To be precise, the SMS and Gen/MD VDP versions of this feature work slightly differently from the TMS9918A version of it. The CSYNC output pin becomes an input

To be precise, the SMS and Gen/MD VDP versions of this feature work slightly differently from the TMS9918A version of it. The CSYNC output pin becomes an input and is used to synchronize video generation, but you need external hardware to mix the video together. This is also true of TMS9918A variants that output separate luminance and color difference signals like the TMS9928A. Generally this mixing is done with the analog video signals (RGB in the case of the SMS and Gen/MD, luminance/color difference for the TMS9928A), but you could use the digital pixel bus. The downside to that is you need external CRAM and video DAC. edit: Technically, the TMS9928A doesn't have the CSYNC as input part and just relies on the two VDPs running in lockstep.