These registers contain the modulos for the odd and even bit planes.
A modulo is a number that is automatically added to the address at
the end of each line, in order that the address then points to the
start of the next line. Since they have separate modulos, the odd
and even bit planes may have sizes that are different from each
other, as well as different from the display window size.
If scan-doubling is enabled, BPL1MOD serves as the primary bitplane
modulos and BPL2MOD serves as the alternate. Lines whose LSBs of
beam counter and DIWSTRT match are designated primary, whereas lines
whose LSBs don`t match are designated alternate.