If the Sega Genesis/MegaDrive could be overclocked so easily, why couldn't the SNES?

Question

I discovered quite a number of videos on youtube demonstrating the effectiveness of over-clocking the CPU of Sega Genesis/MasterDrive. The video features games that are known for their intensive rendering procedures such as Flash Back, Pebble Beach Golf and Hard Drivin'. As one might have guessed, the games ran at a much higher fps rate and looked smoother. It also appears that the actual modification of the console is surprisingly simple, involving connecting the clock pin of the 68000 to new crystal.

This naturally made me wonder how such behavior was possible. It is known that many older 8bit computers such as the C64 where unable to support over clocking since the CPU clock was also tied to the video clock. Any changing of video clock would result in unstable video output. For over clocking to be useful, a system that supports chips with different clocking rates would be needed. Hence, only game logic would be speed up and not core system I/O interface. The Genesis does in fact satisfy this precondition.

Doing a quick search for over clocking on SNES results in a limited success. I can only find examples with the SuperFX chip. At the hardware level, why has the SNES seen so little success in over clocking of games?

Answer 1

The Super NES (SNES) has a much different hardware architecture than the Sega Genesis, and is built around the custom Ricoh 5A22 ASIC. As opposed to the discrete, stock, Motorola 68000 CPU employed in the Sega Genesis/MegaDrive, the SNES ASIC contains multiple components that internally operate at different clock speeds. The ASIC has the 65C816 CPU core, along with the interfaces to the coprocessors and I/O, which are designed to operate at different clock frequencies, but all under the control of the ASIC. So, unlike in the Sega console, where you can simply overclock the CPU, attempting to overclock the SNES ASIC actually affects the speed of multiple component interfaces. There's just no way to overclock the CPU core by itself, because of this high degree of integration within the single chip ASIC.

Besides the fact that the Sega console uses a discrete component CPU, allowing its clock source to be substituted with a faster clock, it also has a split-memory design allowing asynchronous sharing of the bus and RAM between the CPU and the Video Display Processor (VDP). Access to VRAM (other than the VDP using it to draw the display) is fully asynchronous to the CPU and usually done either via the VDP's FIFO queue or through DMA. Thus, the 68000 running at a higher clock speed is only going to be accessing Program RAM or VRAM during blanking. For this reason, the faster CPU processing doesn't break the normal display handling, but does allow the game to run smoother since the CPU can do more when it is in control. Likewise, sound playback will also not be effected by the faster CPU since that is done asynchronously by the Z80 coprocessor, still running at its normal speed.

Answer 2

It's an issue of integration. On the early Genesis systems the custom chips and CPU were physically separate from each other so it was easy to isolate the various clock signals and replace the CPU with a part rated for faster operation.

The SNES has a more tightly integrated design where an ASIC contains both the CPU and other components, making it harder to isolate and control one clock domain (say the CPU) from another (video or sound).

The later revisions of the Genesis hardware have this same limitation as they started merging more custom parts, as well as the CPUs into a smaller amount of larger custom chips to reduce cost. So an overclock modification on a single-custom-chip Genesis 3 is difficult compared to the original hardware with three-custom-chips and a stock CPU.