The
following notes apply
to
both the WAIT instruction and
to
the SKIP instruction,
which
is
described below
in
the "Advanced Topics" section.
HORIZONTAL
BEAM
POSITION
TJl
•.
~'~tat:~,,~'''_''~~~'M:~;$~
The least significant bit is Ilot
used'in
the
~~~p;ri;~n','s~
th~re
~re
.....
'iPo~iti~'~s
av~ilablef~~Copperoperations.
fTiiiS;,;
'i8A".,t;:ti7:~~~~,~jr{it~:lti~i~;f~(t8,,'.J$.
it
.:liig1f:~r'OOalutii~~
Horizon tal
blanking falls in the range of$'OF
to
$35. The
st~n(fard
screen (320 pixels wide) has an
unused horizontal portion of
$04
to
$47 (during which only the background color is
displayed).
VERTICAL
BEAM
POSITION
The
vertical beam position can be resolved to one line, with a maximum value of 255.
There are actually 262 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 posi-
tions), one of the simplest ways to handle this is shown below.
Instruction
[ ... other instructions ... ]
WAIT for position
(0,255)
WAIT for any horizontal position
with vertical position 0 through
6,
covering the last 6 lines of the scan
before vertical blanking occurs.
12
Coprocessor Hardware
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 instruc-
tions can
be
executed.
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