region (there would be two copies of it). But we want you to not use aliasing, so there will only be one copy.
-- Robin Grindley (firstname.lastname@example.org), February 02, 1999.