Thank you CosmicR;

Just wondering about timing the write after the raster beam passed the active display, that is the Vsync interval?

If that what it is, it's not difficult to have the writes execute after that, I can detect when the NeoGeo is starting Vsync using the vblank interrupt, and write at that time.
(Issue is preparing the data to be written beforehand, which take up ram)

Or maybe I missunderstood?