Preset row scan        index 08h  CRTC

Layout  b6-b5  BP   Byte panning
        b4-b0  PRS  Preset row scan

- Byte panning (bits 6-5)
        This field controls the number of bytes to pan during a
        panning operation. The Horizontal pixel panning register
        determines the number of pixels to pan, and this field
        determines the number of bytes to pan. Together, these fields
        allow up to three characters to be panned.

- Preset row scan (bits 4-0)
        The starting row of a character box displayed on the top row
        of characters on a screen. Normally, the value in this
        register is 0, which displays the entire vertical extent of
        the top row of characters.

        Used for smooth scrolling.