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.
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