Small inconsistency with 9938 VDP timings...

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By Grauw

Ascended (8373)

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04-08-2019, 16:54

What systems can “freeze” the VDP / video? It doesn’t seem possible, the CRT beam does not stop, if it did the screen would go black. The only such technology I can think of is Adaptive Sync / FreeSync / G-Sync supported by modern graphics cards and displays.

On the other hand there are several systems (e.g. NES) where the CPU can only write to VRAM while the VDP is in vertical blanking, so we don’t have it so bad really with just a little slow-down.

By shram86

Expert (88)

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04-08-2019, 20:22

I suppose I didn't explain that properly. The CRT of course doesn't freeze (haha), that also doesn't make sense when running hardware on non CRT displays. I was specifically referring to the C64 where I believe there is a way to skip video rendering, or perhaps it was Gameboy where the behavior of LCD is such... I thought I recalled it just setting a bit, but thinking on it more I believe it's more of an interrupt trick.

By Grauw

Ascended (8373)

Grauw's picture

04-08-2019, 21:09

Ah, GameBoy with its LCD sounds like it might be possible to have a variable refresh rate. I’ve certainly seen people change the clock frequency on Youtube which affected the display rate but everything kept running as normal (a CRT would immediately lose sync).

I can’t imagine that in this regard the C64 could do something that the MSX can’t, so if you happen to find more details in the future please post it, it could always be inspiring to see how to apply it to MSX.

By DarkSchneider

Paladin (860)

DarkSchneider's picture

05-08-2019, 11:21

Grauw wrote:

On the other hand there are several systems (e.g. NES) where the CPU can only write to VRAM while the VDP is in vertical blanking, so we don’t have it so bad really with just a little slow-down.

That's very interesting. It is curious that I had the same idea, even if not required, to do on MSX, and that's why my intention for using async drawing.

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