/*
* Copyright (c) 2014 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
/**
* Registers. These are ignored in the current implementation.
*/
private int commandRegister;
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) {
commandRegister = data;
// Bits 0 & 1 control the master reset
if((commandRegister & 0x01) != 0 && (commandRegister & 0x02) != 0) {
reset();
}
// Bit 7 controls receiver IRQ behavior
receiveIrqEnabled = (commandRegister & 0x80) != 0;
// Bits 5 & 6 controls transmit IRQ behavior
transmitIrqEnabled = (commandRegister & 0x20) != 0 && (commandRegister & 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;
}
}