The most significant bit of a signed (2's complement) register.
If the sign bit is set, then the value represented in the register is negative. If it is clear, the value is nonnegative. It is worth noting, then, that for an N-bit long register, there are (2^(N-1)) negative numbers and (2^(N-1)) nonnegative numbers that can be stored. Since the nonnegative numbers include zero, the range of allowable values is (-(2^(N-1))) to ((2^(N-1))-1). The magnitude of the most positive number representable is one less than the magnitude of the most negative number representable.