Re: $c019 softswitch

[greg@apple2.com]

In article <david.925790325@wraith>,
david@uow.edu.au (David Wilson) wrote:
>bobryan9@my-dejanews.com writes:

>> At ftp://ground.ecn.uiowa.edu/apple2/MiscInfo/memory.softswitches there is
>> a list of softswitches.
>>
>> What is this softswitch used for, and what does it do?::
>>
>> RDVBLBAR        =       $C019           ;not VBL (VBL signal low)

> ReaD Vertical BLanking (active low). Bit 7 of this location will be a 0
> when the video beam is in the vertical retrace region. This can be used
> to force updates of the video screen to be synchronised with the 50/60Hz
> refresh - ie you will never get a screen with half old data and half new
> displayed. It can also be used to allow fancy split screen effects (ie
> 1/3 HIRES, 1/3 TEXT, 1/3 LORES or other combinations). It is not a
> softswitch, rather it is a status flag.

Is there demo code available on this?  (I'm envisioning a breakout-style
game with lo-res bricks at top, hi-res paddle, and animated text effects
in the middle.)

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