\ sieve of Eratosthenes in FORTH/6000
\ GLM, October 2026

10000 CONSTANT LIMIT
CREATE FLAGS LIMIT 1+ ALLOT

: SIEVE ( limit -- count )
  DUP 2 < IF DROP 0 EXIT THEN
  DUP FLAGS SWAP 1 FILL
  DUP 2 DO
    I FLAGS + C@ IF
      DUP I 2* ?DO 0 I FLAGS + C! J +LOOP
    THEN
  LOOP
  0 SWAP 2 DO I FLAGS + C@ IF 1+ THEN LOOP ;

: PRIMES ( limit -- )
  DUP SIEVE ." primes below " SWAP . ." : " . CR ;

: FIRST-PRIMES ( limit -- )
  DUP SIEVE DROP
  ." small primes:" CR
  0 SWAP 2 DO I FLAGS + C@ IF I 4 .R 1+ DUP 10 MOD 0= IF CR THEN THEN LOOP
  CR DROP ;

10 PRIMES
100 PRIMES
LIMIT PRIMES

50 FIRST-PRIMES

: SIEVE-US ( -- us )
  CLOCK
  200 0 DO LIMIT SIEVE DROP LOOP
  CLOCK SWAP - 200 / ;

." sieve(10000), 200 passes: " SIEVE-US U. ." us per pass" CR