bit manipulation - Why does a right shift on a signed integer causes an overflow? -

given 8 bits negative integer (signed between -1 , -128), right shift in hla causes overflow , don't understand why. if shifted once, should divide value 2. true positive numbers not negative. why? example if -10 entered result +123.

       program cpy;         #include ("stdlib.hhf")        #include ("hla.hhf")        static      i:int8;       begin cpy;     stdout.put("enter value divide 2: ");     stdin.geti8();     mov(al,i);      shr(1,i); //shift bits 1 position right    if(@o)then   // if overlow    stdout.put("overflow");    endif;     end cpy;  

signed numbers represented 2's complement in binary, plus sign bit "on left". 2's complement of 10 coded on 7 bits 1110110, , sign bit value negative numbers 1.

-10: 1111 0110          ^        |      sign bit   

then shift right (when right shift zeroes added left):

-10 >> 1: 0111 1001             ^             |           sign bit  

your sign bit worth 0 (positive), , 1111011 123 in decimal.