src/main/java/com/loomcom/symon/devices/Acia6551.java

/*
 * Copyright (c) 2016 Seth J. Morabito <web@loomcom.com>
 *
 * Permission is hereby granted, free of charge, to any person obtaining
 * a copy of this software and associated documentation files (the
 * "Software"), to deal in the Software without restriction, including
 * without limitation the rights to use, copy, modify, merge, publish,
 * distribute, sublicense, and/or sell copies of the Software, and to
 * permit persons to whom the Software is furnished to do so, subject to
 * the following conditions:
 *
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
 * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
 * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
 * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 */

package com.loomcom.symon.devices;

import com.loomcom.symon.exceptions.MemoryAccessException;
import com.loomcom.symon.exceptions.MemoryRangeException;

/**
 * This is a simulation of the MOS 6551 ACIA, with limited
 * functionality.  Interrupts are not supported.
 * <p/>
 * Unlike a 16550 UART, the 6551 ACIA has only one-byte transmit and
 * receive buffers. It is the programmer's responsibility to check the
 * status (full or empty) for transmit and receive buffers before
 * writing / reading.
 */
public class Acia6551 extends Acia {

	public static final int ACIA_SIZE = 4;

	static final int DATA_REG = 0;
	static final int STAT_REG = 1;
	static final int CMND_REG = 2;
	static final int CTRL_REG = 3;

	/**
	 * Registers. These are ignored in the current implementation.
	 */
	private int commandRegister;
	private int controlRegister;

	public Acia6551(int address) throws MemoryRangeException {
		super(address, ACIA_SIZE, "ACIA");
	}

	@Override
	public int read(int address) throws MemoryAccessException {
		switch (address) {
		case DATA_REG:
			return rxRead();
		case STAT_REG:
			return statusReg();
		case CMND_REG:
			return commandRegister;
		case CTRL_REG:
			return controlRegister;
		default:
			throw new MemoryAccessException("No register.");
		}
	}

	@Override
	public void write(int address, int data) throws MemoryAccessException {
		switch (address) {
		case 0:
			txWrite(data);
			break;
		case 1:
			reset();
			break;
		case 2:
			setCommandRegister(data);
			break;
		case 3:
			setControlRegister(data);
			break;
		default:
			throw new MemoryAccessException("No register.");
		}
	}

	private void setCommandRegister(int data) {
		commandRegister = data;

		// Bit 1 controls receiver IRQ behavior
		receiveIrqEnabled = (commandRegister & 0x02) == 0;
		// Bits 2 & 3 controls transmit IRQ behavior
		transmitIrqEnabled = (commandRegister & 0x08) == 0 && (commandRegister & 0x04) != 0;
	}

	/**
	 * Set the control register and associated state.
	 *
	 * @param data Data to write into the control register
	 */
	private void setControlRegister(int data) {
		controlRegister = data;
		int rate = 0;

		// If the value of the data is 0, this is a request to reset,
		// otherwise it's a control update.

		if (data == 0) {
			reset();
		} else {
			// Mask the lower three bits to get the baud rate.
			int baudSelector = data & 0x0f;
			switch (baudSelector) {
			case 0:
				rate = 0;
				break;
			case 1:
				rate = 50;
				break;
			case 2:
				rate = 75;
				break;
			case 3:
				rate = 110; // Real rate is actually 109.92
				break;
			case 4:
				rate = 135; // Real rate is actually 134.58
				break;
			case 5:
				rate = 150;
				break;
			case 6:
				rate = 300;
				break;
			case 7:
				rate = 600;
				break;
			case 8:
				rate = 1200;
				break;
			case 9:
				rate = 1800;
				break;
			case 10:
				rate = 2400;
				break;
			case 11:
				rate = 3600;
				break;
			case 12:
				rate = 4800;
				break;
			case 13:
				rate = 7200;
				break;
			case 14:
				rate = 9600;
				break;
			case 15:
				rate = 19200;
				break;
			}

			setBaudRate(rate);
		}
	}

	/**
	 * @return The contents of the status register.
	 */
	@Override
	public int statusReg() {
		// TODO: Parity Error, Framing Error, DTR, DSR, and Interrupt flags.
		int stat = 0;
		if (rxFull && System.nanoTime() >= (lastRxRead + baudRateDelay)) {
			stat |= 0x08;
		}
		if (txEmpty && System.nanoTime() >= (lastTxWrite + baudRateDelay)) {
			stat |= 0x10;
		}
		if (overrun) {
			stat |= 0x04;
		}
		return stat;
	}

	private synchronized void reset() {
		txChar = 0;
		txEmpty = true;
		rxChar = 0;
		rxFull = false;
		receiveIrqEnabled = false;
		transmitIrqEnabled = false;
	}

}