Set_Refresh ($F1A2)

This routine loads the refresh timer (t2) with the value in $C83D- $C83E, and recalibrates the vector generators, thus causing the pen to be left at the origin (0,0). The high order byte for the timer is loaded from $C83E, and the low order byte is loaded from $C83D. The refresh rate is calculated as follows:

rate = (C83E)(C83D) / 1.5 mhz

ENTRY:

DP = $D0

EXIT:

D-reg trashed

Source