ksymon/src/main/java/com/loomcom/symon/devices/Crtc.java

/*
 * Copyright (c) 2016 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;

import java.io.IOException;

/**
 * Simulation of a 6545 CRTC and virtual CRT output.
 */
public class Crtc extends Device {
    // Memory locations in the CRTC address space
    public static final int REGISTER_SELECT          = 0;
    public static final int REGISTER_RW              = 1;

    // Registers
    public static final int HORIZONTAL_DISPLAYED     = 1;
    public static final int VERTICAL_DISPLAYED       = 6;
    public static final int MODE_CONTROL             = 8;
    public static final int SCAN_LINE                = 9;
    public static final int CURSOR_START             = 10;
    public static final int CURSOR_END               = 11;
    public static final int DISPLAY_START_HIGH       = 12;
    public static final int DISPLAY_START_LOW        = 13;
    public static final int CURSOR_POSITION_HIGH     = 14;
    public static final int CURSOR_POSITION_LOW      = 15;


    /*
     * These will determine how the Character ROM is decoded,
     * and are Character ROM dependent.
     */

    // R1 - Horizontal Displayed
    private int horizontalDisplayed;

    // R6 - Vertical Displayed
    private int verticalDisplayed;

    // R9 - Scan Lines: Number of scan lines per character, including spacing.
    private int scanLinesPerRow;

    // R10 - Cursor Start / Cursor Mode
    private int cursorStartLine;
    private boolean cursorEnabled;
    private int cursorBlinkRate;

    // R11 - Cursor End
    private int cursorStopLine;

    // R12, R13 - Display Start Address: The starting address in the video RAM of the displayed page.
    private int startAddress;

    // R14, R15 - Cursor Position
    private int cursorPosition;

    // The size, in bytes, of a displayed page of characters.
    private int pageSize;

    private int currentRegister = 0;

    // Status bits
    private boolean rowColumnAddressing = false;
    private boolean displayEnableSkew = false;
    private boolean cursorSkew = false;

    private Memory memory;

    public Crtc(int deviceAddress, Memory memory) throws MemoryRangeException, IOException {
        super(deviceAddress, deviceAddress + 2, "CRTC");
        this.memory = memory;

        // Defaults
        this.horizontalDisplayed = 40;
        this.verticalDisplayed = 25;
        this.scanLinesPerRow = 9;
        this.cursorStartLine = 0;
        this.cursorStopLine = 7;
        this.startAddress = 0x7000;
        this.cursorPosition = startAddress;
        this.pageSize = horizontalDisplayed * verticalDisplayed;
        this.cursorEnabled = true;
        this.cursorBlinkRate = 500;
    }

    @Override
    public void write(int address, int data) throws MemoryAccessException {
        switch (address) {
            case REGISTER_SELECT:
                setCurrentRegister(data);
                break;
            case REGISTER_RW:
                writeRegisterValue(data);
                break;
        }
    }

    @Override
    public int read(int address) throws MemoryAccessException {
        switch (address) {
            case REGISTER_RW:
                switch (currentRegister) {
                    case CURSOR_POSITION_LOW:
                        return cursorPosition & 0xff;
                    case CURSOR_POSITION_HIGH:
                        return cursorPosition >> 8;
                    default:
                        return 0;
                }
            default:
                return 0;
        }
    }

    @Override
    public String toString() {
        return null;
    }

    public int getCharAtAddress(int address) throws MemoryAccessException {
        // TODO: Row/Column addressing
        return memory.read(address);
    }

    public int getHorizontalDisplayed() {
        return horizontalDisplayed;
    }

    public int getVerticalDisplayed() {
        return verticalDisplayed;
    }

    public int getScanLinesPerRow() {
        return scanLinesPerRow;
    }

    public int getCursorStartLine() {
        return cursorStartLine;
    }

    public int getCursorStopLine() {
        return cursorStopLine;
    }

    public int getCursorBlinkRate() {
        return cursorBlinkRate;
    }

    public boolean isCursorEnabled() {
        return cursorEnabled;
    }

    public int getStartAddress() {
        return startAddress;
    }

    public int getCursorPosition() {
        return cursorPosition;
    }

    public int getPageSize() {
        return pageSize;
    }

    public boolean getRowColumnAddressing() {
        return rowColumnAddressing;
    }

    public boolean getDisplayEnableSkew() {
        return displayEnableSkew;
    }

    public boolean getCursorSkew() {
        return cursorSkew;
    }

    private void setCurrentRegister(int registerNumber) {
        this.currentRegister = registerNumber;
    }

    private void writeRegisterValue(int data) throws MemoryAccessException {
        int oldStartAddress = startAddress;
        int oldCursorPosition = cursorPosition;

        switch (currentRegister) {
            case HORIZONTAL_DISPLAYED:
                horizontalDisplayed = data;
                pageSize = horizontalDisplayed * verticalDisplayed;
                break;
            case VERTICAL_DISPLAYED:
                verticalDisplayed = data;
                pageSize = horizontalDisplayed * verticalDisplayed;
                break;
            case MODE_CONTROL:
                rowColumnAddressing = (data & 0x04) != 0;
                displayEnableSkew = (data & 0x10) != 0;
                cursorSkew = (data & 0x20) != 0;
                break;
            case SCAN_LINE:
                scanLinesPerRow = data;
                break;
            case CURSOR_START:
                cursorStartLine = data & 0x1f;
                // Bits 5 and 6 define the cursor mode.
                int cursorMode = (data & 0x60) >> 5;
                switch (cursorMode) {
                    case 0:
                        cursorEnabled = true;
                        cursorBlinkRate = 0;
                        break;
                    case 1:
                        cursorEnabled = false;
                        cursorBlinkRate = 0;
                        break;
                    case 2:
                        cursorEnabled = true;
                        cursorBlinkRate = 500;
                        break;
                    case 3:
                        cursorEnabled = true;
                        cursorBlinkRate = 1000;
                        break;
                }
                break;
            case CURSOR_END:
                cursorStopLine = data & 0x1f;
                break;
            case DISPLAY_START_HIGH:
                startAddress = ((data & 0xff) << 8) | (startAddress & 0x00ff);
                break;
            case DISPLAY_START_LOW:
                startAddress = ((data & 0xff) | (startAddress & 0xff00));
                break;
            case CURSOR_POSITION_HIGH:
                cursorPosition = ((data & 0xff) << 8) | (cursorPosition & 0x00ff);
                break;
            case CURSOR_POSITION_LOW:
                cursorPosition = (data & 0xff) | (cursorPosition & 0xff00);
                break;
            default:
                break;
        }

        if (startAddress + pageSize > memory.endAddress()) {
            startAddress = oldStartAddress;
            throw new MemoryAccessException("Cannot draw screen starting at selected address.");
        }

        if (cursorPosition > memory.endAddress()) {
            cursorPosition = oldCursorPosition;
            throw new MemoryAccessException("Cannot position cursor past end of memory.");
        }


        notifyListeners();
    }
}