diff --git a/src/audio/apu.h b/src/audio/apu.h
index fef25af..356cf94 100644
--- a/src/audio/apu.h
+++ b/src/audio/apu.h
@@ -12,6 +12,7 @@
 #include "noise.h"
 #include "dmc.h"
 
+// apu_t emulates an NES audio processing unit (APU).
 typedef struct apu_t
 {
 	bus_t* bus;
@@ -46,12 +47,25 @@ typedef struct apu_t
 
 apu_t* apu_create(bus_t* bus, gfx_t* gfx);
 void apu_destroy(apu_t* apu);
+
+// apu_reset performs a soft reset on the APU.
 void apu_reset(apu_t* apu);
+
+// apu_exec executes a single APU cycle.
 void apu_exec(apu_t* apu);
+
+// apu_set_status writes to the APU_STATUS register (0x4015).
 void apu_set_status(apu_t* apu, uint8_t val);
+
+// apu_get_sample cycles the apu_t channels and returns a tone.
 float apu_get_sample(apu_t* apu);
+
+// apu_queue_audio queues audio samples to be played by SDL.
 void apu_queue_audio(apu_t* apu, gfx_t* gfx);
+
+// apu_read_status reads from the APU_STATUS register (0x4015).
 uint8_t apu_read_status(apu_t* apu);
+
 void apu_set_frame_counter_ctrl(apu_t* apu, uint8_t val);
 
 #endif // NES_TOOLS_APU_H
diff --git a/src/bus.h b/src/bus.h
index 70304d8..4643817 100644
--- a/src/bus.h
+++ b/src/bus.h
@@ -60,6 +60,7 @@ struct cpu6502_t;
 struct apu_t;
 struct ppu_t;
 
+// bus_t emulates an NES bus.
 typedef struct
 {
 	// Memory.
@@ -81,10 +82,17 @@ typedef struct
 bus_t* bus_create(mapper_t* mapper);
 void bus_destroy(bus_t* bus);
 
+// bus_write writes val to addr in main memory.
 void bus_write(bus_t* bus, uint16_t addr, uint8_t val);
+
+// bus_read fetches the value at addr from main memory.
 uint8_t bus_read(bus_t* bus, uint16_t addr);
+
+// bus_get_ptr gets a pointer to the value at addr in main memory.
 uint8_t* bus_get_ptr(bus_t* bus, uint16_t addr);
 
+// Eventually set all other NES circuits. bus_read/bus_write will be
+// able to access memory-mapped registers.
 void bus_set_cpu(bus_t* bus, struct cpu6502_t* cpu);
 void bus_set_apu(bus_t* bus, struct apu_t* apu);
 void bus_set_ppu(bus_t* bus, struct ppu_t* ppu);
diff --git a/src/cpu6502.h b/src/cpu6502.h
index 33357a1..e47a45d 100644
--- a/src/cpu6502.h
+++ b/src/cpu6502.h
@@ -13,6 +13,7 @@ enum
 	INTERRUPT_PENDING = 1 << 1
 };
 
+// Enumerates the possible states of the CPU status register.
 enum cpu_flag
 {
 	NEGATIVE  = 1 << 7,
@@ -24,6 +25,7 @@ enum cpu_flag
 	CARRY     = 1
 };
 
+// CPU 6502 opcodes, including unofficial/undocumented.
 enum cpu_opcode
 {
 	// Official opcodes.
@@ -41,18 +43,21 @@ enum cpu_opcode
 	ISB, RLA, RRA, SLO, SRE, SKB, IGN,
 };
 
+// 6502 instruction addressing modes.
 enum cpu_addr_mode
 {
 	NONE, IMPL, ACC, REL, IMT, ZPG, ZPG_X, ZPG_Y,
 	ABS, ABS_X, ABS_Y, IND, IND_IDX, IDX_IND,
 };
 
+// A CPU instruction combines an opcode and an addressing mode.
 struct cpu_instr
 {
 	enum cpu_opcode    opcode;
 	enum cpu_addr_mode mode;
 };
 
