Max scan line          index 09h  CRTC

Layout  b7     2T4  200-to-400 line conversion
        b6     LC   Line compare register bit 9
        b5     VBS  Vertical blank start register bit 9
        b4-b0  MSL  Maximum scan line

- 200-to-400 line conversion (bit 7)
        Allows a 200-line mode to be displayed on 400 display scan
        lines. This permits the 200-line display modes to fill the
        entire screen. Each line is duplicated twice by dividing the
        clock to the row scan counter by 2.

        0 : Normal operation.
        1 : Display 200 lines on the full display.

- Maximum scan line (bits 4-0)
        The number of scan lines in a character row minus one.