The Comparison Enable Bits - Commodore Amiga A1000 Hardware Reference Manual

INSTRUCTION
[ ... other instructions ... ]
WAIT for position (0,255)
WAIT for any horizontal position with
vertical position 0 through 256, covering
the last 6 lines of the scan before vertical
blanking occurs.
NOTE
The vertical is like the horizontal - as there are alternating long and short lines, there are
also long and short fields (interlace only). In NTSC, the fields are 262, then 263 lines and
in PAL, 312,313.
This alteration of lines & fields produces the standard NTSC 4 field repeating pattern:
short field ending on short line
long field ending on long line
short field ending on long line
long field ending on short line
& back to the beginning...
1 horizontal count takes 1 cycle of the system clock. (Processor is twice this)
NTSC- 3,579,545 Hz
PAL- 3,546,895 Hz
genlocked- basic clock frequency plus or minus about 2%.

THE COMPARISON ENABLE BITS

Bits 14-1 are normally set to all 1s. The use of the comparison enable bits is described
later in the "Advanced Topics " section.
- Coprocessor Hardware 19 -
EXPLANATION
At this point, the vertical
counter appears to wrap to 0
because the comparison works
on the least significant bits
of the vertical count.
Thus the total of 256+6 = 262
lines of video beam travel
during which Copper
instructions can be executed.