diff --git a/source/quickerNES/core/cart.hpp b/source/quickerNES/core/cart.hpp
index abf1a42..75d025c 100644
--- a/source/quickerNES/core/cart.hpp
+++ b/source/quickerNES/core/cart.hpp
@@ -34,12 +34,12 @@ class Cart
     uint8_t chr_count; // number of 8K CHR banks
     uint8_t flags;     // MMMM FTBV Mapper low, Four-screen, Trainer, Battery, V mirror
     uint8_t flags2;    // MMMM --XX Mapper high 4 bits
-    uint8_t zero[8];   // if zero [7] is non-zero, treat flags2 as zero
+    uint8_t zero[8];
   };
   static_assert(sizeof(ines_header_t) == 16);
 
   // Load iNES file
-  void load_ines(const uint8_t *buffer)
+  const char *load_ines(const uint8_t *buffer)
   {
     ines_header_t h;
 
@@ -49,7 +49,10 @@ class Cart
       memcpy(&h, &buffer[bufferPos], copySize);
       bufferPos += copySize;
     }
-    if (h.zero[7]) h.flags2 = 0;
+
+    if (memcmp(h.signature, "NES\x1A", 4) != 0)
+      return "Not an iNES file";
+
     set_mapper(h.flags, h.flags2);
 
     // skip trainer
@@ -73,6 +76,8 @@ class Cart
       memcpy(chr(), &buffer[bufferPos], copySize);
       bufferPos += copySize;
     }
+
+    return nullptr;
   }
 
   inline bool has_battery_ram() const { return mapper & 0x02; }
@@ -114,4 +119,4 @@ class Cart
   unsigned mapper;
 };
 
-} // namespace quickerNES
\ No newline at end of file
+} // namespace quickerNES
diff --git a/source/quickerNES/core/core.hpp b/source/quickerNES/core/core.hpp
index afc13fb..95c9813 100644
--- a/source/quickerNES/core/core.hpp
+++ b/source/quickerNES/core/core.hpp
@@ -26,6 +26,10 @@ Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */
 #include <stdexcept>
 #include <string>
 
+#ifdef _QUICKERNES_ENABLE_INPUT_CALLBACK
+extern void (*input_callback_cb)(void);
+#endif
+
 namespace quickerNES
 {
 
@@ -138,7 +142,7 @@ class Core : private Cpu
     return 0;
   }
 
-  void open(Cart const *new_cart)
+  const char *open(Cart const *new_cart)
   {
     close();
     init();
@@ -153,7 +157,7 @@ class Core : private Cpu
     if (mapper == nullptr)
     {
       fprintf(stderr, "Could not find mapper for code: %u\n", mapperCode);
-      exit(-1);
+      return "Unsupported mapper";
     }
 
     // Assigning backwards pointers to cartdrige and emulator now
@@ -165,6 +169,8 @@ class Core : private Cpu
     cart = new_cart;
     memset(impl->unmapped_page, unmapped_fill, sizeof impl->unmapped_page);
     reset(true, true);
+
+    return nullptr;
   }
 
   inline void serializeState(jaffarCommon::serializer::Base &serializer) const
@@ -844,6 +850,10 @@ class Core : private Cpu
         input_state.arkanoid_latch = current_arkanoid_latch;
         input_state.arkanoid_fire = current_arkanoid_fire;
         #endif
+
+		#ifdef _QUICKERNES_ENABLE_INPUT_CALLBACK
+        input_callback_cb();
+		#endif
       }
       input_state.w4016 = data;
       return;
diff --git a/source/quickerNES/core/cpu.cpp b/source/quickerNES/core/cpu.cpp
index 171f9da..e34d830 100644
--- a/source/quickerNES/core/cpu.cpp
+++ b/source/quickerNES/core/cpu.cpp
@@ -477,7 +477,7 @@ uint8_t clock_table[256] = {
 
 
 // This optimization is only possible with the GNU compiler -- MSVC does not allow function alignment
-#ifdef __GNUC__
+#if defined(__GNUC__) && !defined(__clang__)
 __attribute__((optimize("align-functions=1024")))
 #endif
 Cpu::result_t
@@ -1480,8 +1480,7 @@ end:
   CALC_STATUS(temp);
   r.status = temp;
 }
-  
-  this->clock_count = clock_count;
+
   r.pc = pc;
   r.sp = GET_SP();
   r.a = a;
diff --git a/source/quickerNES/core/emu.cpp b/source/quickerNES/core/emu.cpp
index ad513c4..751b12a 100644
--- a/source/quickerNES/core/emu.cpp
+++ b/source/quickerNES/core/emu.cpp
@@ -78,16 +78,19 @@ inline void Emu::clear_sound_buf()
   sound_buf->clear();
 }
 
-void Emu::set_cart(Cart const *new_cart)
+const char *Emu::set_cart(Cart const *new_cart)
 {
   auto_init();
-  emu.open(new_cart);
+  const char *error = emu.open(new_cart);
+  if (error) return error;
 
   channel_count_ = Apu::osc_count + emu.mapper->channel_count();
   sound_buf->set_channel_count(channel_count());
   set_equalizer(equalizer_);
   enable_sound(true);
   reset();
+
+  return nullptr;
 }
 
 void Emu::reset(bool full_reset, bool erase_battery_ram)
@@ -167,10 +170,12 @@ const char *Emu::emulate_frame(uint32_t joypad1, uint32_t joypad2, uint32_t arka
 
 // Extras
 
-void Emu::load_ines(const uint8_t *buffer)
+const char *Emu::load_ines(const uint8_t *buffer)
 {
-  private_cart.load_ines(buffer);
-  set_cart(&private_cart);
+  const char *error = private_cart.load_ines(buffer);
+  if (error) return error;
+
+  return set_cart(&private_cart);
 }
 
 void Emu::write_chr(void const *p, long count, long offset)
@@ -839,4 +844,4 @@ void Emu::RestoreAudioBufferState()
   sound_buf->RestoreAudioBufferState();
 }
 
-} // namespace quickerNES
\ No newline at end of file
+} // namespace quickerNES
diff --git a/source/quickerNES/core/emu.hpp b/source/quickerNES/core/emu.hpp
index bdb7c81..b8e164a 100644
--- a/source/quickerNES/core/emu.hpp
+++ b/source/quickerNES/core/emu.hpp
@@ -22,7 +22,7 @@ class Emu
   // Basic setup
 
   // Load iNES file into emulator and clear recording
-  void load_ines(const uint8_t *buffer);
+  const char* load_ines(const uint8_t *buffer);
 
   // Set sample rate for sound generation
   const char *set_sample_rate(long);
@@ -98,7 +98,7 @@ class Emu
   // Use already-loaded cartridge. Retains pointer, so it must be kept around until
   // closed. A cartridge can be shared among multiple emulators. After opening,
   // cartridge's CHR data shouldn't be modified since a copy is cached internally.
-  void set_cart(Cart const *);
+  const char *set_cart(Cart const *new_cart);
 
   // Pointer to current cartridge, or NULL if none is loaded
   Cart const *cart() const { return emu.cart; }