The Super Game Boy cart is basically this. The cartridge contains actual Game Boy hardware, wired up to render to a frame buffer, and 60 times per second it's copied to the SNES VRAM.
The trick is that the Game Boy display is smaller than the SNES display, so it doesn't have to transfer a complete frame, and so it can be completed within the vertical blanking interval.
Apparently the timing is so close that it's not possible to transfer palette information alongside the video, just black-and-white, which is why there was never a Super Game Boy Color.
The trick is that the Game Boy display is smaller than the SNES display, so it doesn't have to transfer a complete frame, and so it can be completed within the vertical blanking interval.
Apparently the timing is so close that it's not possible to transfer palette information alongside the video, just black-and-white, which is why there was never a Super Game Boy Color.