The Vertical Blank Register (output port $A) contains the line number where vertical blanking will begin. The register must be set to 101 or less. Bit 0 should be set to 0 and the line number should be in bits 1–7. The background color will display from the vertical blank line to the bottom of the screen. This allows the screen RAM that would normally be displayed in that area to be used for scratchpad. If the vertical blank register is set to 0, the entire RAM can be used for scratchpad.

The color register map shows which color registers are used to define colors in different areas of the screen. The map assumes the background color is set to 0. If it were set to 1 then color registers 1 and 5 would be used for background instead of 0 and 4. The color boundary is between bytes X and X-1.

[NM:85-91]