get balance
subtract 100 
write balance 

-----------------------
balance 1000

process1          high priority process2
get balance
subtract 100 
                  get balance
                  add 240 
                  write balance  (1240)
write balance 
(900)



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

binary addition
a = 00111101  = 61
b = 00000111  = 4 + 2 + 1 = 7


convert 00111101 to dec
               1x2^0
+             0x2^1
+            1x2^2
+           1x2^3
+          1x2^4
+         1x2^5
+        0x2^6
+       0x2^7

=
               1x2^0  = 1
             1x2^2  = 4
+           1x2^3  = 8
+          1x2^4  = 16
+         1x2^5  = 32
= 32 + 16 + 8 + 1 = 61 


what is a + b?
    0111111
a   00111101  
b + 00000111  
 --------------
    01000100

convert 01000100 to decimal
               0x2^0
+             0x2^1
+            1x2^2
+           0x2^3
+          0x2^4
+         0x2^5
+        1x2^6
+       0x2^7

= 2^6 + 2^2
= 64 + 4 = 68 


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

suppose that we have -5 
  11111111 11111011
convert to positive
                11111111 11111011

1's complement  00000000 00000100
             +1                 1
                ------------------
2's complement  00000000 00000101

conclude: this is -5

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

  11111111 11111011 = -5
shift right by 1
  _1111111 11111101
what do we put in the left-most bit?
if "normal" shift, we fill it in with a 0.
for a signed number, copy the left-most bit from before to that position
   11111111 11111101   = -3
arithmetic shift