VDP commands in SC4

By Metalion

Paragon (1343)

Metalion's picture

04-06-2020, 16:56


Are the VDP commands totally disabled in SC4 ?
Or is it just some of them ?

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By Grauw

Ascended (9677)

Grauw's picture

04-06-2020, 18:03

On the V9938 VDP commands do not work in modes below screen 5. Actually, I seem to recall reading they do work during blanking, but don’t quote me on that. On the V9958 they added a flag you can set to enable VDP commands to work in these modes, then they behave as if it is screen 8.


Enlighted (5735)

NYYRIKKI's picture

04-06-2020, 20:24

I also remember something like that if you switch screen mode during blanking to SCREEN 5 or 6 you may be able to do something... Is it worth the trouble is another question... On V9958 the feature is pretty useless due to VRAM interleaving that is used on SCREEN 7 and up... Or at least you need to have extremely special need... Maybe ok, if you place color table and pattern generator table 64kB away from each other(?)

By Metalion

Paragon (1343)

Metalion's picture

04-06-2020, 20:29

Thanks, but my problem is of another nature ...

I do a bunch of VDP commands in SC5 and I would have liked to do a screen split SC4/SC5. The problem is that I need 3 frames to do all my VDP commands (without the screen split), and when I do the screen split, the VDP commands are disabled in the SC4 part of the screen. So it does not work.

One solution would be to limit the VDP commands to the SC5 part of the screen, but then it would mean that I would need a lot more frames to finish the job.


Enlighted (6492)

ARTRAG's picture

04-06-2020, 22:40

I did something about using commands in vblank while the rest of the screen is in text


Sadly all commands ends when you change screen mode

By sd_snatcher

Prophet (3435)

sd_snatcher's picture

05-06-2020, 14:46


Sadly all commands ends when you change screen mode

That's sad.

Related question: Anyone knows what happens when you change from sc5 to sc8 when a blitter command is running (and vice-versa)?

By PingPong

Prophet (3653)

PingPong's picture

05-06-2020, 16:38

the memory addressing change because of interleaving?.