What range mean for ant data type?

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C#3.5

I am confused with the size ans range of datatypes.I want to quote example for this:
In sbyte ,its size is 1 byte and range is -128 to 127.
and it is signed datatype. its size is 8 bits.
If I declare variable

sbyte a;
                     a=-128;

Its binary form is 10000000
taking 8 bits but in signed datatype 1 bit is reserved for signed purpose (i.e 0 for positive number and 1 for negative number) here its 1
so binary form will be 110000000

Now, it wont satisfy the range and size which is defined by sbyte
as its size is 9 bits .
This is my interpretation
Please tell me am i right and if wrong,Please rectify me