; This is an implementation of The Sieve of Eratosthenes that finds all the primes between 2 and 255.

#Include services.asm

#Program

    #Base 0x2000

start:
  ; Search the list until we find a prime.
  SETD DataTop
  CCF ; Clear the carry flag. In later cycles, the carry flag will be set at the end of the next loop. We'll want it cleared.
  RSTA
  RSTB
 findPrimeLoop:
  LDB ; Load an element into B.
  BRB foundPrime ; If it's zero, it's a prime.
  INCA ; Increment A, our index.
  INCD ; Increment the Data Pointer.
  BRA end ; If A becomes zero, we've looked through the whole list without finding another prime.
  BRI findPrimeLoop ; Keep searching for the next prime.
   foundPrime:
    ; If we've found a prime, we should print it and mark it off the list so we don't print it again.
    CALL printByteDecimal ; A contains our prime, so we can just call the print subroutine.
    CALL blankSpace ; Put a space afterward to keep things easy to read.
    INIB 0x01 ; Set B to 1.
    STB ; Mark this prime off the list.
     markMultiples:
      ; Now, we mark each multiple of this prime as nonprime until we reach the end of the list.
      PSHD ; Save the Data Pointer to the stack.
      POPB ; Pop its low byte into B.
      ADD ; Add them together.
      PSHQ ; Store the result back onto the stack.
      POPD ; Pop the modified address into the Data Pointer.
      INIB 0x01 ; Set B to 1.
      STB ; Store B to mark the value as nonprime.
      BRC start ; If the previous add overflowed, the next nonprime is outside the range of our list, so start over with a new prime.
      BRI markMultiples ; Otherwise, loop again to mark the next multiple as nonprime.
 end:
  CALL lineFeed ; Print a linefeed to make it look nice.
  SWI osExit ; The program is done, we found all the primes!




#Data

    #Base 0x1000

; The table of our prime candidates. It has to begin on a page boundary: marking walks
; the pointer's low byte and treats the carry out as running off the end of the table,
; which only finds the right end if the table starts on one.

  #Align 0x100
DataTop:
0x01 0x01 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00


#Include print.asm