Code puzzle 4

void f(char *x, char *y, int z) {
  int z2 = (z+7)/8;
  switch(z%8) {
  case 0: do { *x++ = *y++;
  case 7: *x++ = *y++;
  case 6: *x++ = *y++;
  case 5: *x++ = *y++;
  case 4: *x++ = *y++;
  case 3: *x++ = *y++;
  case 2: *x++ = *y++;
  case 1:*x++ = *y++;
  } while(--z2 > 0);
  }
}