public void executeSUB(RTypeInstruction instruction) { 
		
		RegisterFile regFile = RegisterFile.getInstance();
		int result = regFile.getRegisterValue(instruction.getRegSource1())- 
				regFile.getRegisterValue(instruction.getRegSource2());

		regFile.setRegisterValue(instruction.getRegTarget(), result); 
		AppContext.PROGRAM_COUNTER++;
		/* -- Subtract

	Description:
	Subtracts two registers and stores the result in a register
	Operation:
	$d = $s - $t; advance_pc (4);
	Syntax:
	sub $d, $s, $t
	Encoding:
	0000 00ss ssst tttt dddd d000 0010 0010
		 */ 
	}