diff --git a/Makefile b/Makefile
index bcaf68f..aa7b8e9 100644
--- a/Makefile
+++ b/Makefile
@@ -29,7 +29,7 @@ MAPDIR = $(SRCDIR)/map
 
 NESE_SRC_SRCS = f6502.c f6502_opcodes.c
 NESE_SRC_SRCS += nese.c nes.c cart.c mapper.c
-NESE_SRC_SRCS += ppu.c
+NESE_SRC_SRCS += ppu.c apu.c
 NESE_SRC_SRCS += $(OS)/port.c
 NESE_MAP_SRCS = $(notdir $(wildcard $(MAPDIR)/*.c))
 
diff --git a/src/apu.c b/src/apu.c
new file mode 100644
index 0000000..9e47746
--- /dev/null
+++ b/src/apu.c
@@ -0,0 +1,91 @@
+#include "memory.h"
+
+
+void nes_apu_init(nes_APU* apu, nes_APU_Memory* mem, int freq) {
+    // TODO
+}
+
+void nes_apu_hsync(nes_APU* apu, nes_Memory* core_mem) {
+    nes_APU_Memory* mem = &core_mem->apu;
+
+    for (int i = 0; i < core_mem->apu.n_events; ++i) {
+        int reg = core_mem->apu.events[i].reg;
+        int val = core_mem->apu.events[i].val;
+
+        switch (reg) {
+        // TODO: 0x00 - 0x13
+        // TODO: 0x15
+        case 0x17: // Frame Counter
+            apu->frame_reg = val & (    apu_Frame_Mode |
+                                        apu_Frame_Inhibit);
+            apu->frame = 0;
+            apu->frame_delay = 0;
+
+            if (val & apu_Frame_Mode) {
+                // TODO
+                /*
+                nes_apu_clock_quarter_frame(apu);
+                nes_apu_clock_half_frame(apu);
+                */
+            }
+
+            if (val & apu_Frame_Inhibit) {
+                mem->status &= ~apu_Status_Frame_Int;
+            }
+
+            break;
+
+        }
+    }
+
+    core_mem->apu.n_events = 0;
+
+    // Frame advance
+
+    apu->frame_delay += nes_apu_hsync_ticks;
+
+    if (apu->frame_delay >= nes_apu_quarter_frame_ticks) {
+        apu->frame_delay -= nes_apu_quarter_frame_ticks;
+
+        int end = 0;
+        int quarter_frame = 1;
+        int half_frame = 0;
+
+        if (1 == apu->frame) {
+            half_frame = 1;
+        }
+
+        if (apu->frame_reg & apu_Frame_Mode) {
+            if (3 == apu->frame) {
+                quarter_frame = 0;
+            } else if (4 <= apu->frame) {
+                half_frame = 1;
+                end = 1;
+            }
+        } else {
+            if (3 <= apu->frame) {
+                half_frame = 1;
+                end = 1;
+            }
+        }
+
+        if (half_frame) {
+            // TODO
+            //nes_apu_clock_half_frame(apu);
+        }
+
+        if (quarter_frame) {
+            // TODO
+            //nes_apu_clock_quarter_frame(apu);
+        }
+
+        if (end) {
+            if (0 == apu->frame_reg) {
+                mem->status |= apu_Status_Frame_Int;
+            }
+            apu->frame = 0;
+        } else {
+            apu->frame++;
+        }
+    }
+}
diff --git a/src/apu.h b/src/apu.h
new file mode 100644
index 0000000..65fdd96
--- /dev/null
+++ b/src/apu.h
@@ -0,0 +1,105 @@
+#ifndef NESE_APU_H_
+#define NESE_APU_H_
+
+#include <stdint.h>
+
+
+#define nes_apu_hsync_ticks         (4U)
+#define nes_apu_quarter_frame_ticks (262U)
+
+
+typedef struct {
+    uint8_t reg;
+    uint8_t val;
+} nes_APU_Event;
+
+#define APU_MAX_EVENTS (64U)
+
+typedef enum __attribute__ ((__packed__)) {
+    apu_Status_Square_0  = 0b00000001,
+    apu_Status_Square_1  = 0b00000010,
+    apu_Status_Triangle  = 0b00000100,
+    apu_Status_Noise     = 0b00001000,
+    apu_Status_DMC       = 0b00010000,
+    apu_Status_Frame_Int = 0b01000000,
+    apu_Status_DMC_Int   = 0b10000000,
+} nes_apu_Status;
+
+typedef struct {
+    nes_APU_Event events[APU_MAX_EVENTS];
+    uint8_t n_events;
+    nes_apu_Status status;
+} nes_APU_Memory;
+
+
+typedef enum __attribute__ ((__packed__)) {
+    apu_Frame_Inhibit = 0b01000000,
+    apu_Frame_Mode    = 0b10000000,
+} nes_apu_Frame;
+
+
+typedef enum {
+    apu_Channel_Square_0 = 0,
+    apu_Channel_Square_1 = 1,
+    apu_Channel_Triangle = 2,
+    apu_Channel_Noise    = 3,
+    apu_Channel_DMC      = 4,
+} nes_apu_Channel_Index;
+
+// Applies to Square and Noise
+typedef enum {
+    apu_Envelope_Volume   = 0b00001111,
+    apu_Envelope_Constant = 0b00010000,
+    apu_Envelope_Halt     = 0b00100000,
+    apu_Envelope_Duty     = 0b11000000,
+} nes_apu_Envelope_Reg_0_Mask;
+
+typedef enum {
+    apu_Square_Shift    = 0b00000111,
+    apu_Square_Negate   = 0b00001000,
+    apu_Square_Period   = 0b01110000,
+    apu_Square_Enable   = 0b10000000,
+} nes_apu_Square_Reg_1_Mask;
+
+typedef enum {
+    apu_Triangle_Count  = 0b01111111,
+    apu_Triangle_Halt   = 0b10000000,
+} nes_apu_Triangle_Reg_0_Mask;
+
+typedef enum {
+    apu_Noise_Period    = 0b00001111,
+    apu_Noise_Mode      = 0b10000000,
+} nes_apu_Noise_Reg_2_Mask;
+
+typedef enum {
+    apu_Noise_Length    = 0b11111000,
+} nes_apu_Noise_Reg_3_Mask;
+
+typedef enum {
+    apu_DMC_Period      = 0b00001111,
+    apu_DMC_Loop        = 0b01000000,
+    apu_DMC_IRQ_Enable  = 0b10000000,
+} nes_apu_DMC_Reg_0_Mask;
+
+typedef enum {
+    apu_Channel_Reload  = 0b00000001,
+    apu_Channel_Start   = 0b00000010,
+} nes_apu_Channel_Flag;
+
+typedef struct {
+    nes_apu_Frame frame_reg;
+    int frame;
+    int frame_delay;
+
+    uint8_t reg[20];
+
+    // TODO: Channel data
+} nes_APU;
+
+struct nes_Memory;
+
+void nes_apu_init(nes_APU* apu, nes_APU_Memory* mem, int freq);
+void nes_apu_hsync(nes_APU*, struct nes_Memory*);
+
+
+#endif // NESE_APU_H_
diff --git a/src/f6502.c b/src/f6502.c
index aecc487..1c9538d 100644
--- a/src/f6502.c
+++ b/src/f6502.c
@@ -90,15 +90,21 @@ static inline uint8_t f6502_read(nes_Memory* mem,
 
