BCDbyte tba             ; BCDbyte converts the packed BCD byte in B
        anda    #$0F    ; to the equivalent hex value. 
        andb    #$F0    ; The result is returned in both A and B
        lsrb            ; No other registers or memory are affected
        aba             ; Equivalent C code:
        lsrb            ;
        lsrb            ;   int BCDbyte(int i) 
        aba             ;     {return ((i&0xF0)>>4)*10 + (i&0x0F);}
        tab
        rts
BCDword psha            ; BCDword converts the packed BCD word in D
        xgdx            ; to the equivalent hex value.
        pulb            ; The result is returned in D.
        jsr     BCDbyte ; The X register is destroyed.
        ldab    #100    ; Equivalent C code:
        mul             ;
        xgdx            ;   int BCDword(int i) {
        jsr     BCDbyte ;     return BCDbyte((i&0xFF00)>>8) * 100 + 
        abx             ;            BCDbyte(i&0x00FF);
        xgdx            ;   }
        rts
------------------------------------------------------------------------------
struct Date {           // Note: This sturcture does not generate code
  int year;             //       It only declares what a Date looks like
  char month;           // January = 1, etc.
  char day;
};

Date dates[3] = {0x2001, 0x10, 0x31,  // Note values are initialized in BCD
                 0x1900, 0x01, 0x01, 
                 0x1776, 0x07, 0x04};
char moonAge[3];

// In the following use integer division (discard remainders)
// X%Y means X modulo Y; i.e., the remainder when X is divided by Y

int getAge(Date* d) {   // Compute the Age of the Moon (days since new)
  int C;                // Note that C, T, Y, M, and D are local variables
  int T;                // (they live on the stack)

  int Y  = BCDword(d->year);
  char M = BCDbyte(d->month);
  char D = BCDbyte(d->day);

  if (M <= 2) {
    M = M + 12; 
    Y = Y - 1;
  }
  M = M - 2;
  C = Y / 100;                                   // Discard remainder
  T = 11 * (Y%19) + 38 + C/3 + C/4 - C + M + D;
  return T%30; 
}

void main(void) {
  char i;
  for (i=0; i<3; i++) 
    moonAge[i] = getAge(&dates[i]);     // Pass the ADDRESS of date on stack
}                                       //   return value on stack, too.