Array Storing in Memory

Question

byte[] arr=new byte[1];

arr[0]=255;

----7------6--------5--------4----------3---------2-------1-------0---

|   1   |    1     |    1    |     1       |    1    |      1    |    1    |   1   |

----------------------------------------------------------------------------

is each and every byte have singed bit....right...? but in above binary how we know this is out signed bit , although in the above binary last bit means (MSB) is 1 means our value is negative , but we have puted value is positive.....?

any one can help me... plz