Deep IIci rejuvenation, possible ROM select problem — #15

Post #15 by trag

Source Forum	68kMLA
Category	Hardware
Post Date	Mon, 1 Mar 2010 - 17:41
Original URL	https://68kmla.org/bb/threads/deep-iici-rejuvenation-possible-rom-select-problem.19657/

Post

With ROM, it skips the "real" A0 and A1. This is because each address on the address bus is sent to all 4 ROM chips. Because there are 4 8-bit ROM chips, each address requested from ROM returns 32 bits. But as we know, each single byte (each 8 bits) has an address, not each 32 bits. So by skipping the first 2 address bits, the physical address sent to the ROM changes each 4 bytes instead of each byte. These missing address bits are probably used elsewhere to grab a specific 8-bit portion of the 32-bit data returned from ROM.
"real" A19, which logically should be A17 to the ROM chips,

Right. My mistake. A0 and A1 are only uaed for bytewise addressing and then only on a couple of earlier machines. With four X8 ROM chips, you've got a 4 byte word and so A0 and A1 are omitted and address lines are tied to the ROM chips starting at A2.

There's actually a note in TGTTMFH about A0 and A1. I should have thought of that but I typed my previous message out in a bit of a hurry.