Subject: Re: [stella] Reverse engineering the TIA (Was: Midified VCS...) From: "Roger Williams" <mer02@xxxxxxxxxxxxx> Date: Thu, 6 Dec 2001 19:37:20 -0800 |
From: Oliver Achten <achten@xxxxxx> > I intend to redesign it using 74xx IC's (don't laugh...) . Preferably > F-Types. It's tempting, but then I'm the guy who is seriously thinking of writing a compiler, so what the hey. To each his own windmill to tilt at, and all that :-) > > Well, i looked at the counters, and they indeed consist of 6 self-feedbacked > shift registers, driven by a two phase clock , using some kind of wired-and > logic to decode various states. I read in some background articles that this arrangement is called a "polynomial counter" and allegedly has the advantage of much reduced silicon over standard ripple counters. >Fortunately, J. Miner wrote at the H-Sync > counter, which number causes some kind of action (H-SYNC start, etc.) The H-SYNC > counter is clocked 1/4 the clock of the master clock (3.58Mhz). Once I > figured out in what order it counts, this counter could be replaced by a standard > binary counter, since it really is not important in what order the H-Sync > counter counts (as long as your supply logic decodes the same states...). > Unfortunately, this is not valid for the player counters (because of the > sideeffects)..... Actually I think your H-sync and player counters can be of any type as long as they are of the _same_ type. The alleged disadvantage of the polynomial counter is that your output doesn't emerge in any sensible order, which is why only a "reset" and no preset was provided. (Given what was later done with table lookups, this may have been a very shortsighted move on Atari's part.) > The bus logic and sync decoding are not really a problem to do in TTL. The > Playfield registers are tricky. Could be replaced by latches and a flip-flop > chain, which is logically anded with the outputs of the latches. There should be some prepackaged MSI shift registers that would work here. 74165 or 74166 maybe? > The H-Move counters... Any idea? Any presettable counter wired to cut off its own clk input (along with that fed to the "extra cycles" input of the position register) at an appropriate point. >Well... Lets go to the next sheet... ;-) Looks like you have your work cut out for you. I doubt if you'll be able to get it exactly compatible with a reasonable amount of effort but you could probably get it close enough to work with most games. --Roger Williams ---------------------------------------------------------------------------------------------- Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/
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