Simulator is just about ready for real-world testing now. Added a simulated

MOS6551 ACIA at address $C000 which does buffered input and output via the
console. Updated the README with a bit more documentation, and bumped the
version number to 0.1 because I'm impatient.

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

@@ -0,0 +1,124 @@
+package com.loomcom.symon.devices;
+
+import com.loomcom.symon.exceptions.*;
+import com.loomcom.symon.util.*;
+
+/**
+ * This is a simulation of the MOS 6551 ACIA, with limited
+ * functionality.  Interrupts are not supported.
+ *
+ * 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.  However, in the simulation we maintain two
+ * small buffers of 256 characters, since we only wake up to check for
+ * keyboard input and do output every 500 instructions.
+ */
+public class Acia extends Device {
+
+  public static final int ACIA_SIZE = 4;
+  public static final int BUF_LEN = 256;
+
+  static final int DATA_REG = 0;
+  static final int STAT_REG = 1;
+  static final int CMND_REG = 2;
+  static final int CTRL_REG = 3;
+
+  /** Register addresses */
+  private int baseAddress;
+
+  /** Registers. These are ignored in the current implementation. */
+  private int commandRegister;
+  private int controlRegister;
+
+  /** Read/Write buffers */
+  private FifoRingBuffer rxBuffer = new FifoRingBuffer(BUF_LEN);
+  private FifoRingBuffer txBuffer = new FifoRingBuffer(BUF_LEN);
+
+  public Acia(int address) throws MemoryRangeException {
+    super(address, ACIA_SIZE, "ACIA");
+    this.baseAddress = address;
+  }
+
+  @Override
+  public int read(int address) throws MemoryAccessException {
+    switch (address) {
+    case DATA_REG:
+      try {
+        return rxRead();
+      } catch (FifoUnderrunException ex) {
+        throw new MemoryAccessException("Buffer underrun");
+      }
+    case STAT_REG:
+      return ((rxBuffer.isEmpty() ? 0x00 : 0x08) |
+              (txBuffer.isEmpty() ? 0x10 : 0x00));
+    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:
+      commandRegister = data;
+      break;
+    case 3:
+      controlRegister = data;
+      break;
+    default:
+      throw new MemoryAccessException("No register.");
+    }
+  }
+
+  @Override
+  public String toString() {
+    return "ACIA@" + String.format("%04X", baseAddress);
+  }
+
+  public int rxRead() throws FifoUnderrunException {
+    return rxBuffer.pop();
+  }
+
+  public void rxWrite(int data) {
+    rxBuffer.push(data);
+  }
+
+  public int txRead() throws FifoUnderrunException {
+    return txBuffer.pop();
+  }
+
+  public void txWrite(int data) {
+    txBuffer.push(data);
+  }
+
+  /**
+   * @return true if there is character data in the TX register.
+   */
+  public boolean hasTxChar() {
+    return !txBuffer.isEmpty();
+  }
+
+  /**
+   * @return true if there is character data in the RX register.
+   */
+  public boolean hasRxChar() {
+    return !rxBuffer.isEmpty();
+  }
+
+  private void reset() {
+    txBuffer.reset();
+    rxBuffer.reset();
+  }
+
+}