diff --git a/src/gamegear.c b/src/gamegear.c
index 5fc760e..6426c12 100644
--- a/src/gamegear.c
+++ b/src/gamegear.c
@@ -24,6 +24,7 @@ GameGear* gamegear_create()
     // mmu_init(&gg->mmu, ...);
     z80_init(&gg->cpu, &gg->mmu);
     gg->powered = false;
+    gg->exc_buffer[0] = '\0';
     return gg;
 }
 
@@ -57,7 +58,7 @@ void gamegear_load(GameGear *gg, ROM *rom)
 
     Powering on the GameGear executes boot code (e.g. clearing memory and
     setting initial register values) and starts the clock. Powering it off
-    stops the clock.
+    stops the clock and clears any exception data.
 
     Setting the power state to its current value has no effect.
 */
@@ -71,7 +72,7 @@ void gamegear_power(GameGear *gg, bool state)
         z80_power(&gg->cpu);
         gg->last_tick = get_time_ns();
     } else {
-        // TODO: free exception buffer
+        gg->exc_buffer[0] = '\0';
     }
     gg->powered = state;
 }
@@ -105,15 +106,33 @@ bool gamegear_simulate(GameGear *gg)
 
     This function returns a const pointer to a buffer holding a human-readable
     exception string (although it may be cryptic to an end-user). The buffer is
-    owned by the GameGear object and should not be freed - it lasts until the
-    GameGear's power state is changed. If no exception flag is set, this
-    function returns NULL.
+    owned by the GameGear object and should not be freed - its contents last
+    until the GameGear's power state is changed. If no exception flag is set,
+    this function returns NULL.
 */
 const char* gamegear_get_exception(GameGear *gg)
 {
+#define SET_EXC(...) snprintf(gg->exc_buffer, GG_EXC_BUFF_SIZE, __VA_ARGS__);
     if (!gg->powered)
         return NULL;
 
-    // TODO: return ptr to exception buffer
-    return "Unknown exception";
+    if (!gg->exc_buffer[0]) {
+        if (gg->cpu.except) {
+            switch (gg->cpu.exc_code) {
+                case Z80_EXC_NOT_POWERED:
+                    SET_EXC("CPU not powered")
+                    break;
+                case Z80_EXC_UNIMPLEMENTED_OPCODE:
+                    SET_EXC("Unimplemented opcode: 0x%02X", gg->cpu.exc_data)
+                    break;
+                default:
+                    SET_EXC("Unknown exception")
+                    break;
+            }
+        } else {
+            return NULL;
+        }
+    }
+    return gg->exc_buffer;
+#undef SET_EXC
 }
diff --git a/src/gamegear.h b/src/gamegear.h
index 759714f..4577aa2 100644
--- a/src/gamegear.h
+++ b/src/gamegear.h
@@ -10,6 +10,8 @@
 #include "rom.h"
 #include "z80.h"
 
+#define GG_EXC_BUFF_SIZE 128
+
 /* Structs */
 
 typedef struct {
@@ -17,6 +19,7 @@ typedef struct {
     Z80 cpu;
     bool powered;
     uint64_t last_tick;
+    char exc_buffer[GG_EXC_BUFF_SIZE];
 } GameGear;
 
 /* Functions */
diff --git a/src/z80.c b/src/z80.c
index f572548..3b6e5ad 100644
--- a/src/z80.c
+++ b/src/z80.c
@@ -22,6 +22,8 @@ void z80_init(Z80 *z80, MMU *mmu)
 {
     z80->mmu = mmu;
     z80->except = true;
+    z80->exc_code = Z80_EXC_NOT_POWERED;
+    z80->exc_data = 0;
 }
 
 /*
@@ -86,18 +88,7 @@ static inline uint8_t get_interrupt_mode(Z80 *z80)
     return 2;
 }
 
-/* ------------------------------------------------------------------------- */
-
-static uint8_t z80_inst_nop(Z80 *z80) {
-    z80->regfile.pc++;
-    return 4;
-}
-
-static uint8_t (*instruction_lookup_table[256])(Z80*) = {
-    z80_inst_nop
-};
-
-/* ------------------------------------------------------------------------- */
+#include "z80_instructions.inc"
 
 /*
     Emulate the given number of cycles of the Z80, or until an exception.
@@ -108,18 +99,15 @@ static uint8_t (*instruction_lookup_table[256])(Z80*) = {
 */
 bool z80_do_cycles(Z80 *z80, double cycles)
 {
-    if (z80->except)
-        return true;
-
     cycles -= z80->pending_cycles;
-    while (cycles > 0) {
+    while (cycles > 0 && !z80->except) {
         // uint8_t opcode = mmu_read_byte(&z80->mmu, z80->regfile.pc);
         uint8_t opcode = 0x00;
-        cycles -= (*instruction_lookup_table[opcode])(z80) - 2;
+        cycles -= (*instruction_lookup_table[opcode])(z80, opcode) - 2;
     }
 
     z80->pending_cycles = -cycles;
-    return false;
+    return z80->except;
 }
 
 #ifdef DEBUG_MODE
diff --git a/src/z80.h b/src/z80.h
index 2b17b0d..cd35501 100644
--- a/src/z80.h
+++ b/src/z80.h
@@ -8,6 +8,9 @@
 
 #include "mmu.h"
 
+#define Z80_EXC_NOT_POWERED          0
+#define Z80_EXC_UNIMPLEMENTED_OPCODE 1
+
 /* Structs */
 
 typedef struct {
@@ -23,6 +26,7 @@ typedef struct {
     Z80RegFile regfile;
     MMU *mmu;
     bool except;
+    uint8_t exc_code, exc_data;
     double pending_cycles;
 } Z80;
 
diff --git a/src/z80_instructions.inc b/src/z80_instructions.inc
new file mode 100644
index 0000000..8523f59
--- /dev/null
+++ b/src/z80_instructions.inc
@@ -0,0 +1,34 @@
+/* Copyright (C) 2014-2015 Ben Kurtovic <ben.kurtovic@gmail.com>
+   Released under the terms of the MIT License. See LICENSE for details. */
+
+/*
+    This file contains code to implement the Z80 instruction set. Since there
+    are a lot of functions, it is kept separate from the main z80.c file. It is
+    included in the middle of z80.c and should not be compiled separately.
+*/
+
+/*
+    0x00: NOP
+*/
+static uint8_t z80_inst_nop(Z80 *z80, uint8_t opcode)
+{
+    (void) opcode;
+    z80->regfile.pc++;
+    return 4;
+}
+
+/*
+    Unimplemented opcode handler.
+*/
+static uint8_t z80_inst_ERR(Z80 *z80, uint8_t opcode)
+{
+    z80->except = true;
+    z80->exc_code = Z80_EXC_UNIMPLEMENTED_OPCODE;
+    z80->exc_data = opcode;
+    return 2;
+}
+
+static uint8_t (*instruction_lookup_table[256])(Z80*, uint8_t) = {
+    z80_inst_nop,
+    z80_inst_ERR
+};