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Add a YM2612 stub implementation with just timers and status registers so that games that depend on it can run.
author Mike Pavone <pavone@retrodev.com>
date Sun, 05 May 2013 22:56:42 -0700
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children cc39629e8d06
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a8ee7934a1f8 Add a YM2612 stub implementation with just timers and status registers so that games that depend on it can run.
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1 #include <string.h>
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2 #include "ym2612.h"
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3
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4 #define BUSY_CYCLES 17
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5 #define TIMERA_UPDATE_PERIOD 144
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6
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7 #define REG_TIMERA_HIGH 0x3 // 0x24
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8 #define REG_TIMERA_LOW 0x4 // 0x25
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9 #define REG_TIMERB 0x5 // 0x26
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10 #define REG_TIME_CTRL 0x6 // 0x27
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11
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12 #define BIT_TIMERA_ENABLE 0x1
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13 #define BIT_TIMERB_ENABLE 0x2
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14 #define BIT_TIMERA_OVEREN 0x4
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15 #define BIT_TIMERB_OVEREN 0x8
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16 #define BIT_TIMERA_RESET 0x10
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17 #define BIT_TIMERB_RESET 0x20
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18
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19 #define BIT_STATUS_TIMERA 0x1
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20 #define BIT_STATUS_TIMERB 0x2
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21
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22 void ym_init(ym2612_context * context)
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23 {
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24 memset(context, 0, sizeof(*context));
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25 }
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26
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27 void ym_run(ym2612_context * context, uint32_t to_cycle)
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28 {
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29 uint32_t delta = to_cycle - context->current_cycle;
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30 //Timers won't be perfect with this, but it's good enough for now
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31 //once actual FM emulation is in place the timers should just be
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32 //decremented/reloaded on the appropriate ticks
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33 uint32_t timer_delta = to_cycle / TIMERA_UPDATE_PERIOD;
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34 if (context->part1_regs[REG_TIME_CTRL] & BIT_TIMERA_ENABLE) {
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35 if (timer_delta > context->timer_a) {
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36 if (context->part1_regs[REG_TIME_CTRL] & BIT_TIMERA_OVEREN) {
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37 context->status |= BIT_STATUS_TIMERA;
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38 }
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39 uint32_t rem_delta = timer_delta - (context->timer_a+1);
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40 uint16_t timer_val = (context->part1_regs[REG_TIMERA_HIGH] << 2) | (context->part1_regs[REG_TIMERA_LOW] & 0x3);
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41 context->timer_a = timer_val - (rem_delta % (timer_val + 1));
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42 } else {
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43 context->timer_a -= timer_delta;
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44 }
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45 }
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46 timer_delta /= 16; //Timer B runs at 1/16th the speed of Timer A
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47 if (context->part1_regs[REG_TIME_CTRL] & BIT_TIMERB_ENABLE) {
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48 if (timer_delta > context->timer_b) {
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49 if (context->part1_regs[REG_TIME_CTRL] & BIT_TIMERB_OVEREN) {
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50 context->status |= BIT_STATUS_TIMERB;
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51 }
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52 uint32_t rem_delta = timer_delta - (context->timer_b+1);
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53 uint8_t timer_val = context->part1_regs[REG_TIMERB];
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54 context->timer_b = timer_val - (rem_delta % (timer_val + 1));
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55 } else {
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56 context->timer_a -= timer_delta;
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57 }
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58 }
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59 context->current_cycle = to_cycle;
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60 if (to_cycle >= context->write_cycle + BUSY_CYCLES) {
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61 context->status &= 0x7F;
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62 }
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63 }
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64
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65 void ym_address_write_part1(ym2612_context * context, uint8_t address)
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66 {
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67 if (address >= 0x21 && address < 0xB7) {
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68 context->selected_reg = context->part1_regs + address - 0x21;
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69 } else {
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70 context->selected_reg = NULL;
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71 }
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72 }
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73
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74 void ym_address_write_part2(ym2612_context * context, uint8_t address)
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75 {
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76 if (address >= 0x30 && address < 0xB7) {
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77 context->selected_reg = context->part1_regs + address - 0x30;
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78 } else {
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79 context->selected_reg = NULL;
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80 }
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81 }
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82
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83 void ym_data_write(ym2612_context * context, uint8_t value)
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84 {
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85 if (context->selected_reg && !(context->status & 0x80)) {
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86 *context->selected_reg = value;
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87 context->write_cycle = context->current_cycle;
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88 context->selected_reg = NULL;//TODO: Verify this
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89 }
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90 }
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91
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92 uint8_t ym_read_status(ym2612_context * context)
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93 {
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94 return context->status;
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95 }
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96