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#include <string.h>
#include "ym2612.h"
#define BUSY_CYCLES 17
#define TIMERA_UPDATE_PERIOD 144
#define REG_TIMERA_HIGH 0x3 // 0x24
#define REG_TIMERA_LOW 0x4 // 0x25
#define REG_TIMERB 0x5 // 0x26
#define REG_TIME_CTRL 0x6 // 0x27
#define BIT_TIMERA_ENABLE 0x1
#define BIT_TIMERB_ENABLE 0x2
#define BIT_TIMERA_OVEREN 0x4
#define BIT_TIMERB_OVEREN 0x8
#define BIT_TIMERA_RESET 0x10
#define BIT_TIMERB_RESET 0x20
#define BIT_STATUS_TIMERA 0x1
#define BIT_STATUS_TIMERB 0x2
void ym_init(ym2612_context * context)
{
memset(context, 0, sizeof(*context));
}
void ym_run(ym2612_context * context, uint32_t to_cycle)
{
uint32_t delta = to_cycle - context->current_cycle;
//Timers won't be perfect with this, but it's good enough for now
//once actual FM emulation is in place the timers should just be
//decremented/reloaded on the appropriate ticks
uint32_t timer_delta = to_cycle / TIMERA_UPDATE_PERIOD;
if (context->part1_regs[REG_TIME_CTRL] & BIT_TIMERA_ENABLE) {
if (timer_delta > context->timer_a) {
if (context->part1_regs[REG_TIME_CTRL] & BIT_TIMERA_OVEREN) {
context->status |= BIT_STATUS_TIMERA;
}
uint32_t rem_delta = timer_delta - (context->timer_a+1);
uint16_t timer_val = (context->part1_regs[REG_TIMERA_HIGH] << 2) | (context->part1_regs[REG_TIMERA_LOW] & 0x3);
context->timer_a = timer_val - (rem_delta % (timer_val + 1));
} else {
context->timer_a -= timer_delta;
}
}
timer_delta /= 16; //Timer B runs at 1/16th the speed of Timer A
if (context->part1_regs[REG_TIME_CTRL] & BIT_TIMERB_ENABLE) {
if (timer_delta > context->timer_b) {
if (context->part1_regs[REG_TIME_CTRL] & BIT_TIMERB_OVEREN) {
context->status |= BIT_STATUS_TIMERB;
}
uint32_t rem_delta = timer_delta - (context->timer_b+1);
uint8_t timer_val = context->part1_regs[REG_TIMERB];
context->timer_b = timer_val - (rem_delta % (timer_val + 1));
} else {
context->timer_a -= timer_delta;
}
}
context->current_cycle = to_cycle;
if (to_cycle >= context->write_cycle + BUSY_CYCLES) {
context->status &= 0x7F;
}
}
void ym_address_write_part1(ym2612_context * context, uint8_t address)
{
if (address >= 0x21 && address < 0xB7) {
context->selected_reg = context->part1_regs + address - 0x21;
} else {
context->selected_reg = NULL;
}
}
void ym_address_write_part2(ym2612_context * context, uint8_t address)
{
if (address >= 0x30 && address < 0xB7) {
context->selected_reg = context->part1_regs + address - 0x30;
} else {
context->selected_reg = NULL;
}
}
void ym_data_write(ym2612_context * context, uint8_t value)
{
if (context->selected_reg && !(context->status & 0x80)) {
*context->selected_reg = value;
context->write_cycle = context->current_cycle;
context->selected_reg = NULL;//TODO: Verify this
}
}
uint8_t ym_read_status(ym2612_context * context)
{
return context->status;
}
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