David Empson wrote:
Tristan Mumford <xtristan.xmumford@xgmail.xcom> wrote:Michael J. Mahon wrote:<Whack on side of head> Thanks for reminding me that it *has* to bank switch the slot space, since that's the only way it can be compatible! So the only different code is the "reset" 256 bytes, and after it runs, the slot code will be identical to either the 13-sector boot ROM or the 16-sector boot ROM...Unless I'm mistaken, which is not entirely unheard of, I should just be able to attack it by just poking to 0x180 (sorry about the C style hex notation. Force of habit) and BSAVEing each 256 byte page. Sure they are probably duplicates of the Apple disk ROMs, but it can't hurt to look, right? Jorge said that it had a 2k ROM. I'm going to believe this statement. It also means I'm curious of its contents, and how much of it can be accessed. I'd be surprised if it's more than just the 3 pages, but curiosity drives me!2KB is the smallest EPROM size that was readily available at the time (a 2708 or equivalent). They are probably only using three pages.
Actually, the 2708 was a 1K EPROM, but they were never as popular as the 2716, so often 2716s were both more readily available at the same or lower cost. I agree that there are most likely only three pages in use.
The rest may be addressable: it is likely to be a multi-bit latch written at $Cx80, with the bank register being cleared by a hardware reset. Bank 0 will therefore be the reset code, and it seems that bank 2 and 3 contain the 16/13 sector boot code. Bank 1 will probably exist but be unused (unless needed to extend the code in bank 0) and banks 4-7 might be unimplemented, leaving half the ROM inaccessible. Oh well, I'm sure someone will find a use for my "copy the C8 area" code snippet, and it will be in the archives. :-)
Thanks, David--now it's in my archives, too. ;-) -michael NadaNet networking for Apple II computers! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it's seriously underused."