_FLETCHER'S CHECKSUM_
by John Kodis


Example 1: 

(a)

        integer i, sum1, sum2;

        sum1 = 0;
        sum2 = 0;
        for i from 1 to message_length do
            sum1 = ( sum1 + message[i] ) modulo 255;
            sum2 = ( sum2 + sum1       ) modulo 255;
        end for.


(b)

        check1 = 255 - (( sum1 + sum2   ) modulo 255);
	message[message_length+1] = check1;
        check2 = 255 - (( sum1 + check1 ) modulo 255);
        message[message_length+2] = check2;




Example 2: 


(a)

        register unsigned char *ptr;
        register short int i, len, sum1, sum2;
        
        sum1 = sum2 = 0;
        for (i=0; i<len; i++)
            {
            sum1 += *ptr++;
            if (sum1 >= 255) sum1 -= 255;
            sum2 += sum1;
            if (sum2 >= 255) sum2 -= 255;
            }
        

(b)

                moveq   #0,d5
                move.w  d5,d4   ;sum1 = sum2 = 0;
                moveq   #0,d6
                bra.s   L_29    ;for(i=0; i<len; i++) {
        L_28    moveq   #0,d0
                move.b  (a3)+,d0
                add.w   d0,d4   ;  sum1 += *ptr++;
                cmp.w   #255,d4
                blt.s   L_32
                sub.w   #255,d4 ;  if(sum1 >= 255) sum1 -= 255;
        L_32    add.w   d4,d5   ;  sum2 += sum1;
                cmp.w   #255,d5
                blt.s   L_36
                sub.w   #255,d5 ;  if(sum2 >= 255) sum2 -= 255;
        L_36    addq.w  #1,d6
        L_29    cmp.w   d7,d6
                blt.s   L_28    ;}



Example 3:

(a)
        register unsigned char *ptr;
        register short int sum1, len;
        register unsigned long int sum2;
        
        sum1 = sum2 = 0;
        while (len--)
            {
            sum1 += *ptr++;
            if (sum1 >= 255) sum1 -= 255;
            sum2 += sum1;
            }
        sum2 %= 255;

(b)	loop 	moveq	#0,d3
		move.b	(a3)+,d3
		add.w	d3,d0	; sum1 += *ptr++;

		cmp.w	#255,d0
		blt.s	no sub
		sub.w	#255,d0	; if (sum1 >= 255) sum1 -= 255;
		
	no_sub	add.1	d0,d1	; sum2 += sum1;
		dbra	d2,loop	; while (len--);



Example 4:

        ; Register         Usage
	; D0 		the first checksum value (sum1)
        ; D1 		the second checksum value (sum2)
        ; D2		the length in bytes of the buffer (len)
        ; D4		contains zero, for the addx.b instructions
	; A0 		pointer into the buffer (ptr)

        loop    add.b   (a0)+,d0   ; sum1 += *ptr++;
                addx.b  d4,d0      ; if (sum1 >= 256) sum1 += 1;
                add.l   d0,d1      ; sum2 += sum1;
                dbra    d2,loop    ; while (len--);