Re: $c019 softswitch

[david@uow.edu.au (David Wilson)]

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.
--
David Wilson  School of IT & CS, Uni of Wollongong, Australia  david@uow.edu.au