diff --git a/app/src/main/kotlin/android/emu6502/CPU.kt b/app/src/main/kotlin/android/emu6502/CPU.kt
index 0fa578c..782aef3 100644
--- a/app/src/main/kotlin/android/emu6502/CPU.kt
+++ b/app/src/main/kotlin/android/emu6502/CPU.kt
@@ -287,4 +287,8 @@ class CPU(val memory: Memory) : Display.Callbacks {
   override fun onDraw() {
     executionLock?.countDown()
   }
+
+  fun triggerIRQ() {
+    throw NotImplementedError()
+  }
 }
\ No newline at end of file
diff --git a/app/src/main/kotlin/android/emu6502/nes/Cartridge.kt b/app/src/main/kotlin/android/emu6502/nes/Cartridge.kt
index 3fef9b2..95c9e90 100644
--- a/app/src/main/kotlin/android/emu6502/nes/Cartridge.kt
+++ b/app/src/main/kotlin/android/emu6502/nes/Cartridge.kt
@@ -6,10 +6,10 @@ data class Cartridge(
     // @formatter:off
     val pgr: IntArray,                        // PRG-ROM banks
     val chr: IntArray,                        // CHR-ROM banks
-    val sram: IntArray = IntArray(0x2000),    // Save RAM
     val mapper: Byte,                         // mapper type
     var mirror: Int,                          // mirroring mode
-    val battery: Byte                         // battery present
+    val battery: Byte,                        // battery present
+    val sram: IntArray = IntArray(0x2000)     // Save RAM
     // @formatter:on
 ) {
 
diff --git a/app/src/main/kotlin/android/emu6502/nes/INESFileParser.kt b/app/src/main/kotlin/android/emu6502/nes/INESFileParser.kt
index 342e62e..b5f679d 100644
--- a/app/src/main/kotlin/android/emu6502/nes/INESFileParser.kt
+++ b/app/src/main/kotlin/android/emu6502/nes/INESFileParser.kt
@@ -42,7 +42,8 @@ internal class INESFileParser {
         // read chr-rom bank(s)
         val chr = ByteArray(inesFileHeader.numCHR.toInt() * 8192)
         stream.read(chr)
-        return Cartridge(pgr, chr, mapper.toByte(), mirror.toByte(), battery)
+        return Cartridge(pgr.map(Byte::toInt).toIntArray(), chr.map(Byte::toInt).toIntArray(),
+            mapper.toByte(), mirror, battery)
       }
     }
   }
diff --git a/app/src/main/kotlin/android/emu6502/nes/PPU.kt b/app/src/main/kotlin/android/emu6502/nes/PPU.kt
index 6846dfb..28f194a 100644
--- a/app/src/main/kotlin/android/emu6502/nes/PPU.kt
+++ b/app/src/main/kotlin/android/emu6502/nes/PPU.kt
@@ -3,17 +3,17 @@ package android.emu6502.nes
 class PPU(
     // @formatter:off
 
-    val cycle:                  Int,            // 0-340
-    val scanLine:               Int,            // 0-261, 0-239=visible, 240=post, 241-260=vblank, 261=pre
-    val frame:                  Int,            // frame counter
+    val cycle:                  Int = 0,            // 0-340
+    val scanLine:               Int = 0,            // 0-261, 0-239=visible, 240=post, 241-260=vblank, 261=pre
+    val frame:                  Int = 0,            // frame counter
 
     // PPU registers
-    val v:                      Int,            // current vram address (15 bit)
-    val t:                      Int,            // temporary vram address (15 bit)
-    val x:                      Byte,           // fine x scroll (3 bit)
-    val w:                      Byte,           // write toggle (1 bit)
-    val f:                      Byte,           // even/odd frame flag (1 bit)
-    val register:               Byte,
+    val v:                      Int = 0,            // current vram address (15 bit)
+    val t:                      Int = 0,            // temporary vram address (15 bit)
+    val x:                      Byte = 0,           // fine x scroll (3 bit)
+    val w:                      Byte = 0,           // write toggle (1 bit)
+    val f:                      Byte = 0,           // even/odd frame flag (1 bit)
+    val register:               Byte = 0,
 
