Fetch_Execute.java

      //  Fetch_Execute.java
      
      import java.io.*;
      
      /**   The algorithm of a CPU's Fetch-Execute cycle expressed in Java.
        *   This code compiles and runs; making a full computer out of it is
        *   left as an exercise for the student!
        */
        public class Fetch_Execute
        {
          static int[]  Memory = new int[ 1024*1024 ];
          static int    IR, PC;
      
          //  main()
          //  --------------------------------------------------------------
          /**
            *   Loads a program into the computer's memory and executes it.
            */
            public static void main( String[] args ) throws IOException
            {
              //  Load a program into the computer's memory
              if ( args.length != 1 )
              {
                System.err.println( "Usage: java Fetch_Execute <file>" );
                System.exit( 1 );
              }
              BufferedReader br = new BufferedReader(
                                                new FileReader( args[0] ) );
              String buf = null;
      
              while ( (buf = br.readLine()) != null )
              {
                String[] ad = buf.split( " " ); // Separate address/data
                int addr = Integer.parseInt( ad[0] );
                int data = Integer.parseInt( ad[1] );
                Memory[ addr ] = data;
              }
      
              //  Here's the fetch-execute cycle
              //  ----------------------------------------------------------
              PC = 0;
              while ( true )
              {
                IR = Memory[ PC++ ];
                execute();
              }
            }
      
            //  execute()
            //  ------------------------------------------------------------
            /**
              *   Executes the instruction in the IR.
              *   Assume instructions are 32 bits long with a seven bit
              *   op code in bits 24:30.
              */
              static void execute( )
              {
                int op_code = (IR & 0x7F000000) >> 24;
                switch ( op_code )
                {
                  case 0x01:
                    //  Execute an instruction with op code of 0x01 ...
                    break;
      
                  case 0x02:
                    //  Execute an instruction with op code of 0x02 ...
                    break;
      
                   // Other cases for other op codes ...
      
                }
              }
        }