$00200180 ; a
+$00900090 ; b
==========
 $00b00210 ; a+b, raw binary addition

 $00100010 ; nybbles with detected carries, calculated by some fixed shifting & masking operations
 $00600060 ; multiplied by 6, this is the number to add to do the fixup

 $00b00210 ; raw binary a+b
+$00600060 ; fixup value
=========
 $01100270 ; the decimal-adjusted addition

if (p & D_FLAG)
{
    temp = A + operand + c;

    /* Detect nybbles that have binary overflowed (16-18) */
    WORD nybbleFlags = (A ^ operand ^ temp) >> 4;

    /* Detect decimally overflowing nybbles (10-15) */
    nybbleFlags |= (((temp>>1)&temp)>>2) | (((temp>>2)&temp)>>1);

    /* Flags have been shifted to bit0 of each nybble; mask out all other bits */
    nybbleFlags &= 0x11111111;

    while(nybbleFlags != 0)
    {
       /* Convert each nybble from a 0/1 flag to the fixup number, and add it in */
       temp += nybbleFlags * 6;
   
       /* Test again to see if any nybbles are 10 ($a).  This is the only overflow that can ripple at this point. */
       nybbleFlags = (((temp>>2)&temp)>>1) & 0x11111111;
    }
}

static void adc()
{
    DWORD
        temp = A + operand;
    if (p & D_FLAG)
    {
        DWORD         /* bias (A + operand) to detect any nybble carries */
            t2 = (temp + 0x66666666UL) ^ A ^ operand;
                            /* apply corrections to the affected nybbles */
        t2 = ((t2 >> 3) & 0x22222222UL) * 3 + temp;
                /* bias ((A + operand) + c) to detect any nybble carries */
        temp = (t2 + c + 0x66666666UL) ^ t2 ^ c;
                            /* apply corrections to the affected nybbles */
        temp = ((temp >> 3) & 0x22222222UL) * 3 + t2 + c;       
                                      /* detect tens-complement overflow */
        if ((A < 0x50000000UL) == (operand < 0x50000000UL)) &&
          (((WORD) temp < 0x50000000UL) != (operand < 0x50000000UL))
            setV();
        else
            clrV();
    }
    else
    {
        temp += c;
        if ((A & BIT_31) == (operand & BIT_31)) &&
          ((temp & BIT_31) != (operand & BIT_31))
            setV();
        else
            clrV();
    }
    operand = (WORD) temp;
    c = (WORD) (temp >> 32);
    lda();
}