Reset0Ref ($F354)

This routine zeros the integrators, and resets the pen back to the origin. It leaves the integrators in zero mode, so nothing can be drawn until a move is done, or $D00C is set to 0xCE to bring /ZERO high. This routine must be called every so often, to prevent your vectors from getting out of whack.

ENTRY:

DP = $D0

EXIT:

D-reg trashed

Source