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Forums > C64 Coding > Bus Noise
2017-07-25 02:46

Registered: Apr 2012
Posts: 13
Bus Noise

D800/Color Ram Hi-Bits (xxxx0000) are allegedly not connected. Thus: fuzz.

Well but actually why then this:

(one is C64 with reloaded board, other one is vice, just to make sure (sadly vice returns what? some prerecorded signals?))

This is anything but random. So then what is it? Any clues?

2017-07-25 03:01

Registered: Dec 2001
Posts: 8211
what you read from unconnected i/o is the last value that was on the bus, usually what the VIC fetched in the previous cycle. it IS indeed anything but random, it is 100% predictable. VICE (x64sc) in fact emulates this correctly :)

tests are here: https://sourceforge.net/p/vice-emu/code/HEAD/tree/testprogs/C64..
2017-07-25 16:42

Registered: Dec 2001
Posts: 8211
mmmh now i think about it again... i am not entirely sure if that applies to the color ram under all conditions. a test program would be needed to verify this :) a simple loop like you used doesnt really give useful results however, what you need to do is testing this with a stable raster irq, so that the reads always happen at the same well defined cycles.
2017-07-25 20:54

Registered: Apr 2012
Posts: 13
Ah! Good idea. I had been trying to read stuff as fast as i could, but i had always thought about syncing, too. Will now do that. The other possibility i imagined would be reading out tons of data to an external target and then checking for patterns but to do that, i am waiting for a certain hardware add-on that currently unpublicly is being developed (i hope, still).
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