I've wondered this since I was a kid but never took the time or money to buy multiple cartridges and experiment. When you lock-on S&K to S3, there are two extra save slots. My assumption has always been that those save slots were always there on the S3 cartridge, hidden, waiting to be used by S&K. Is this correct, or does the S&K cartridge have a little extra battery backup memory?
Only Sonic 3 has SRAM, but it allows for independent saves for both S3 and S3&K. It's just that it copies your Sonic 3 saves over to the S3&K slots the first time you connect the cartridges.
Oh dang, that would allow you to fight Big Arm and then continue to the S&K levels with the same save. I wish I knew that when I was little.
More specifically, there's 14 over the two games: S3 has 6 save slots while S3&K has 8, meaning that once it copies over the data for use in S3&K you have two spare at the end. This makes sense if you think about it - you get 2 slots each for Sonic, Tails, and Sonic & Tails, then there's 2 free for 2 Knuckles playthroughs.
Yup. One runthrough without Emeralds, and one with. I hope that's how everyone else did it anyway (Although I was always a bit miffed that the unique charater pictures for each save would get replaced with a universal "All Clear" pic when you'd done all your super emeralds. I liked that Super Knuckles face!)
That's why my "perfect save" for those eight slots has long been this setup from left-to-right: SUPER EMERALDS: Knuckles, Sonic, Tails CHAOS EMERALDS: Sonic, Knuckles NO EMERALDS: Knuckles, Sonic, Tails Tails doesn't have a unique "all-emeralds" portrait (or a Chaos transformation of any kind), so this gets you all of 'em. Though it IS still odd that even once you get all the Super Emeralds on a complete file, it doesn't update the portrait until you complete the game again.
This makes a lot more sense than how I used to do it! I used to count 'Sonic & Tails' as a seperate file and disregard the middle 'Chaos but not Super Emeralds' run. But I've seen the error of my ways. The next time S3&K gets a re-release/port (hah) I might do it like this.
Fascinating. So the Sonic 3 Cart has 14 saves the whole time. That's truly fascinating. I knew they planned for Sonic and Knuckles the whole time but I didn't realize exactly to what extent! This also means that Sonic 3 is 'aware' of the Super Emeralds too... to the extent that it has at least one extra byte ready for each S3&K save slot to hold 7 extra boolean values.
Not really. Sonic 3 just has way more SRAM space in the cart than it ever uses, and Sonic & Knuckles borrows some of that blank space. Sonic 3 itself never touches anything in that region, and has a fairly comprehensively different format for its own saves, which have no room for super emeralds, and still map Flying Battery between Carnival Night and Icecap, for example. When you load up Sonic 3 & Knuckles and have no S3&K save data, it reads the Sonic 3 format, and writes equivalent saves out into the separate space in its own format. The two sets of saves are never linked again, unless you delete all your S3&K saves and restart.
There is a video of someone managing to break the game with debug mode so badly that the data select gains about 100 save slots. It's too bad there isn't a disassembly of Sonic 3 alone because there is a bit of leftover data from Sonic & Knuckles still present in the ROM. One thing for sure is that the super emerald special stages don't seem to exist or their respective palettes and changing the emerald count to 14 simply grants Super Sonic. Unless of course the 8th special stage is actually one of the super emerald speical stages and not just an unused design of some sort.
You realise that the Sonic 3 & Knuckles ROM is literally the Sonic 3 ROM added to the end of the Sonic & Knuckles ROM, right? The Sonic 3 & Knuckles disassembly therefore includes the leftover data in the Sonic 3 alone part.
I don't think it does, it's purely a Sonic & Knuckles disassembly, with all the necessary data (and some code maybe?) that's needed for S3K to function added on when you build it with the "Sonic3_Complete" flag enabled.
It is linked in the Sonic 3 & Knuckles chronology and endings article (though it's easy to miss), which is itself linked in a few places, including the S3&KOmni template which is not used in any pages now, as articles have been changed to use S3Omni and S&KOmni templates instead.