Vertical Line Count Offset Register

Number	TBRegisterNumber::$64
Readable	TBRegisterReadable::Yes
Writable	TBRegisterWritable::Yes
Short Description	ShortDesc::Offset numbering of raster lines in copper/interrupt/active register

Since core 3.1.5 (new register):

Bits 7-0 form offset value 0..255, the offset is added to Copper, Video Line Interrupt and Active Video Line readings.

Normally the ULA's pixel row 0 aligns with vertical line count 0. With a non-zero offset, the ULA's pixel row 0 will align with the vertical line offset.

Eg, if the offset is 32 then video line 32 will correspond to the first pixel row in the ULA and video line 0 will align with the first pixel row of the Tilemap and Sprites (in the top border area). Then Copper WAIT command waiting for line 0 will happen already in top border area.

(also Copper mode %11 restarting CPC=0 at pixel [0,0] will be offset - yet to be confirmed by Allen or VHDL source - TODO)

Since a change in offset takes effect when the ULA reaches row 0, the change can take up to one frame to occur.

(also related: {{#ask: TBRegisterNumber::$1E }} ($1E), {{#ask: TBRegisterNumber::$1F }} ($1F), {{#ask: TBRegisterNumber::$22 }} ($22), {{#ask: TBRegisterNumber::$23 }} ($23) and Copper)