Hi;
I've recently been reading a lot about the Master System and its TMS-based VDP. It's almost exactly a TMS with one extra mode. It takes a completely different approach from the V9938 to increase video bandwidth over the TMS though, by switching to a 16-bit data bus but otherwise retaining essentially the same line timing as the TMS. Not identical but clearly a minor adaptation of the same thing.
One of the other interesting things it does is to generate sprite collision and overflow flags even in the border. Not just the horizontal border, but also the vertical and while the screen is blanked. That's despite also offering improved memory access availability while blanked or in the vertical border.
So it seems that Sega's designers implemented some light-touch sprites-only fetch pattern for blank and vertical border periods. But that's while generally hewing as closely as possible to the TMS' scheduling and functionality.
If you think about it, this would actually be a useful feature for the TMS — it means that if you unblank the display partway down, sprites are correct from the very first output line. Which was possibly Sega's motivation but there's no evidence either way.
So I was just curious:
- is it known whether the TMS generates sprite collisions in the horizontal and/or vertical border?
- supposing it does, is it possible that it isn't refreshing RAM "multiple times" as Mattias postulates, but spending some of those 'refresh' accesses on sprite activity?
Apologies for the probably-stupid question; I've been unable to answer it for myself.
