bn_addw
bn_addw(a[0], carry, &carry, &r[0]);
bn_addw(b[0], carry, &carry, &r[0]);
bn_addw(a, b, &r1, &r0);
bn_addw(r0, c, &carry, &r0);
bn_addw(r0, x << BN_BITS4, &carry, &r0);
bn_addw(r0, x << BN_BITS4, &carry, &r0);
bn_addw(r0, c, &carry, &r0);
bn_addw(r0, d, &carry, &r0);
bn_addw(c1, x1, &carry, &r1);
bn_addw(c0, x0, &carry, &r0);
bn_addw(c1, x1 + carry, &r2, &r1);
bn_addw(c2, r2, &carry, &r2);
bn_addw(r0, x0, &carry, &r0);
bn_addw(r1, x1 + carry, &carry, &r1);
bn_addw(r[1], carry, &carry, &r[1]);