+// 6502 instruction lookup table.
 static const struct cpu_instr cpu_instr_lookup[256] =
 {
 //  HI\LO        0x0          0x1          0x2             0x3           0x4             0x5         0x6           0x7           0x8           0x9           0xA        0xB            0xC             0xD          0xE           0xF
@@ -75,6 +80,7 @@ static const struct cpu_instr cpu_instr_lookup[256] =
 	/*  0xF  */  {BEQ, REL}, {SBC, IND_IDX}, NIL_OP,     {ISB, IND_IDX}, {NOP, ZPG_X}, {SBC, ZPG_X}, {INC, ZPG_X}, {ISB, ZPG_X}, {SED, IMPL}, {SBC, ABS_Y}, {NOP, IMPL}, {ISB, ABS_Y}, {NOP, ABS_X}, {SBC, ABS_X}, {INC, ABS_X}, {ISB, ABS_X}
 };
 
+// cycle times corresponding to each instruction in cpu_instr_lookup.
 static const uint8_t cpu_cycle_lookup[256] =
 {
         // HI/LO 0  1  2  3  4  5  6  7  8  9  A  B  C  D  E  F
@@ -105,6 +111,7 @@ enum cpu_interrupt
 	IRQ
 };
 
+// cpu6502_t emulates a 6502 microprocessor.
 typedef struct cpu6502_t
 {
 	uint8_t  state;
@@ -134,9 +141,17 @@ typedef struct cpu6502_t
 
 cpu6502_t* cpu_create(bus_t* bus);
 void cpu_destroy(cpu6502_t* cpu);
+
+// cpu_reset performs a soft-reset.
 void cpu_reset(cpu6502_t* cpu);
+
+// cpu_exec executes a single CPU cycle.
 void cpu_exec(cpu6502_t* cpu);
+
+// cpu_dma_suspend suspends the CPU for 513 cycles for DMA transfer.
 void cpu_dma_suspend(cpu6502_t* cpu);
+
+// cpu_interrupt queues an interrupt.
 void cpu_interrupt(cpu6502_t* cpu, enum cpu_interrupt interrupt);
 
 #endif // NES_TOOLS_CPU6502_H
diff --git a/src/emulator.h b/src/emulator.h
index f4083c4..acf5ed4 100644
--- a/src/emulator.h
+++ b/src/emulator.h
@@ -10,18 +10,20 @@
 #include "gfx.h"
 #include "timerx.h"
 
-// Frame rate in Hz
+// Frame rate in Hz.
 #define NTSC_FRAME_RATE 60
 #define PAL_FRAME_RATE  50
 
 // Turbo keys toggle rate (Hz): value should be a factor of FRAME_RATE
-// and should never exceed FRAME_RATE for best result
+// and should never exceed FRAME_RATE for best result.
 #define NTSC_TURBO_RATE 30
 #define PAL_TURBO_RATE  25
 
 // Sleep time when emulator is paused in milliseconds.
 #define IDLE_SLEEP 50
 
+// emulator_t tracks the state of the NES emulator. It encapsulates
+// all significant NES circuits (CPU, PPU, APU, BUS).
 typedef struct
 {
 	cpu6502_t* cpu;
@@ -42,9 +44,16 @@ typedef struct
 
 } emulator_t;
 
+
 emulator_t* emulator_create(mapper_t* mapper);
+void emulator_destroy(emulator_t* emu);
+
+// emulator_reset reinitializes the emulator's state (equivalent to
+// soft-resetting the NES).
 void emulator_reset(emulator_t* emu);
+
+// emulator_exec executes the emulator. It enters a loop that stops
+// when the user closes the window or exits the process.
 void emulator_exec(emulator_t* emu);
-void emulator_destroy(emulator_t* emu);
 
 #endif // NES_TOOLS_EMULATOR_H
diff --git a/src/gfx.h b/src/gfx.h
index db27de9..0d09b26 100644
--- a/src/gfx.h
+++ b/src/gfx.h
@@ -3,6 +3,7 @@
 
 #include "system.h"
 
+// gfx_t interfaces the SDL window/renderer.
 typedef struct
 {
 	// Window metadata.
@@ -22,8 +23,12 @@ typedef struct
 
 } gfx_t;
 
+// gfx_create allocates a new gfx_t, creating a window with the
+// specified width, height, and scaling factor.
 gfx_t* gfx_create(int width, int height, float scale);
-void gfx_render(gfx_t* gfx, const uint32_t* buffer);
 void gfx_destroy(gfx_t* gfx);
 
+// gfx_render writes the texture stored in buffer to the screen.
+void gfx_render(gfx_t* gfx, const uint32_t* buffer);
+
 #endif // NES_TOOLS_GFX_H
diff --git a/src/joypad.h b/src/joypad.h
index d618999..81ba19c 100644
--- a/src/joypad.h
+++ b/src/joypad.h
@@ -3,6 +3,8 @@
 