     case 0x4000:
         switch (addr & 0x1FU) {
+        case 0x15:
+            return mem->apu.status;
+
         case 0x16:
         case 0x17:
-        {   nes_Gamepad* gamepad = &mem->input.gamepads[addr & 1];
             return ((addr >> 8) & nes_controller_bus_mask) |
-                  read_gamepad_bit(gamepad);
-        }   break;
+                   read_gamepad_bit(
+                            &mem->input.gamepads[addr & 1]
+                        );
 
         default:
-            // TODO: APU Reg
+            if (mem->mapper.read_apu) {
+                return mem->mapper.read_apu(&mem->mapper, mem,
+                                            addr & 0x1FU);
+            }
         }
         break;
 
@@ -246,8 +252,40 @@ static inline int f6502_write(nes_Memory* mem,
         break;
 
     case 0x4000:
-        // TODO: APU
         switch (addr & 0x1FU) {
+        case 0x00:
+        case 0x01:
+        case 0x02:
+        case 0x03:
+        case 0x04:
+        case 0x05:
+        case 0x06:
+        case 0x07:
+        case 0x08:
+        case 0x09:
+        case 0x0A:
+        case 0x0B:
+        case 0x0C:
+        case 0x0D:
+        case 0x0E:
+        case 0x0F:
+        case 0x10:
+        case 0x11:
+        case 0x12:
+        case 0x13:
+        case 0x15:
+        case 0x17:
+            if (mem->apu.n_events >= APU_MAX_EVENTS) {
+                LOGE("APU event buffer full");
+            } else {
+                mem->apu.events[mem->apu.n_events++] =
+                    (nes_APU_Event){
+                        .reg = addr & 0x1FU,
+                        .val = val,
+                    };
+            }
+            break;
+
         case 0x14:  // OAM DMA
         {   uint8_t* src = NULL;
             // OAM DMA
@@ -285,6 +323,12 @@ static inline int f6502_write(nes_Memory* mem,
                 mem->input.gamepads[1].shift = 0;
             }
             break;
+
+        default:
+            if (mem->mapper.write_apu) {
+                ret = mem->mapper.write_apu(&mem->mapper, mem,
+                                            addr, val);
+            }
         }
         break;
 
diff --git a/src/linux/port.c b/src/linux/port.c
index 98b7139..c2a48f9 100644
--- a/src/linux/port.c
+++ b/src/linux/port.c
@@ -15,7 +15,8 @@
 #include "log.h"
 
 
-/* OS-specific file operations
+/*
+ * OS-specific file operations
  * Memory mapping specifically needs to be ported for each OS
  */
 
