Hello! Rhialto wrote: > Sounds much like what the DWW board is doing. I posted about that in > this very mailing list. It has 8 banks of 1KB. It (most likely) used the > offset into character cell memory, as output by the CRTC, as index into > a 1 KB block of bitmap memory, combining it with the vertical offset > into the character (0-7) (also as output by the CRTC) to select which bank. > (Except that the DWW was designed before the CRTC was used, but I'm sure > the same counters were present in the design anyway) Yes. However, this board is even more simple - no PIA, no I/O chips whatsoever, just some TTL glue logic. Maybe it uses some CRTC address outputs as signal lines (like the CBM-II does) because otherwise I don't know how it would even turn the graphics on. > Yes, that is what I read elsewhere too. The "extra 32K" aka the > "inaccessible 32K". Not that they're totally inaccessible: those 32 K > are the second part of the non-expansion RAM (the other 64 KB of the 128 > KB are expansion RAM), and therefore would map from $8000-$FFFF if there > were not ROM and I/O there. Yes, this is probable. So far I looked at the initialization code at $9000, and the first thing it does, it copies some routines to RAM at $8800. So most probably the bitmap data is in the extra RAM, and the graphics operations require banking out the ROMs - thus the routines itself must be copied to that RAM. I still do not know how it banks them out, though. > And in fact you can access parts of it: $8xxx is normal screen > (character cell) memory. $9xxx and $Axxxx can be easily jumpered to be > RAM as well (if you don't have EPROMs installed in the sockets of the > same address). And iirc there is also a 'jumper' option (but not so > easily accessible) to connect a user port line to the NOROM line and > make it possible to map out all ROMs, thereby uncovering even more of > that RAM). Well, there are ROMs at $9000 and $A000 of course, the HiRes routines and BASIC extension :-) > Given this, the interesting issue gets to be: Where is the bitmap, and > how is it accessed by the CPU? Yes, that's what I'm trying to figure out :-) > Given the resolution you mention (512 x 256) it would need 16 KB of > memory, or half of that 32 KB space. If normal text output would remain > working, and perhaps they thought it was nice if the $9xxx and $Axxx > would keep working as it was, it should be at $C000-$FFFF. Somehow. My guess too. And yes, the normal screen still works independently of the graphics - if you turn out the graphics, the text screen remains as it was (and vice versa). > I wonder why that resolution was chosen, and not 640 x 200 (which is 8 x > (80 x 25)). That would make it much more compatible with the text > screen. Maybe to retain compatibility with boards based on the Thomson chip, which used that resolution? Regards, Michau. Message was sent through the cbm-hackers mailing listReceived on 2012-09-30 08:00:04
Archive generated by hypermail 2.2.0.