src/main/java/com/loomcom/symon/devices/Acia6850.java

/*
 * Copyright (c) 2016 Seth J. Morabito <web@loomcom.com>
 *                    Maik Merten <maikmerten@googlemail.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 Motorola 6850 ACIA, with limited
 * functionality.  Interrupts are not supported.
 * <p/>
 * Unlike a 16550 UART, the 6850 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 Acia6850 extends Acia {

	public static final int ACIA_SIZE = 2;

	static final int STAT_REG = 0; // read-only
	static final int CTRL_REG = 0; // write-only

	static final int RX_REG = 1; // read-only
	static final int TX_REG = 1; // write-only

	public Acia6850(int address) throws MemoryRangeException {
		super(address, ACIA_SIZE, "ACIA6850");
		setBaudRate(2400);
	}

	@Override
	public int read(int address) throws MemoryAccessException {
		switch (address) {
		case RX_REG:
			return rxRead();
		case STAT_REG:
			return statusReg();

		default:
			throw new MemoryAccessException("No register.");
		}
	}

	@Override
	public void write(int address, int data) throws MemoryAccessException {
		switch (address) {
		case TX_REG:
			txWrite(data);
			break;
		case CTRL_REG:
			setCommandRegister(data);
			break;
		default:
			throw new MemoryAccessException("No register.");
		}
	}

	private void setCommandRegister(int data) {
		// Bits 0 & 1 control the master reset
		if ((data & 0x01) != 0 && (data & 0x02) != 0) {
			reset();
		}

		// Bit 7 controls receiver IRQ behavior
		receiveIrqEnabled = (data & 0x80) != 0;
		// Bits 5 & 6 controls transmit IRQ behavior
		transmitIrqEnabled = (data & 0x20) != 0 && (data & 0x40) == 0;
	}

	/**
	 * @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 |= 0x01;
		}
		if (txEmpty && System.nanoTime() >= (lastTxWrite + baudRateDelay)) {
			stat |= 0x02;
		}
		if (overrun) {
			stat |= 0x20;
		}

		return stat;
	}

	private synchronized void reset() {
		overrun = false;
		rxFull = false;
		txEmpty = true;
	}

}