     // $2000 PPUCTRL
     val flagNameTable:          Boolean = false, // 0: $2000; 1: $2400; 2: $2800; 3: $2C00
@@ -31,7 +31,7 @@ class PPU(
     val flagShowSprites:        Boolean = false, // 0: hide; 1: show
     val flagRedTint:            Boolean = false, // 0: normal; 1: emphasized
     val flagGreenTint:          Boolean = false, // 0: normal; 1: emphasized
-    val flagBlueTint:           Boolean = false // 0: normal; 1: emphasized
+    val flagBlueTint:           Boolean = false, // 0: normal; 1: emphasized
 
     // $2002 PPUSTATUS
     val flagSpriteZeroHit:      Boolean = false,
diff --git a/app/src/main/kotlin/android/emu6502/nes/mappers/AOROM.kt b/app/src/main/kotlin/android/emu6502/nes/mappers/AOROM.kt
index b495886..7c300dc 100644
--- a/app/src/main/kotlin/android/emu6502/nes/mappers/AOROM.kt
+++ b/app/src/main/kotlin/android/emu6502/nes/mappers/AOROM.kt
@@ -1,13 +1,13 @@
 package android.emu6502.nes.mappers
 
 class AOROM : Mapper {
-  override fun write(address: Int, value: Byte) {
+  override fun write(address: Int, value: Int) {
   }
 
   override fun step() {
   }
 
-  override fun read(address: Int): Byte {
+  override fun read(address: Int): Int {
     throw NotImplementedError()
   }
 }
\ No newline at end of file
diff --git a/app/src/main/kotlin/android/emu6502/nes/mappers/CNROM.kt b/app/src/main/kotlin/android/emu6502/nes/mappers/CNROM.kt
index f478d08..e61fcbc 100644
--- a/app/src/main/kotlin/android/emu6502/nes/mappers/CNROM.kt
+++ b/app/src/main/kotlin/android/emu6502/nes/mappers/CNROM.kt
@@ -1,13 +1,13 @@
 package android.emu6502.nes.mappers
 
 class CNROM : Mapper  {
-  override fun read(address: Int): Byte {
-    throw NotImplementedError()
-  }
-
-  override fun write(address: Int, value: Byte) {
+  override fun write(address: Int, value: Int) {
   }
 
   override fun step() {
   }
+
+  override fun read(address: Int): Int {
+    throw NotImplementedError()
+  }
 }
\ No newline at end of file
diff --git a/app/src/main/kotlin/android/emu6502/nes/mappers/MMC1.kt b/app/src/main/kotlin/android/emu6502/nes/mappers/MMC1.kt
index 99695d2..9304432 100644
--- a/app/src/main/kotlin/android/emu6502/nes/mappers/MMC1.kt
+++ b/app/src/main/kotlin/android/emu6502/nes/mappers/MMC1.kt
@@ -1,13 +1,13 @@
 package android.emu6502.nes.mappers
 
 class MMC1 : Mapper  {
-  override fun read(address: Int): Byte {
-    throw NotImplementedError()
-  }
-
-  override fun write(address: Int, value: Byte) {
+  override fun write(address: Int, value: Int) {
   }
 
   override fun step() {
   }
+
+  override fun read(address: Int): Int {
+    throw NotImplementedError()
+  }
 }
\ No newline at end of file
diff --git a/app/src/main/kotlin/android/emu6502/nes/mappers/UNROM.kt b/app/src/main/kotlin/android/emu6502/nes/mappers/UNROM.kt
index 38e44b1..87de66c 100644
--- a/app/src/main/kotlin/android/emu6502/nes/mappers/UNROM.kt
+++ b/app/src/main/kotlin/android/emu6502/nes/mappers/UNROM.kt
@@ -1,13 +1,13 @@
 package android.emu6502.nes.mappers
 
 class UNROM : Mapper  {
+  override fun write(address: Int, value: Int) {
+  }
+
   override fun step() {
   }
 
-  override fun write(address: Int, value: Byte) {
-  }
-
-  override fun read(address: Int): Byte {
+  override fun read(address: Int): Int {
     throw NotImplementedError()
   }
 }
\ No newline at end of file