Re: uniplus disks imaged

From: Ray Arachelian <ray_at_email.domain.hidden>
Date: Thu, 20 Mar 2008 11:35:51 -0400

gilles wrote:

>> The even bytes are junk, usually 0xFF is a good value to return there.  
>> Offset 24 is the even byte for 0xfcc017,

> well, I was talking about offset 24 as seen by the 6504, it makes
> 0xfcc049
> as seen by the 68k.
> I agree about even bytes... but Xenix makes word read/writes so
> depending on
> the memory handling of the emulator, even address must be handled or
> not...

Ah... ok... 0xfcc049 means "drive present (FF=yes)" - so you'll want to return 0xffff if it's read as a word... I think I recall Xenix checked for this sort of thing too - but for the disk present flag at 0xfcc041 instead.

I pretty much return 0xff or 0xf-something for non-implemented memory as a rule (except for those spaces that should cause bus errors.) I sometimes use 0xf7 or some other value, but almost always use odd values incase there's a bug that uses that as a pointer - in which case, it'll get caught as an odd-address exception.

Also be aware that some I/O spaces repeat, for example the VIA and SCC regs show up at multiple addresses so you need to figure out a binary AND mask when accessing them...
i.e. (addr & 0x79)

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