our number  10001111
what is this value?

1's comp    01110000
2's comp   +       1
           ---------
            01110001

 1*2^6 + 1*2^5 + 1*2^4 + 0 + 0 + 0 + 1*2^0
= 64 + 32 + 16 + 0 + 0 + 0 + 1
= 113

 -113

Say that we sign extend our number
11111111 10001111

1's comp  00000000 01110000
2's comp +                1 
         ------------------
          00000000 01110001

= 113

-113


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15 
   0000 1111

------

   a = 5
   b = 6
   ; exchahge a and b's values
   ; a beginner might say:
   a = b
   b = a
   ; does not work
   temp = a
   a = b
   b = temp
   ; this will exchange the values