Commodore 128D - Page 330

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PEN
—Return X and Y coordinates of the light pen in control port 1.
PEN(n)
where n = 0 PEN returns the X coordinate of VIC light pen position,
n = 1 PEN returns the Y coordinate of VIC light pen position,
n = 2 PEN returns the X coordinate of the 80 column display,
n = 3 PEN returns the Y coordinate of the 80 column display,
n = 4 PEN returns the (80-column) light pen trigger value.
Note that, like sprite coordinates, the VIC PEN value is not scaled
and uses real coordinates, not graphic bit map coordinates. The X
position is given as an even number, ranging from approximately 60
to 320, while the Y position can be any number from 50 to 250.
These are the visible screen coordinate ranges, where all other val
ues are not visible on the screen. A value of zero for either position
means the light pen is off screen and has not triggered an interrupt
since the last read. Note that COLLISION need not be active to use
PEN. A white background is usually required to stimulate the light
pen. PEN values vary from CRT to CRT, and one light pen to another..
NOTE: The VIC (40 columns) provides only 9 bits of resolution, so the
accuracy is limited to two pixels.
Unlike the 40 column (VIC) screen, the 80 column (8563) coordinates
are character row and column positions and not pixel coordinates
like the VIC screen. Both the 40 and 80 column screen coordinate
values are approximate and vary, due to the nature of light pens. The
80-column values are not valid until PEN(4) is true (non-zero).
EXAMPLES:
10 PRINT PEN(0);PEN(1) Displays the X and Y coordinates of
the light pen (for the 40 column
screen).
10 DO UNTIL PEN(4):LOOP Ensures the values are valid (for the
80 column screen).
20X=PEN(2)
30Y = PEN(3)
40 REM:REST OF PROGRAM
—Return the value of pi (3.14159265)
\ i
EXAMPLE:
PRINT w This returns the result 3.14159265.
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BASIC 7.0 ENCYCLOPEDIA-Basic Functions