I believe the rules for emulating another console, is the console your emulating it on has to have about 10x the processing power.
rules?? none, sorry. these kinds of answers are bogus, and don't help to stop children from asking. You need to give them the truth, not some thought up answer to shut up pondering minds. that reminds me of the problems with the american public school system. Teachers just feed bullshit to make there jobs as simple as possible, they just don't care about bettering anything other than theyre union contracts.
simply put, there is no set rules to emulation. that's the benifit of software coding; its all in the hands of the coder(s) how to program, some code faster routines given the coding libraries (for example look at snes GBA emulation), some code very poorly (look at a majority of early emulator releases).
just to give a living example of why these "rules" are bullshit, look at the xbox emulation on 360. now just for hypethetical purposes i'm going to use 10x as a rule. and say for our purposes, that the xbox gpu doesnt need emulation (hahaha) and so all that needs to be emulated is the intel 733 megahertz processor. now the 360's three core single process 3.0 gighertz processor cannot handle 7.33 gigherts of 32 bit single core code at once... but i enjoy many xbox games on my 360. namely halo2, GTA san andreas, and crimson skies.
i will say that an n64 runs at 93.75 megahertz. the Nintendo ds runs at 66 megahertz with a seperate 33 megahertz. now im not going into it but you cant add the two together to get 99 megahertz but it doesnt work like that. not to mention the nintendo 64 was exactly that: It is a 64-bit processor, in that it has 64-bit registers, a 64-bit instruction set, and 64-bit internal data paths. The nintendo ds contains no 64 bit processoes, no 64-bit registers, no 64-bit instruction set, and no 64 bit internal data path. If you are old enough to remember, the first n64 emulators on pcs had most of there processing power used to interpret and rehash these 64 bit instuction sets down to 32 for an intel processor which would typically run around 600-755 megahertz. There was slowdown... thats just beginning to rub the surface of why n64 emulation on nds will maybe never happen. please notice i say maybe, that's because anything can happen with software coding, and so as i have learned, never, ever, ever, say never.