100 // The Sieve of Eratosthenes
  110 //
  120 nPrimes = WIDTH * (HEIGHT - 1) - 1
  130 PRINT nPrimes
  140 DIM primes(nPrimes)
  150 FOR i = 0 TO nPrimes CYCLE 
  160   primes(i) = TRUE
  170 REPEAT 
  180 CLS 
  190 PROC display(nPrimes)
  200 WAIT (2)
  210 PROC sieve(nPrimes)
  220 PROC display(nPrimes)
  230 END 
  500 //
  510 // Procedure to display the primes array
  520 //
  530 DEF PROC display(num)
  540 LOCAL i
  550 HVTAB (0, 0)
  560 FOR i = 0 TO num CYCLE 
  570   IF primes(i) THEN 
  580     PRINT "*";  
  590   ELSE 
  600     PRINT " ";  
  610   ENDIF 
  620 REPEAT 
  630 ENDPROC 
 1000 //
 1010 // Procedure to calculate the Sieve
 1020 //   Uses the global array primes()
 1030 //
 1040 DEF PROC sieve(num)
 1050 LOCAL i, limit, pointer
 1060 // Initialise
 1070 primes(0) = FALSE
 1080 primes(1) = FALSE
 1090 limit = SQRT (num)
 1100 pointer = 1
 1110 CYCLE 
 1120   // Find the next number flagged as a prime
 1130   pointer = pointer + 1
 1140   WHILE  NOT primes(pointer) CYCLE 
 1150     pointer = pointer + 1
 1160     IF pointer > limit THEN ENDPROC 
 1170   REPEAT 
 1180   // Now strike-off all its multiples
 1190   FOR i = pointer + pointer TO num STEP pointer CYCLE 
 1200     primes(i) = FALSE
 1210   REPEAT 
 1220   PROC display(num)
 1230   WAIT (0.5)
 1240 REPEAT