Why does this code crash

[pmgraphics]

This bit of code prints the variable value for I and Y, as the joystick is moved left, right, up, and down. It all works fine, until moving the joystick up, decreasing Y. As the Y variable value decreases eventually the program crashes. I'm using Altirra 3.1.

 

10 I=10:Y=12

100 STK=PEEK(54016)

110 IF STK=247 THEN POKE SCREEN+I+20*Y,0:I=I+0.1:IF I>=18 THEN I=18

120 IF STK=251 THEN POKE SCREEN+I+20*Y,0:I=I-0.1:IF I<=1 THEN I=1

130 IF STK=254 THEN POKE SCREEN+I+20*Y,0:Y=Y-0.1:IF Y<=1 THEN Y=1

140 IF STK=253 THEN POKE SCREEN+I+20*Y,0:Y=Y+0.1:IF Y>=22 THEN Y=22

150 PRINT I,Y

160 GOTO 100

 

[MaPa]

Where is defined SCREEN variable? It's not defined so it's value is zero so you are poking zero value in address area of 21-458 (zero page with OS/BASIC variables, page one - stack etc.). No wonder it crashes

Edited May 4, 2019 by MaPa

[Wrathchild]

You've not initialised SCREEN which you are poking too, what effect are you after, looks like you are trying to clear a previous value?

[EDIT] - ha, same time as MaPa

Edited May 4, 2019 by Wrathchild

[pmgraphics]

I missed including this line in the small program:

5 SCREEN=PEEK(88)+256*PEEK(89)

 

Once I included that the little program did not crash (thanks). This was to test something I tried in my larger program to slow the character down as it moves with the joystick. The game is a graphic mode 1 with redefined characters like many in the old Compute! magazines.

 

The actual program increases and decreases I and Y by 1 to move the character one space. I tried to slow the moving down by increasing I and Y by 0.5. It worked for left and right, but going up and down the character takes large jumps to the left or right. The the little program was to see what I and Y were doing as I moved the joystick. With the little program moving up and down (Y) did not change the value of "I" (left and right), as I expected it was doing.