Commodore Plus 4 - Page 404

392 Using Peripheral Devices
20 Enter high-resolution graphic mode.
30 Draw a circle.
40 Save the circle in C$.
50 Erase the circle.
60 Set the initial coordinate values.
70 Draw the circle.
80 Erase the circle.
90 Look for the space bar.
100 Get the current value of JOY. If neutral, go back to line 70.
110 Check for fire button. If not, go on to line 130.
120 Draw the circle.
130 Use the value of JOY (fire button status removed) to change the
coordinates.
140 Go back to line 70.
150 Write HIT KEY on the screen.
160 Wait for a key, then return to the text mode and stop.
170-190 Data for change in x- and ^-coordinates based on JOY value.
Using Joysticks from Machine Language
In machine language, the joysticks must be read through the keyboard latch at
$FF08 on the graphics chip. Since the normal system interrupt service routine
uses this register for keyboard scanning, interrupts would normally be turned off
during this process. For joystick port 1, store $FA to $FF08 and then read $FF08.
For joystick port 2, use $FD. Usually, this should be done at least twice and the
results compared to avoid spurious readings. Additional debouncing, such as
reading the ports somewhat infrequently, may be required to provide smooth
joystick action.
The results of reading the keyboard latch are slightly different for the two
ports. In particular, port 1 uses bit 6 for the fire button switch, whereas port 2 uses
bit 7. The relationships between bit and switch are as follows:
Bit Switch
0 Up
1 Down