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initial (and untested) implementation of a Motorola 6850 ACIA

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Maik Merten 2014-07-15 18:45:13 +02:00
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src/main/java/com/loomcom/symon/devices/Acia6850.java Normal file
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/*
* Copyright (c) 2008-2013 Seth J. Morabito <sethm@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 Device {
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
/**
* Register addresses
*/
private int baseAddress;
/**
* Registers. These are ignored in the current implementation.
*/
private int commandRegister;
private boolean receiveIrqEnabled = false;
private boolean transmitIrqEnabled = false;
private boolean overrun = false;
private long lastTxWrite = 0;
private long lastRxRead = 0;
private int baudRate = 0;
private long baudRateDelay = 0;
/**
* Read/Write buffers
*/
private int rxChar = 0;
private int txChar = 0;
private boolean rxFull = false;
private boolean txEmpty = true;
public Acia6850(int address) throws MemoryRangeException {
super(address, address + ACIA_SIZE - 1, "ACIA");
this.baseAddress = address;
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;
}
/*
* Calculate the delay in nanoseconds between successive read/write operations, based on the
* configured baud rate.
*/
private long calculateBaudRateDelay() {
if (baudRate > 0) {
// TODO: This is a pretty rough approximation based on 8 bits per character,
// and 1/baudRate per bit. It could certainly be improved
return (long)((1.0 / baudRate) * 1000000000 * 8);
} else {
return 0;
}
}
/**
* @return The simulated baud rate in bps.
*/
public int getBaudRate() {
return baudRate;
}
/**
* Set the baud rate of the simulated ACIA.
*
* @param rate The baud rate in bps. 0 means no simulated baud rate delay.
*/
public void setBaudRate(int rate) {
this.baudRate = rate;
this.baudRateDelay = calculateBaudRateDelay();
}
/**
* @return The contents of the status register.
*/
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;
}
@Override
public String toString() {
return "ACIA6850@" + String.format("%04X", baseAddress);
}
public synchronized int rxRead() {
lastRxRead = System.nanoTime();
rxFull = false;
overrun = false;
return rxChar;
}
public synchronized void rxWrite(int data) {
// when receiving while full: overrun
if(rxFull) {
overrun = true;
}
rxFull = true;
if (receiveIrqEnabled) {
getBus().assertIrq();
}
rxChar = data;
}
public synchronized int txRead() {
txEmpty = true;
if (transmitIrqEnabled) {
getBus().assertIrq();
}
return txChar;
}
public synchronized void txWrite(int data) {
lastTxWrite = System.nanoTime();
txChar = data;
txEmpty = false;
}
/**
* @return true if there is character data in the TX register.
*/
public boolean hasTxChar() {
return !txEmpty;
}
/**
* @return true if there is character data in the RX register.
*/
public boolean hasRxChar() {
return rxFull;
}
private synchronized void reset() {
overrun = false;
}
}