int  -int   2's comp
0000  0     0    0
0001  1     1    1
0010  2     2    2
0011  3     3    3
0100  4     4    4
0101  5     5    5
0110  6     6    6
0111  7     7    7
1000  8    -0   -8
1001  9    -1   -7
1010  10   -2   -6
1011  11   -3   -5
1100  12   -4   -4
1101  13   -5   -3
1110  14   -6   -2
1111  15   -7   -1

"int" is if we treat each value as unsigned (always positive)
"-int" is if we use the left-most bit to indicate sign, with the other bits
  to represent magnitude
"2's comp" is if we represent the numbers in 2's complement form. 

Using "-int":
if -1 is 1001, and we add this to 2 0010, the result is 1011 which is "-3"
under this representation. This makes no sense: -1 + 2 != -3

  1001
  0010
-------
  1011

Using "2's comp":
-1 1111 add to 2 (0010), we get 0001 which is 1. This makes sense: -1 + 2 = 1

   110
   1111
   0010
--------
 1  0001


To find the 2's comp :
   1. find the 1's comp
   2. add 1

e.g. to find -3, we start with 3 (0011)
  1's comp : 1100
  add 1    : 1101 


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

Shift
              00111001
shift left:   01110010 

 0011 shift left 0110 shift left 1100
dec 3               6              12

shift right:
 0101 0010    0001 

-----------------
convert 00111001 to dec
               1x2^0
+             0x2^1
+            0x2^2
+           1x2^3
+          1x2^4
+         1x2^5
+        0x2^6
+       0x2^7

=
               1x2^0  = 1
+           1x2^3  = 8
+          1x2^4  = 16
+         1x2^5  = 32
= 32 + 16 + 8 + 1 = 57 

What if the binary value starts with 1 in the left-most bit?
  Find 2's complement, 
  then do the above conversion,
  then remember to write the answer with a minus-sign.

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

OR Truth table
a b| a OR b
-------------
0 0| 0
0 1| 1
1 0| 1
1 1| 1

XOR Truth table
a b| a XOR b
-------------
0 0| 0
0 1| 1
1 0| 1
1 1| 0

AND Truth table
a b| a AND b
------------
0 0| 0
0 1| 0
1 0| 0
1 1| 1

NOT Truth table
 c| NOT(c)
-------
 0| 1
 1| 0