These registers contain the modulos for the odd and
even bit planes. A modulo is a number that is automa-
itcally 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 seperate 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.