10 REM *** "BLACKBOARD.BAS" *******
20 BASE=2*4096:POKE 53272,PEEK(53272)OR8:REM PUT HIGH-RES MAP AT 8192
30 POKE 53265,PEEK(53265)OR32:REM ENTER HIGH-RES BIT-MAP MODE
40 FOR I=BASE TO BASE+7999:POKE I,0:NEXT:REM CLEAR BIT MAP
50 FOR I=1024 TO 2023:POKE I,16:NEXT I:REM BLACK BACKGROUND, WHITE LINE
60 GOTO 200
80 REM ****** PLOT ROUTINE **********
90 CHAR=INT(HPSN/8)
100 ROW=INT(VPSN/8)
110 LINE=VPSN AND 7
120 BYTE=BASE+ROW*320+8*CHAR+LINE
130 BIT=7-(HPSN AND 7)
140 POKE BYTE,PEEK(BYTE) OR (2^BIT)
150 RETURN
200 REM **** DRAW VERTICAL LINE *******
220 FOR VPSN=0 TO 199:REM PLOT LINE FROM TOP TO BOTTOM OF SCREEN
225 FOR HPSN=159 TO 160
230 GOSUB 80
240 NEXT HPSN:NEXT VPSN
245 REM *** DRAW HORIZONTAL LINE ******
250 VPSN=100:REM HALFWAY DOWN SCREEN
260 FOR HPSN=0 TO 319:REM PLOT LINE FROM LEFT SIDE TO RIGHT SIDE OF SCREEN
270 GOSUB 80
280 NEXT HPSN
290 GOTO 290