The hack pre-defined symbol table.

symbol_table.rb

  @@symbols = {
                'SP'     =>     0,
                'LCL'    =>     1,
                'ARG'    =>     2,
                'THIS'   =>     3,
                'THAT'   =>     4,
                'R0'     =>     0,
                'R1'     =>     1,
                'R2'     =>     2,
                'R3'     =>     3,
                'R4'     =>     4,
                'R5'     =>     5,
                'R6'     =>     6,
                'R7'     =>     7,
                'R8'     =>     8,
                'R9'     =>     9,
                'R10'    =>    10,
                'R11'    =>    11,
                'R12'    =>    12,
                'R13'    =>    13,
                'R14'    =>    14,
                'R15'    =>    15,
                'SCREEN' => 16384,
                'KBD'    => 24576
               }