I have to agree with Uniserver about the post I'm quoting a fragment of below. Not cool, dude. Everyone goes through the experience now and then of feeling like someone "stole my idea!", but... seriously, you're basing this on posting a question that was itself phrased as "What would take to get this accomplished?" in a thread about someone else actually doing the legwork to get the idea working on related hardware? Seems to me that if you *had* lifted a finger to do this yourself first, which clearly you didn't, BBraun and Dougg would have excellent grounds for b****ing about you "stealing" *their* idea.
(Not that I think it's particularly likely they would have objected unless you did something egregious like stolen their code without giving them credit.)
If you'd really wanted to participate in building this I suspect they would have let you in if you'd showed more interest in the idea than a single forum post. Did you even try?
Now, my current idea is adding more RAM to an Old Mac and give it some use somehow. In seeing the memory maps, I have to disagree that the 6522 and other VIAs on the Mac board takes up 1 MEG of memory space when these same exact same chips were on 8bit machines before the Mac and they only took up 1K or less of physical space! 1KB, compared to 1MB is a big difference...
Stupid question: Do you actually fully understand how address bus decoding works? I'll be short here: A single VIA, if fully decoded, would only occupy 16 bytes of address space, so, yes, you could fit thousands of them into 4MB. However, *MANY* cheap computers, and the Mac very much falls into this category, don't fully decode their address buses, in order to save on logic. Instead they'll carve out a bigger block of memory, call it "I/O" space, and simply cram some peripherals in there in such a way that address lines are used directly as chip selects. Think about it: you have a 24 bit address bus. (Okay, technically the 68000 has a 23 bit bus and an even/odd byte line, but it's effectively the same.) That means to fully decode an arbitrary 16 byte (4 bit) space you need 20 gates. If instead of doing that you say "eh, forget that, I'm swimming in address space so I'll just decode the four address lines and use the fifth for the chip select" you'll get exactly what you get in the the Mac, which is one little chip chewing up half a megabyte of address space. Strictly speaking the VIA is still only using 16 addresses, but those addresses are going to be repeated 2^15 times through that block. To share that space with RAM you'd have to:
A: Add decoding hardware to the Mac's motherboard to decode the other, what did I say, 15 address lines you didn't bother with? (I'd have to look at the schematic again to completely refresh my memory.) If you don't then the VIA (Or IWM, or SCC, or whatever) is going to be enabled at the same time the RAM is, and:
B: Bear in mind that although *hopefully* any software that addresses the VIA will only use the 16 contiguous addresses starting at the base of the big smeary block it resides in to talk to it, it's very possible that ill-behaved software that for some reason takes advantage of the address duplication may exist.
Seriously, this is why half the Mac Plus' address space is occupied by peripherals that should really only consume a few hundred bytes of real address space. I'm a little mystified myself sometimes why Apple designed it to be *this* sloppy, when they could have added something like a single 74LS154 to the board to provide some chip selects for peripherals inside a smaller space. (They were already using PALs anyway, one more PAL and they could have easily fit ROM and peripherals into, say, the top 4MB leaving 12MB for RAM.) But that's how it is, and that means that a plan to add more than 4MB of RAM to a 128k through Plus would have to do some serious remedial work on the motherboard. (Or, at least, hang an alternate memory system off the CPU socket that includes the additional decoding in front of the motherboard, which would essentially be treated like a "peripheral".)
Page 10 of this book has a good explanation as to why one VIA and two PIOs take up 2k of address space in a Commodore PET instead of 24 bytes. The Mac is this on steroids.
