Bit plane modulo (even planes)
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 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.