Subject: RE: [stella] Stella 2.0 Alpha 2 available From: "B. Watson" <atari@xxxxxxxxxxxxxx> Date: Wed, 29 Jun 2005 09:11:36 -0400 |
On Wed, 29 Jun 2005, Thomas Jentzsch wrote: > > Fred wrote: >> Clearing random bits by ANDing with random values, to be exact. Fred, did you ever try exclusive OR instead of AND in your code? > But when you repeat this code multiple times, wouldn't then all registers > end cleared completely? > BTW: When does the code get executed? During VSYNC? It gets executed once per frame, after the last scanline where VSYNC is active. At least it does in CVS Stella: I think Fred's version would have to do the same thing, the way he's described it. This means that the 6502 will run for a full frame, storing and modifying RAM and the TIA as usual, in between runs of the randomizing code. This should keep everything from being reset to zero and staying there... The TIA registers aren't being randomized exactly the way RAM is: the TIA locations access different regs depending on whether you're reading or writing (e.g. $04 is NUSIZ0 when writing, or CXM0FB when reading). The frying code reads from each location, ANDs the result with random bits, then writes it back to the same location. When reading, the unconnected bits (such bits 0-5 of CXM0FB) take whatever's on the data bus (usually the location being read)... Hm, and reading from $1x, $2x, or $3x actually means to read from $0x... much funkiness going on there, really. I wonder whether the TIA scrambling actually affects the game state much. Seems like the TIA state is ephemeral, whereas persistent changes would involve RAM. Will try looping only over RAM and see what that does. -- B. Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://stella.biglist.com
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