10 WIDTH 80
20 SCREEN 2
30 X1=0
40 Y1=0
50 X2=640
60 Y2=264
70 C=8
80 A=1
90 FOR I=1 TO 88
100 LINE (X1,Y1)-(X2,Y2),C,B
110 IF C>1 THEN C=8
120 X1=X1+1
130 Y1=Y1+1
140 X2=X2-1
150 Y2=Y2-1
160 IF C=8 THEN C=1 ELSE C=8
170 IF C<1 THEN C=8
180 IF I=A+8 THEN GOSUB 210
190 NEXT I
200 IF INKET$="" THEN GOTO 200 ELSE SCREEN 1
210 C=6
220 A=I
230 RETURN