The esoteric language BF is so simple even an unexpanded Vic20 can run an interpreter for it. The following program interprets BF:
10 INPUT "INPUT THE BF PROGRAM";B$
20 DIM M%(256)
30 MP=1
40 PC=1
50 IF PC>LEN(B$) THEN END
51 C$=MID$(B$,PC,1)
60 IF C$="+" THEN GOTO 1000
70 IF C$="-" THEN GOTO 2000
80 IF C$="." THEN GOTO 3000
90 IF C$="," THEN GOTO 4000
100 IF C$="<" THEN GOTO 5000
110 IF C$=">" THEN GOTO 6000
120 IF C$="[" THEN GOTO 7000
130 IF C$="]" THEN GOTO 8000
140 PC=PC+1
150 GOTO 50
1000 M%(MP)=M%(MP)+1
1010 GOTO 140
2000 M%(MP)=M%(MP)-1
2010 GOTO 140
3000 PRINT CHR$(M%(MP))
3010 GOTO 140
4000 INPUT "INPUT NEXT CHARACTER ",K$
4010 M%(MP)=ASC(LEFT$(K,1))
4020 GOTO 140
5000 MP=MP-1
5010 GOTO 140
6000 MP=MP+1
6010 GOTO 140
7000 GOTO 140
8000 IF M%(MP)=0 THEN GOTO 140
8001 N=1
8005 PC=PC-1
8010 IF MID$(B$,PC,1)="[" THEN N=N-1
8020 IF MID$(B$,PC,1)="]" THEN N=N+1
8030 IF N>0 THEN GOTO 8005
8040 GOTO 140