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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);
}
}
}
}
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