/*
* Copyright (c) 2014 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
*/
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;
}
}