clean up intruction instantiation and fix tests

lib/arm/constants.rb

@@ -18,12 +18,12 @@ module Arm
       :tst => 0b1000,
 
       :b => 0b1010,
-      :bl => 0b1011,
-      :bx => 0b00010010
+      :call=> 0b1011
     }
     #return the bit patter that the cpu uses for the current instruction @attributes[:opcode]
     def op_bit_code
-      OPCODES[@attributes[:opcode]] or throw "no code found for #{@attributes[:opcode].inspect}"
+      bit_code = OPCODES[@attributes[:opcode]] 
+      bit_code or raise "no code found for #{@attributes[:opcode].inspect}"
     end
 
     #codition codes can be applied to many instructions and thus save branches