Be sure to have read through the 6502 primer . (Maybe you have already. I don't remember if you've said.) It was written after our years of seeing the same problems and questions come up over and over on the forum.
Thank you! I remember cruising down this a couple of months back but had forgotten ...

Thanks guys. My particular concern (which may not be the correct concern!) is the parameter called THR in the MOS/Commodore datasheets--the 10ns period that begins when Phi2 falls to .4V during which the data being supplied to the 6502 is expected to remain valid.

To answer Garth Wilson's question ...

Sooooo...

I created the following thread a couple of weeks back regarding building an NES music player:
http://forum.6502.org/viewtopic.php?f=12&t=5099

I set off down this path with high hopes, but as soon as I got to where I was dumping music to play, I found that far too many of my favorite game ...

Another possibility is to forget NMI and simply use RST only. It means one less vector that needs to be mapped properly, and again that might be helpful.

This is another thing I'd wondered about, but set aside for fear that asserting RST might reinitialize certain internal audio registers, and ...

- yes, in NMOS 6502s, including Ricoh's 2A03, the stack pointer, together with A, X and Y, are indeterminate at power on.

Ah! Thank you for this. This hadn't turned up in my own searches, and I'd certainly have been pulling my hair out--what little I have left, anyway!--if I'd built the thing and ...

If it were imperative to gain another byte then the double-duty operand/opcode would be worth it. But merely to save a diode... I dunno.

That's a very reasonable perspective, but I think we're already deep in the weeds (thanks to you :wink:). This project screams "abuse" right from the get-go ...

I hadn't taken the diode pruning too seriously in the beginning, but it's a fun exercise, yes. And while certainly not needed, the $0002 NMI vector is yet another diode saved as well, so...*shrug*... why not?

Hi Jeff -- Ah! You beat me to it. I was considering the now extremely underutilized second '138 and how to get rid of it. To make sure I understand the hijacking concern (which, according to the nesdev forums, is a valid concern on the 2A03): The BRK in the NMI ISR would be considered safe because ...

Aaron, anyone who notices opportunities to do something bizarre -- and worries about missing such opportunities! -- is good folks as far as I'm concerned. :) Welcome! I think I like your inline operands even better than your diode matrix!

But you've made a minor booboo, I think. You said, on RTI ...

Now that's an interesting idea and an interesting circuit! I'm a big fan of diode ROMs, perhaps partly because they are rarely seen. (I think it would be good for you to repost that in the hardware thread with a suitable title - you might not catch your audience in this thread!)

I'll certainly do ...

Thank you both for the kind welcome and the quick replies!

Well, assuming your microprocessor (MPU) reset circuit works properly, all of the MPU's outputs would be deasserted at reset and hence your glue logic should not be asserting any /OE or /WE inputs on your memory and I/O hardware. Can you ...

Hello! I'm Aaron -- First post! Woo-hoo!

I'm not a credentialed EE, just an enthusiastic electronics hobbyist who's worked with microcontrollers for a while and has finally resolved to take on 6502 project. I've got what I believe to be a good (if somewhat weird) design worked out, but my question ...