View source for Vertical Line Count Offset Register

{{NextRegister
|Number=$64
|Readable=Yes
|Writable=Yes
|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 vertical line 32 will correspond to the first pixel row in the ULA and vertical line 0 will align with the first pixel row of the [[Tilemap]] and [[Sprites]].

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: {{NextRegNo|$1E}},  {{NextRegNo|$1F}},  {{NextRegNo|$22}},  {{NextRegNo|$23}} and [[Copper]])