1 files changed, 142 insertions, 0 deletions
diff --git a/mknes_apu.c b/mknes_apu.c
new file mode 100644
index 0000000..98552ed
--- /dev/null
+++ b/mknes_apu.c
@@ -0,0 +1,142 @@
+
+
+static inline void ppu_tick(struct nes_state *state);
+static uint8_t memory_read(struct nes_state *state, uint32_t offset);
+static uint8_t memory_read_dma(struct nes_state *state, uint32_t offset);
+
+// DMC frequency table (NTSC)
+static const uint16_t dmc_rate_table[16] = {
+	428, 380, 340, 320, 286, 254, 226, 214,
+	190, 160, 142, 128, 106,  85,  72,  54
+};
+
+// $4015 write
+static void apu_write4015(struct nes_state *state, uint8_t val) {
+	struct apu_state *apu = &state->apu;
+
+	if(val & 0x10) {
+		if(!apu->dmc_dma_enabled || apu->dmc_bytes_remaining == 0) {
+			apu->dmc_current_addr = 0xC000 + ((uint16_t)apu->dmc_sample_addr << 6);
+			apu->dmc_bytes_remaining = ((uint16_t)apu->dmc_sample_len << 4);
+		}
+		apu->dmc_dma_enabled = 1;
+	} else {
+		apu->dmc_dma_enabled = 0;
+	}
+}
+
+// $4015 read
+static uint8_t apu_read4015(struct nes_state *state) {
+	struct apu_state *apu = &state->apu;
+	uint8_t result = 0;
+
+	if(apu->dmc_bytes_remaining > 0) {
+		result |= 0x10;
+	}
+	if(apu->irq_pending) {
+		result |= 0x40;
+		apu->irq_pending = 0;
+	}
+	return result;
+}
+
+// $4010–$4013, $4015 write
+static void apu_write(struct nes_state *state, uint16_t addr, uint8_t val) {
+	struct apu_state *apu = &state->apu;
+
+	switch(addr) {
+		case 0x4010: {
+			apu->dmc_irq_enable = (val >> 7) & 1;
+			apu->dmc_loop_flag = (val >> 6) & 1;
+			apu->dmc_freq_index = val & 0x0F;
+		} break;
+		case 0x4011: {
+			// DAC write ignored
+		} break;
+		case 0x4012: {
+			apu->dmc_sample_addr = val;
+		} break;
+		case 0x4013: {
+			apu->dmc_sample_len = val;
+		} break;
+		case 0x4015: {
+			apu_write4015(state, val);
+		} break;
+	}
+}
+
+// APU tick
+static inline void apu_tick(struct nes_state *state) {
+	struct apu_state *apu = &state->apu;
+
+	apu->frame_cycle++;
+
+	if(apu->mode == 0) {
+		if(apu->frame_cycle == 7457) {
+			// Quarter frame
+		} else if(apu->frame_cycle == 14913) {
+			// Quarter frame
+			// Half frame
+		} else if(apu->frame_cycle == 22371) {
+			// Quarter frame
+		} else if(apu->frame_cycle == 29829) {
+			// Half frame
+			if(!apu->irq_inhibit) {
+				apu->irq_pending = 1;
+				state->cpu.irq_pending = 1;
+			}
+		} else if(apu->frame_cycle >= 29830) {
+			apu->frame_cycle = 0;
+		}
+	} else {
+		if(apu->frame_cycle == 7457) {
+			// Quarter frame
+		} else if(apu->frame_cycle == 14913) {
+			// Quarter frame
+			// Half frame
+		} else if(apu->frame_cycle == 22371) {
+			// Quarter frame
+		} else if(apu->frame_cycle == 29829) {
+			// Half frame
+		} else if(apu->frame_cycle == 37281) {
+			// Quarter frame
+		} else if(apu->frame_cycle >= 37282) {
+			apu->frame_cycle = 0;
+		}
+	}
+
+	if(apu->dmc_dma_enabled && apu->dmc_bytes_remaining > 0) {
+		apu->dmc_sample_timer++;
+		if(apu->dmc_sample_timer >= dmc_rate_table[apu->dmc_freq_index]) {
+			apu->dmc_sample_timer = 0;
+
+			uint8_t val = memory_read(state, apu->dmc_current_addr);
+			(void)val;
+
+			apu->dmc_current_addr++;
+			if(apu->dmc_current_addr == 0x0000) {
+				apu->dmc_current_addr = 0x8000;
+			}
+
+			apu->dmc_bytes_remaining--;
+
+			if(apu->dmc_bytes_remaining == 0) {
+				if(apu->dmc_loop_flag) {
+					apu->dmc_current_addr = 0xc000 + ((uint16_t)apu->dmc_sample_addr << 6);
+					apu->dmc_bytes_remaining = ((uint16_t)apu->dmc_sample_len << 4);
+				} else {
+					if(apu->dmc_irq_enable) {
+						apu->irq_pending = 1;
+						state->cpu.irq_pending = 1;
+					}
+					apu->dmc_dma_enabled = 0;
+				}
+			}
+
+			for(uint32_t i = 0; i < 4; i++) {
+				state->cpu.cycles++;
+				ppu_tick(state);
+			}
+		}
+	}
+}