Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!wuarchive!kuhub.cc.ukans.edu!markv From: markv@kuhub.cc.ukans.edu Newsgroups: comp.sys.amiga.programmer Subject: Re: CIA register Message-ID: <1991May16.103515.30814@kuhub.cc.ukans.edu> Date: 16 May 91 15:35:15 GMT References: <1991May14.164909.19186@NCoast.ORG> <1991May15.183635.24595@wehi.dn.mu.oz> <91135.204206FERZAN@TREARN.BITNET> Organization: University of Kansas Academic Computing Services Lines: 35 In article , phil@adam.adelaide.edu.au (Phil Kernick) writes: > FERZAN@TREARN.BITNET writes: > >> Can anyone explain the memory overlay bit >> in the CIAA chip ? >> But I think this bit sets the ROM start address to 0 >> Really ? > It probably does. This is just sheer speculation, but I suspect that it > is set when the machine is reset, and points the ROM at 0 so the > processor will be able to pick up the initial PC from address 0 and SSP > from address 4. > It almost certainly resets after this and the ROM initialises all of the > standard 68000 vectors and then starts the OS. Yes, the overlay bit maps the ROMs to 0. That's ALL it does. Now, you likely will crash long and hard after that, but you dont have to. The ROMs at 0 dont have a full exception table, since that is one of the first things that gets done, so any interrupts or exceptions will die hard (the OS turns off interrupts when it resets). But if you deal with the exception problem, and make sure your code isn't running out of the first 512 of RAM, then you can twiddle it to your hearts content. Of course there is no need to since the ROM is accessable normally. However, this trick can be used to look at the boot ROMs in a 3000 or 1000 (the 1000 requires other magic too, because of the WCS), since its the *real* ROM that gets mapped in, not the ROM image. -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Mark Gooderum Only... \ Good Cheer !!! Academic Computing Services /// \___________________________ University of Kansas /// /| __ _ Bix: mgooderum \\\ /// /__| |\/| | | _ /_\ makes it Bitnet: MARKV@UKANVAX \/\/ / | | | | |__| / \ possible... Internet: markv@kuhub.cc.ukans.edu ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~