10 REM Waiting by the clock of the computer
20 FOR N=5 TO 1 STEP -1
30   PRINT N
40   S=50:GOSUB 1000
50 NEXT N
60 PRINT "Go!"
70 END
1000 REM Wait S fiftieths of a second (S up to 255)
1010 POKE 20,0
1020 IF PEEK(20)<S THEN 1020
1030 RETURN