Implement clip quirks (#55)

Author: craigthomas

src/main/java/ca/craigthomas/chip8java/emulator/components/CentralProcessingUnit.java

@@ -118,6 +118,9 @@ public class CentralProcessingUnit extends Thread
     // Whether index quirks are enabled
     private boolean indexQuirks = false;
 
+    // Whether clip quirks are enabled
+    private boolean clipQuirks = false;
+
     CentralProcessingUnit(Memory memory, Keyboard keyboard, Screen screen) {
         this.random = new Random();
         this.memory = memory;
@@ -179,6 +182,15 @@ public void setIndexQuirks(boolean enableQuirk) {
         indexQuirks = enableQuirk;
     }
 
+    /**
+     * Sets the clipQuirks to true or false.
+     *
+     * @param enableQuirk a boolean enabling clip quirks or disabling clip quirks
+     */
+    public void setClipQuirks(boolean enableQuirk) {
+        clipQuirks = enableQuirk;
+    }
+
     /**
      * Fetch the next instruction from memory, increment the program counter
      * to the next instruction, and execute the instruction.
@@ -883,14 +895,16 @@ private void drawExtendedSprite(int xPos, int yPos, int bitplane, int activeInde
 
                     for (int xIndex = 0; xIndex < 8; xIndex++) {
                         int xCoord = xPos + xIndex + (xByte * 8);
-                        xCoord = xCoord % screen.getWidth();
+                        if ((!clipQuirks) || (xCoord < screen.getWidth())) {
+                            xCoord = xCoord % screen.getWidth();
 
-                        boolean turnedOn = (colorByte & mask) > 0;
-                        boolean currentOn = screen.getPixel(xCoord, yCoord, bitplane);
+                            boolean turnedOn = (colorByte & mask) > 0;
+                            boolean currentOn = screen.getPixel(xCoord, yCoord, bitplane);
 
-                        v[0xF] += (turnedOn && currentOn) ? (short) 1 : (short) 0;
-                        screen.drawPixel(xCoord, yCoord, turnedOn ^ currentOn, bitplane);
-                        mask = (short) (mask >> 1);
+                            v[0xF] += (turnedOn && currentOn) ? (short) 1 : (short) 0;
+                            screen.drawPixel(xCoord, yCoord, turnedOn ^ currentOn, bitplane);
+                            mask = (short) (mask >> 1);
+                        }
                     }
                 } else {
                     v[0xF] += 1;
@@ -912,20 +926,22 @@ private void drawNormalSprite(int xPos, int yPos, int numBytes, int bitplane, in
         for (int yIndex = 0; yIndex < numBytes; yIndex++) {
             short colorByte = memory.read(activeIndex + yIndex);
             int yCoord = yPos + yIndex;
-            yCoord = yCoord % screen.getHeight();
-
-            short mask = 0x80;
-
-            for (int xIndex = 0; xIndex < 8; xIndex++) {
-                int xCoord = xPos + xIndex;
-                xCoord = xCoord % screen.getWidth();
+            if ((!clipQuirks) || (yCoord < screen.getHeight())) {
+                yCoord = yCoord % screen.getHeight();
+                short mask = 0x80;
+                for (int xIndex = 0; xIndex < 8; xIndex++) {
+                    int xCoord = xPos + xIndex;
+                    if ((!clipQuirks) || (xCoord < screen.getWidth())) {
+                        xCoord = xCoord % screen.getWidth();
 
-                boolean turnedOn = (colorByte & mask) > 0;
-                boolean currentOn = screen.getPixel(xCoord, yCoord, bitplane);
+                        boolean turnedOn = (colorByte & mask) > 0;
+                        boolean currentOn = screen.getPixel(xCoord, yCoord, bitplane);
 
-                v[0xF] |= (turnedOn && currentOn) ? (short) 1 : (short) 0;
-                screen.drawPixel(xCoord, yCoord, turnedOn ^ currentOn, bitplane);
-                mask = (short) (mask >> 1);
+                        v[0xF] |= (turnedOn && currentOn) ? (short) 1 : (short) 0;
+                        screen.drawPixel(xCoord, yCoord, turnedOn ^ currentOn, bitplane);
+                        mask = (short) (mask >> 1);
+                    }
+                }
             }
         }
     }

src/main/java/ca/craigthomas/chip8java/emulator/components/Emulator.java

@@ -54,7 +54,7 @@ public class Emulator
      * screen scale, a cycle time of 0, a null rom, and trace mode off.
      */
     public Emulator() {
-        this(1, 0, null, false, "#000000", "#666666", "#BBBBBB", "#FFFFFF", false, false, false, false);
+        this(1, 0, null, false, "#000000", "#666666", "#BBBBBB", "#FFFFFF", false, false, false, false, false);
     }
 
     /**
@@ -71,6 +71,7 @@ public Emulator() {
      * @param shiftQuirks whether to enable shift quirks or not
      * @param logicQuirks whether to enable logic quirks or not
      * @param jumpQuirks whether to enable logic quirks or not
+     * @param clipQuirks whether to enable clip quirks or not
      */
     public Emulator(
             int scale,
@@ -84,7 +85,8 @@ public Emulator(
             boolean shiftQuirks,
             boolean logicQuirks,
             boolean jumpQuirks,
-            boolean indexQuirks
+            boolean indexQuirks,
+            boolean clipQuirks
     ) {
         if (color0.length() != 6) {
             System.out.println("color_0 parameter must be 6 characters long");
@@ -147,6 +149,7 @@ public Emulator(
         cpu.setLogicQuirks(logicQuirks);
         cpu.setJumpQuirks(jumpQuirks);
         cpu.setIndexQuirks(indexQuirks);
+        cpu.setClipQuirks(clipQuirks);
 
         // Load the font file into memory
         InputStream fontFileStream = IO.openInputStreamFromResource(FONT_FILE);

src/main/java/ca/craigthomas/chip8java/emulator/runner/Arguments.java

@@ -47,4 +47,7 @@ public class Arguments
 
     @Parameter(names={"--index_quirks"}, description="enable index quirks")
     public Boolean indexQuirks = false;
+
+    @Parameter(names={"--clip_quirks"}, description="enable clip quirks")
+    public Boolean clipQuirks = false;
 }

src/main/java/ca/craigthomas/chip8java/emulator/runner/Runner.java

@@ -39,7 +39,8 @@ public static void main(String[] argv) {
                 args.shiftQuirks,
                 args.logicQuirks,
                 args.jumpQuirks,
-                args.indexQuirks
+                args.indexQuirks,
+                args.clipQuirks
         );
         emulator.start();
     }