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The SuperFX chip was an additional RISC processor includes in some Super Famicom cartridges to generate 3D graphics, the most famous example being the game Starfox.
However it's performance wasn't particularly impressive, with games on other 16 bit machines such as the Amiga demonstrating higher frame rates with similar levels of detail (e.g. the game No Second Prize).
What were the factors limiting the performance of the SuperFX chip for 3D graphics? Calculation speed, memory bandwidth, transfer speed when copying data to the Super Famicom for display?
There are two main limitations with SuperFX that limited its performance and the level of 3D gaming that could thus be accomplished (e.g how many polygons/second).
The chip itself only provided limited computational abilities. Mostly, it allowed up to 16x16 bit signed and unsigned multiplies with adequate working register set. This made it far faster than the SNES's own CPU for polygon math and bitmap scaling, but it is not capable of the parallelism or SIMD instructions that would become common on more advanced GPU's. These computational limits set an upper-bound on the number of polygons in a frame.
The memory architecture was not unified, and there was a lot of latency in transferring data from the SuperFX working framebuffer RAM to the SNES VRAM. It would typically take 2 to 3 frames just to transfer one frame worth of data from the RAM on the cartridge (which the SuperFX chip was rendering to) over to the VRAM in the SNES. This bandwidth limitation alone could limit the system to around 20 FPS, depending on how large of a framebuffer the game needed for the 3D calculations.
SuperFX chip was limited, among other things, in power consumption - the console could not really give a lot of power to the cartridge port.
The memory architecture significantly limited the performance not only by the HDMA speed of 2.7MB / s, but also by access to ROM / RAM. The code from the 512-byte cache executed, in general, almost three times faster.
The SNES graphic data format with elements of bitplanes in a tile engine forced the architecture of the chip to be complicated and significantly limited the performance in some games.