Cx4
The Cx4 chip is a math coprocessor that was used by Capcom to perform general trigonometric calculations for wireframe effects, sprite positioning and rotation. It is known for its role in mapping and transforming wireframes in Capcom's second and third Mega Man X series games. It is based on the Hitachi HG51B169 DSP.
A Cx4 self-test screen can be accessed by holding the 'B' button on the second controller upon system start-up in both Mega Man X2 and Mega Man X3. In both the PlayStation 2 and Nintendo GameCube versions of Mega Man X Collection, this self-test screen is still accessible in Mega Man X2 (although differently accessed due to the remapped controller configuration), but not in Mega Man X3, because Mega Man X Collection features the 32-bit CD version of the game and not the SNES version.
The Cx4 chip has been emulated in several popular SNES emulators, such as ZSNES and Snes9x, allowing both games to be playable with those emulators given that the user has their ROM images. In June 2011, bsnes added low-level emulation of the chip, significantly improving emulation accuracy compared to the high-level emulation approach used by previous emulators.
Read more about this topic: List Of Super NES Enhancement Chips