Alternative Method for Computing Two's Complement Binary -> Decimal
No error. in fact, if you add the binary representation of (-38) and 38 as unsigned 7-bit integer without overflowing, you'll get $10000000_2$, then due to overflow the lowest 7 bits is preserved, resulting 0. This property holds for all $0&lt;x&lt;2^6$, ie. denoting $u(x)$ the unsigned value from 2's component representation of $x$, then $$u(-x) + x = 2^7, (x&gt;0)$$ thus $$\begin{align}-x =&amp; u(-x)-2^7\\=&amp;(u(-x)-2^6)+(-2^6)\end{align}$$ ie. subtract the highest bit, and add back the minus bit. This can be extended from 7-bit to n-bit.