```
The vertical beam position can be resolved to one line, with a maximum
value of 255. There are actually 262 NTSC (312 PAL) possible vertical
positions. Some minor complications can occur if you want something to
happen within these last six or seven scan lines. Because there are only
eight bits of resolution for vertical beam position (allowing 256
different positions), one of the simplest ways to handle this is shown
below.

Copper Instruction           Explanation
--------------------          -----------
WAIT  for position (0,255)   At this point, the vertical
counter appears to wrap to 0
because the comparison works
on the least significant bits
of the vertical count

WAIT  for any horizontal       Thus the total of 256 + 6 = 262
position with vertical        lines of video beam travel during
position 0 through 5,         which  Copper instructions  can be
covering the last 6 lines     executed
of the scan before
vertical blanking  occurs.

Note that the vertical is like the horizontal.
----------------------------------------------
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, then 313 lines.  This alternation of lines and
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
and back to the beginning...

One horizontal count takes one 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%

```

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