/**
 * Implementation of a SaM Processor. See the Processor
 * information for more information. 
 */
public class SamProcessor implements Processor {

	public final static byte REGISTERS = 5;
	protected int[] registers = new int[] { 0, 0, Memory.STACKLIMIT, 0, 0 };

	protected Program prg;
	protected Sys sys;

	public SamProcessor(Sys sys) 
		{ this.sys = sys; }

	/* Loads Program and registers instructions with the System */
	public void load(Program prog) {
		prg = prog;
		for (int i = 0; i < prog.getLength(); i++) 
			{ prog.getInst(i).setSystem(sys); }
	}

	public Program getProgram() 
		{ return prg; }

	/* Execution */
	public void step() throws SystemException 
		{ prg.getInst(registers[PC]).exec(); }

	public void run() throws SystemException 
		{ while (registers[HALT] == 0) step(); }

	/* Registers */
	public void reset() {
		for (int i = 0; i < REGISTERS; i++)
			registers[i] = 0;
		registers[HP] = Memory.STACKLIMIT;
	}

	public int get(byte reg) 
		{ return registers[reg]; }

	public int[] getRegisters() {
		int[] regs = new int[REGISTERS];
		System.arraycopy(registers, 0, regs, 0, REGISTERS);
		return regs;
	}

	public void set(byte reg, int value) throws SystemException {
		verify(reg, value);
		registers[reg] = value;
	}

	public int inc(byte reg) throws SystemException {
		verify(reg, registers[reg] + 1);
		return ++registers[reg];
	}

	public int dec(byte reg) throws SystemException {
		verify(reg, registers[reg] - 1);
		return --registers[reg];
	}

	private void verify(byte reg, int value) throws SystemException {
		switch (reg) {
			case PC :
				if (value < 0 || value > prg.getLength() - 1)
					throw new SystemException("Invalid instruction index, PC = " + value);
				break;
			case SP :
				if (value < 0)
					throw new SystemException("Stack Underflow, SP = " + value);
				if (value > Memory.STACKLIMIT - 1)
					throw new SystemException("Stack Overflow, SP = " + value);
				break;
			case FBR :
				if (value < 0 || value > Memory.MEMORYLIMIT - 1)
					throw new SystemException("Invalid frame address, FBR = " + value);
				break;
			default : //User has no control over HALT and HP, handle those where necessary 
				//so that they do not overflow/underflow 
		}
	}

}