 #include "system.h"
 
+// Each button's status is enumerated as a single bit in the joypad's
+// status attribute.
 enum
 {
 	TURBO_B  = 1 << 9,
@@ -17,6 +19,7 @@ enum
 	BUTTON_A = 1
 };
 
+// joypad_t maintains the state of an NES joypad.
 typedef struct
 {
 	uint8_t  strobe;
@@ -26,10 +29,23 @@ typedef struct
 
 } joypad_t;
 
+// joypad_create initializes a new joypad_t, where player specifies
+// the joypad number (two joypads may be used).
 joypad_t joypad_create(uint8_t player);
+
+// joypad_read reads from the joypad as though it were mapped to
+// main memory (address 0x4016 - 0x4017).
 uint8_t joypad_read(joypad_t* joy);
+
+// joypad_write writes to the joypad as though it were mapped to
+// main memory (address 0x4016).
 void joypad_write(joypad_t* joy, uint8_t data);
+
+// joypad_update updates the status of the joypad according to an SDL
+// keyboard event.
 void joypad_update(joypad_t* joy, SDL_Event* event);
+
+// joypad_trigger_turbo triggers the turbo button.
 void joypad_trigger_turbo(joypad_t* joy);
 
 #endif // NES_TOOLS_JOYPAD_H
diff --git a/src/mapper.h b/src/mapper.h
index 68737ab..c7dcff8 100644
--- a/src/mapper.h
+++ b/src/mapper.h
@@ -3,12 +3,18 @@
 
 #include "system.h"
 
+// The NES console had two main variants based on different TV display
+// standards: NTSC (used primarily in Japan/USA) and PAL (used in
+// Europe, Australia, etc.). The variant is specified in iNES file
+// format.
 enum tv_system
 {
 	NTSC = 0,
 	PAL
 };
 
+// Nametable mirroring configuration. See:
+// https://www.nesdev.org/wiki/Mirroring#Nametable_Mirroring
 enum mirroring
 {
 	NO_MIRRORING,
@@ -17,6 +23,7 @@ enum mirroring
 	FOUR_SCREEN
 };
 
+// mapper_t stores data for an iNES mapper/cartridge.
 typedef struct
 {
 	uint8_t* chr_rom;
@@ -39,16 +46,30 @@ typedef struct
 
 } mapper_t;
 
+// mapper_from_file creates a mapper_t instance from a '.nes' file.
 mapper_t* mapper_from_file(const char* path);
 void mapper_destroy(mapper_t* mapper);
 
+// mapper_read_rom fetches data from CPU-addressable locations in the
+// mapper circuit's memory. If an address is invalid, it returns bus.
 uint8_t mapper_read_rom(mapper_t* mapper, uint8_t bus, uint16_t addr);
+
+// mapper_write_rom writes to a CPU-addressable location in the mapper
+// circuit's memory.
 void mapper_write_rom(mapper_t* mapper, uint16_t addr, uint8_t val);
 
+// mapper_read_prg decodes a CPU-addressable address to a value in the
+// mapper's PRG ROM memory.
 uint8_t mapper_read_prg(mapper_t* mapper, uint16_t addr);
+
+// mapper_read_prg decodes a CPU-addressable address and writes a
+// value in the mapper's PRG ROM memory.
 void mapper_write_prg(mapper_t* mapper, uint16_t addr, uint8_t val);
 
+// mapper_read_chr fetches the value at addr from the mapper's CHR ROM.
 uint8_t mapper_read_chr(mapper_t* mapper, uint16_t addr);
+
+// mapper_read_chr writes val to the mapper's CHR ROM at the given addr.
 void mapper_write_chr(mapper_t* mapper, uint16_t addr, uint8_t val);
 
 #endif // NES_TOOLS_MAPPER_H
diff --git a/src/ppu.h b/src/ppu.h
index d37982f..0150d4e 100644
--- a/src/ppu.h
+++ b/src/ppu.h
@@ -36,6 +36,7 @@ enum
 	Y_SCROLL_BITS   = 0x73E0
 };
 
+// ppu_t emulates an NES picture processing unit (PPU).
 typedef struct ppu_t
 {
 	size_t frames;
@@ -91,21 +92,47 @@ extern uint32_t ppu_palette[64];
 
 ppu_t* ppu_create(bus_t* bus);
 void ppu_destroy(ppu_t* ppu);
+
+// ppu_reset performs a soft reset on the ppu.
 void ppu_reset(ppu_t* ppu);
 
