This is something relating to Clownacy's post earlier: As the VDP manual claims, DMA may fail if two conditions aren't met: The destination address write must be a word write The write must be from work RAM (no d0 move to VDP directly!) As Clownacy's post suggests, he found an occasional crash with the first condition not met. Well, it seems I have found a crash where the second condition was not met (the first wasn't initially either, but it got ruled out pretty quickly as not causing the bug). For some context: While working for a screen, we noticed something strange happening; random tiles appearing and the game eventually freezing, sometimes with the Z80 continuing to work, sometimes not. This screen used a lot of DMA's to update everything necessary. As it turns out, it occurs in a very specific routine, and after a lot of experimentation, loading the whole DMA command longword into RAM, and then loading it from RAM word by word fixed these issues. After further experimentation, only the last word was required to be written from RAM, as the manual suggests. What is strange about this, is that this bug only occured on my 88' (model series 98M) model 1 Japanese SEGA Mega Drive. My model 1 Genesis did not exhibit this behaviour. Similarly, changing the base game to write the entire command directly into VDP did also not trigger this bug at all. It looks as if this is an actual VDP bug at the very least with early revisions of Mega Drives, but it either needs very specific circumstances to trigger or it triggers only very occasionally.