Classic games consoles played their games from cartridges, plastic bricks that held a PCB with the game code on it ready to be run by the console hardware. You might therefore expect them to be an easy prospect for emulation, given that the code can be extracted from whatever ROM they contain. But as anyone with an interest in the subject will tell you, some cartridges included extra hardware to boost the capabilities of their games, and this makes the job of an emulator significantly more complex.
[Byuu] has penned an article exploring this topic across a variety of consoles, with in-depth analyses of special-case cartridges. We see the obvious examples such as the DSP coprocessors famously used on some SNES games, as well as Nintendo’s Super Game Boy that contained an entire Game Boy on a chip.
But perhaps more interesting are the edge-case cartridges which didn’t contain special hardware. Capcom’s Rockman X had a copy protection feature that sabotaged the game if it detected RAM at a frequently used save game address emulated by copiers. Unfortunately this could also be triggered accidentally, so every one of the first generation Rockman X cartridges had a manually attached bodge wire that a faithful emulator must replicate. There is also the case of the Sega Genesis F22 Interceptor, which contained an 8-bit ROM where most cartridges for this 68000-powered platform had a 16-bit part. Simple attempts to copy this cartridge result in the upper 8 bits having random values due to the floating data lines, which yet again an emulator must handle correctly.
It’s a subject with a variety as huge as the number of console developers and their games, and a field in which new quirks are constantly being unearthed. While most of us don’t spend our time peering into dusty cartridges, we’re grateful for this insight into that world.
We’ve visited the world of emulators a few times before, such as when we looked at combatting in-game lag.
In the mid-90’s Nintendo released the Super Game Boy, a cartridge for the SNES which allowed you to play Game Boy games on the big screen. Each cartridge was in fact an entire Game Boy with the appropriate hardware to present it in a way the host console could interface with, but missing some of the hardware a standalone Game Boy would include like a link port to connect it to another system. This mod fixes this limitation by bridging the correct pins out from the CPU to a breakout board which includes the link port connector. For general background on what’s going on here, check out 
