u8 in RAM at 0xE1E0 called SCANLINE_COUNT
76543210
CCCCCCCC
C: R. VCOUNT# (Current VCOUNT#)
NOTE: Values of 240+, are actually negative values, before the first scanline is rendered.
Values of 0..239 are on-screen scanlines.
u8 in RAM at 0xE1E2 called SYSTEM_COPPER_TIMING
76543210
xxxxTTTT
T: RW 0..15 = 128*n cycles will be wasted before beginning to render a scanline. Wasted cycles are great for the copper,
since it can use that time to setup video regs, palette colors, and do DMA's, before the first pixel of the scanline
is computed. Of course, those wasted cycles are not available for rendering more complicated scenes, but even at maximum
delay, you should still be (just) able to render an entire scanline full of sprites. Note also, that this value can
be written by the copper, so different scanlines could dedicate different amounts of time to the copper.
u8 in RAM at 0xE1E3 called FRAME_COUNT
76543210
CCCCCCCC
This register increments on every new frame. It is tied to the VBlank.