@@ -42,7 +43,8 @@ static int nese_file_size(FILE* file) {
 }
 
 
-/* Platform-specific allocation
+/*
+ * Platform-specific allocation
  * GPU and CPU refer to emulator memory spaces
  * Note: GPU (and possibly CPU) regions may need
  * to be placed into internal ram for performance
@@ -61,7 +63,8 @@ void* nese_alloc(int size) {
 }
 
 
-/* SDL-specific init and entry point
+/*
+ * SDL-specific init and entry point
  * This should be maximally reusable across platforms
  */
 
@@ -84,6 +87,14 @@ typedef struct {
 } platform_data;
 
 
+/* Input */
+
+int nese_update_input(void* plat_data, nes_Input* input) {
+    // Gamepad states are already updated in nese_frame_ready()
+    return 0;
+}
+
+
 static const int sdl_alt_start_key = SDLK_RETURN;
 
 static const int sdl_keycodes[nes_controller_num_buttons] = {
@@ -138,6 +149,9 @@ static int process_events(nes* sys) {
     return status;
 }
 
+
+/* Time / Video */
+
 static SDL_Color nes_palette[64] = {
    {0x80,0x80,0x80}, {0x00,0x00,0xBB}, {0x37,0x00,0xBF}, {0x84,0x00,0xA6},
    {0xBB,0x00,0x6A}, {0xB7,0x00,0x1E}, {0xB3,0x00,0x00}, {0x91,0x26,0x00},
@@ -278,11 +292,18 @@ int nese_frame_ready(void* plat_data) {
     return status;
 }
 
-int nese_update_input(void* plat_data, nes_Input* input) {
-    // Gamepad states are already updated in nese_frame_ready()
-    return 0;
+
+/* Audio */
+
+int nese_get_audio_frequency(void*) {
+    return 44100;
 }
 
+// TODO: Audio functions
+
+
+/* Platform Data */
+
 static int plat_init(platform_data* plat) {
     int status = SDL_Init(
             SDL_INIT_EVENTS |
diff --git a/src/memory.h b/src/memory.h
index b23bb9e..c905191 100644
--- a/src/memory.h
+++ b/src/memory.h
@@ -3,6 +3,7 @@
 
 #include <stdint.h>
 
+#include "apu.h"
 #include "input.h"
 #include "mapper.h"
 #include "ppu.h"
@@ -33,6 +34,7 @@ struct nes_Memory {
     nes_PPU_Memory ppu;
     nes_Mapper mapper;
     nes_Input input;
+    nes_APU_Memory apu;
 #endif
 };
 typedef struct nes_Memory nes_Memory;
diff --git a/src/nes.c b/src/nes.c
index 2053452..153b35e 100644
--- a/src/nes.c
+++ b/src/nes.c
@@ -7,10 +7,11 @@
 #include "log.h"
 
 
-void nes_init(nes* sys) {
+void nes_init(nes* sys, void* plat) {
     f6502_init(&sys->core);
     nes_ppu_init(&sys->ppu, &sys->core.memory.ppu);
-    // TODO: Init APU
+    nes_apu_init(&sys->apu, &sys->core.memory.apu,
+                 nese_get_audio_frequency(plat));
 }
 
 void nes_reset(nes* sys) {
@@ -47,7 +48,7 @@ static int nes_vsync(nes* sys, void* plat) {
 static int nes_hsync(nes* sys, void* plat) {
     int status = 0;
 
-    // TODO: APU sync
+    nes_apu_hsync(&sys->apu, &sys->core.memory);
 
     if (sys->ppu.scanline < nes_ppu_postrender_line) {
         if (sys->ppu.scanline < nes_ppu_visible_line) {
diff --git a/src/nes.h b/src/nes.h
index 17bd9c2..e4114d9 100644
--- a/src/nes.h
+++ b/src/nes.h
@@ -11,10 +11,10 @@ typedef struct {
     const ines_Header* cart_header;
     f6502_Core core;
     nes_PPU ppu;
-    // TODO: APU
+    nes_APU apu;
 } nes;
 
-void nes_init(nes*);
+void nes_init(nes*, void*);
 void nes_reset(nes*);
 void nes_done(nes*);
 int nes_loop(nes*, void*);
diff --git a/src/nese.c b/src/nese.c
index ff4d2e3..fa45112 100644
--- a/src/nese.c
+++ b/src/nese.c
@@ -6,7 +6,7 @@
 
 
 int nese_start(nes* sys, void* plat) {
-    nes_init(sys);
+    nes_init(sys, plat);
 
     nes_reset(sys);
 
diff --git a/src/port.h b/src/port.h
index 4d91896..122ef28 100644
--- a/src/port.h
+++ b/src/port.h
@@ -10,6 +10,7 @@ int nese_frame_start(void*, uint8_t background);
 int nese_line_ready(void*, uint8_t* buffer, int line);
 int nese_frame_ready(void*);
 int nese_update_input(void*, nes_Input*);
+int nese_get_audio_frequency(void*);
 
 void* nese_alloc_gpu(int);
 void* nese_alloc_cpu(int);