+// ppu_exec executes a single PPU cycle.
 void ppu_exec(ppu_t* ppu);
 
+// ppu_read_status emulates reading from PPU_STATUS (0x2003).
 uint8_t ppu_read_status(ppu_t* ppu);
+
+// ppu_read emulates reading from PPU_DATA (0x2008).
 uint8_t ppu_read(ppu_t* ppu);
+
+// ppu_set_ctrl sets the PPU control register (0x2000).
 void ppu_set_ctrl(ppu_t* ppu, uint8_t ctrl);
+
+// ppu_write writes to PPU_DATA (0x2008).
 void ppu_write(ppu_t* ppu, uint8_t val);
+
+// ppu_dma triggers an OAM DMA transfer that takes ~513 CPU cycles.
 void ppu_dma(ppu_t* ppu, uint8_t addr);
+
+// ppu_set_scroll writes to the scroll register (0x2005).
 void ppu_set_scroll(ppu_t* ppu, uint8_t val);
+
+// ppu_set_addr writes to the PPU_ADDR register (0x2006).
 void ppu_set_addr(ppu_t* ppu, uint8_t addr);
+
+// ppu_set_oam_addr writes to the OAM_ADDR register (0x2003).
 void ppu_set_oam_addr(ppu_t* ppu, uint8_t addr);
+
+// ppu_read_oam reads from the OAM_DATA register (0x2004).
 uint8_t ppu_read_oam(ppu_t* ppu);
+
+// ppu_write_oam writes to the OAM_DATA register (0x2004).
 void ppu_write_oam(ppu_t* ppu, uint8_t val);
+
+// ppu_read_vram reads from the PPU's internal video memory.
 uint8_t ppu_read_vram(ppu_t* ppu, uint16_t addr);
+
+// ppu_read_vram writes to the PPU's internal video memory.
 void ppu_write_vram(ppu_t* ppu, uint16_t addr, uint8_t val);
 
 #endif // NES_TOOLS_PPU_H
diff --git a/src/snapshot.h b/src/snapshot.h
index 2a3b8bb..bd3047c 100644
--- a/src/snapshot.h
+++ b/src/snapshot.h
@@ -4,6 +4,7 @@
 #include "system.h"
 #include "emulator.h"
 
+// snapshot_t stores the emulator's state at a some point in time.
 typedef struct
 {
 	cpu6502_t* cpu;
@@ -19,9 +20,17 @@ typedef struct
 
 } snapshot_t;
 
+// snapshot_create copies the emulator's state into a new snapshot_t.
 snapshot_t* snapshot_create(emulator_t* emu);
+
+// snapshot_update copies an emulator's state into an already
+// initialized snapshot_t struct.
 void snapshot_update(snapshot_t* snap, emulator_t* emu);
+
+// snapshot_restore writes the snapshot's state into the given emulator.
 void snapshot_restore(snapshot_t* snap, emulator_t* emu);
+
+// snapshot_destroy safely deallocates a snapshot_t instance.
 void snapshot_destroy(snapshot_t* snap);
 
 #endif // NES_TOOLS_SNAPSHOT_H
diff --git a/src/timerx.h b/src/timerx.h
index d67fc0c..d48cf17 100644
--- a/src/timerx.h
+++ b/src/timerx.h
@@ -3,6 +3,8 @@
 
 #include "system.h"
 
+// timerx_t implements a timer that measures the execution time of
+// emulator routines and then sleeps for the remaining cycle time.
 typedef struct
 {
 	struct timespec start;
@@ -12,11 +14,25 @@ typedef struct
 
 } timerx_t;
 
+// timerx_create returns an initialized timerx with the given cycle
+// period in nanoseconds.
 timerx_t timerx_create(uint64_t period);
+
+// timerx_mark_start marks the start of a cycle period.
 void timerx_mark_start(timerx_t* timer);
+
+// timerx_mark_end marks the end of a cycle period.
 void timerx_mark_end(timerx_t* timer);
+
+// timerx_adjusted_wait sleeps for (period - execution time)
+// nanoseconds.
 int timerx_adjusted_wait(timerx_t* timer);
+
+// timerx_wait waits for the specified period (in milliseconds).
 int timerx_wait(uint64_t period_ms);
+
+// timerx_get_diff returns the elapsed time between timerx_mark_start
+// and timerx_mark_end.
 double timerx_get_diff(timerx_t* timer);
 
 #endif // NES_TOOLS_TIMERX_H