Posted By
siz
on 2023-04-01
09:02:11
|  Re: New Project: Hannes-compatible 256k RAM Expansion
Yeah, I was not aware that this goes under the TED (my 6502plus4 is a CPU replacement).
Today I had a little time to check Dream World. Yes, it really fails on machines with more than 256k RAM. The problem is that the power-up value of the RAM bank controller ($fd16) is $ff and for 256k RAM only the lower two bits are used but for 512k it's 3 bits.
This demo doesn't follow the rule that unused bits should be kept at their default values for future compatibility and changes bit 2 of the register (A18) from 1 to 0 - resulting in a crash. I've patched the loader to try it out but the 1541 loader also seems to be broken and I didn't have the patience to dig out a 1551 and real floppy disks to try that.
By putting a breakpoint in YaPE to catch $fd16 writes shows that this demo really uses the 256k expansion.
A quick workaround that came to my mind:
from BASIC:
MONITOR
>fd16 80
gfff6
And then load the demo. That way the RAM address bits will be initialized as the demo assumes. But that one fails too so there must be some other values it uses to write $fd16.
The conclusion is: the demo needs to be patched to work on machines with more than 256k memory. And on a 1541. :/
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