Array Storing in Memory

byte[] arr=new byte[1];

arr[0]=255;

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

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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