What to set in V9938 register 9?

By Sdw

Resident (50)

Sdw's picture

19-08-2015, 23:32

I want to enable 212 line mode. Simple, I just set the highest bit of VDP register #9, and it works fine.
However, there are a lot of other bits in this register that I'm unsure of. Simultaneous mode? Dot clock?
What am I supposed to set in these registers? As for now, I just set 0 to everything (except PAL mode, I want 50Hz) but is this correct? Is there no way to keep whatever there is, and just modify the bit I want?

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By Manuel

Ascended (15802)

Manuel's picture

19-08-2015, 23:35

AFAIK what is written to the VDP registers should be mirrored in part of the system variables. So you could read out the value there and write it with the proper bit set. (And update it there too, or use the BIOS routine to write it, which I assume updates it for you.)

However, I'm not an expert at all, so this could be wrong.

By madscient

Master (179)

madscient's picture

20-08-2015, 18:45

You can find the mirror in &HFFE8.

See below:

By Grauw

Ascended (8507)

Grauw's picture

20-08-2015, 20:25

Indeed you should read (and update) the mirror value.

See also: http://map.grauw.nl/resources/msxsystemvars.php#RG9SAV