CCS64 Forum

Character ROM problem
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Author:  Danny B [ Wed Nov 10, 2004 8:41 pm ]
Post subject:  Character ROM problem

I am using CCS64 v2.0 B (release date feb 2002) and I had some problems with using the $9000 area for graphics for my demo. So I made a smaller test prg that tries to use this area, like in my demo. The problem is that CCS64 shows the char set (possibly from ROM), when it should show the RAM area in $9000. Of course I disabled ROM with loading #$36 into $01. But it doesn't work on CCS64. VICE shows it correctly, though. Maybe this bug (if it is one) has already been fixed on the newest version of CCS64, but I can't run CCS64 v3.0 cause of the limits of my PC.
I can send you my test program, along with its source, so maybe you can give me some answers on this

Author:  Stuart Toomer [ Thu Nov 11, 2004 7:35 pm ]
Post subject: 

Please e-mail me your test program and I will confirm whether this has been fixed in CCS64 V3.0 BETA or not.

Author:  StevenRoy [ Sun Nov 28, 2004 7:05 am ]
Post subject: 

If I remember correctly, a real C64 behaves the same way. The VIC chip sees the character ROM in the 1000-1FFF memory range and there's no way to disable that. I think the 9000-9FFF range behaves the same way. Thus, there is no way to use that area for graphics. (It does seem strange to me, though, that the 5000-5FFF and D000-DFFF ranges don't have that. The RAM there is just fine for graphics.)

Pity that neither of my real C64s are functional any more, otherwise I'd make sure I got this right. (I'm not even sure I still have them; they may have been discarded since I can do much more with CCS64.)

Author:  zaphod [ Sat Mar 12, 2005 6:03 am ]
Post subject: 

To use the RAM is those areas, you must *switch out* the character generator rom by twiddling some zero page bits. By doing so, you of course lose access to the character set, unless you have another copy stored in RAM somewhere.

Author:  markhealey [ Wed Oct 05, 2011 8:12 pm ]
Post subject:  Re: Character ROM problem

I do beleive your problem is actually how a real c64 behaves - the vic chip will always see the character rom at $9000, whether you have the rom switched out or not - you can use this area of 'shadow ram' for code quite happily, but the Vic chip will always see the rom, so using it for graphics is a no no unfortunatly.
You will also have this problem using bank 0 with memory $1000-$2000.

so, if you want to use a full 16k bank for graphics, you need to use bank 1 ($4000-$7fff).

That's the theory anyway, I'm about to see if this actually works by changin around the memory map of a game i've been making for c64 - I didn't realise this was a problem as it was working fine on an older emulator, but now i've started using a more up to date (and more accurate I presume) emulator, some of my sprites were corrupt, and I beleive it's down to this issue.

BTW, I got this info from here ... organizing

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