I suspect your puzzlement results from one or both of the following:
First, the MSB is is the most significant bit of the entire machine word, whose size in bits is usually a multiple of $8$; the 1
at the beginning of 1011
would only be the MSB if you were using $4$-bit words, which I don't think has been done for several decades at least. That it's the first bit in the textual representation of the number is just due to the fact that we leave off leading zeros in textual representations.
Second, and perhaps more importantly, that the representation is unsigned doesn't mean that the MSB is 0
. Rather, it means that the MSB is used like all the other bits for representing a value, (namely the value $2^{l-1}$, where $l$ is the number of bits in the word), instead of having a special meaning for representing the sign.
For example, the range of an $8$-bit signed integer is $-128_{10}$ to $127_{10}$, corresponding to hexadecimal representations 80
to 7F
, respectively; the range of an $8$-bit unsigned integer is $0_{10}$ to $255_{10}$, corresponding to hexadecimal representations 00
to FF
, respectively. The hexadecimal representation FF
of the value $255$ is an example of an unsigned representation with MSB 1
.