mirror of
https://github.com/ekeeke/Genesis-Plus-GX.git
synced 2024-11-12 22:05:06 +01:00
452 lines
12 KiB
C
452 lines
12 KiB
C
/*
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SN76489 emulation
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by Maxim in 2001 and 2002
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converted from my original Delphi implementation
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I'm a C newbie so I'm sure there are loads of stupid things
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in here which I'll come back to some day and redo
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Includes:
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- Super-high quality tone channel "oversampling" by calculating fractional positions on transitions
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- Noise output pattern reverse engineered from actual SMS output
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- Volume levels taken from actual SMS output
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07/08/04 Charles MacDonald
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Modified for use with SMS Plus:
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- Added support for multiple PSG chips.
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- Added reset/config/update routines.
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- Added context management routines.
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- Removed SN76489_GetValues().
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- Removed some unused variables.
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25/04/07 Eke-Eke (Genesis Plus GX)
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- Removed stereo GG support (unused)
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- Made SN76489_Update outputs 16bits mono samples
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- Replaced volume table with VGM plugin's one
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05/01/09 Eke-Eke (Genesis Plus GX)
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- Modified Cut-Off frequency (according to Steve Snake: http://www.smspower.org/forums/viewtopic.php?t=1746)
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24/08/10 Eke-Eke (Genesis Plus GX)
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- Removed multichip support (unused)
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- Removed alternate volume table, panning & mute support (unused)
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- Removed configurable Feedback and Shift Register Width (always use Sega ones)
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- Added linear resampling using Blip Buffer (based on Blargg's implementation: http://www.smspower.org/forums/viewtopic.php?t=11376)
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01/09/12 Eke-Eke (Genesis Plus GX)
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- Added generic Blip-Buffer support internally, using common Master Clock as timebase
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- Re-added stereo GG support
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- Re-added configurable Feedback and Shift Register Width
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- Rewrote core with various optimizations
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*/
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#include "shared.h"
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#define PSG_MCYCLES_RATIO (16 * 15)
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/* Initial state of shift register */
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#define NoiseInitialState 0x8000
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/* Value below which PSG does not output */
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/*#define PSG_CUTOFF 0x6*/
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#define PSG_CUTOFF 0x1
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/* original Texas Instruments TMS SN76489AN (rev. A) used in SG-1000, SC-3000H & SF-7000 computers */
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#define FB_DISCRETE 0x0006
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#define SRW_DISCRETE 15
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/* SN76489AN clone integrated in Sega's VDP chips (315-5124, 315-5246, 315-5313, Game Gear) */
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#define FB_SEGAVDP 0x0009
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#define SRW_SEGAVDP 16
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typedef struct
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{
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/* Configuration */
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int PreAmp[4][2]; /* stereo channels pre-amplification ratio (%) */
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int NoiseFeedback;
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int SRWidth;
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/* PSG registers: */
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int Registers[8]; /* Tone, vol x4 */
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int LatchedRegister;
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int NoiseShiftRegister;
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int NoiseFreq; /* Noise channel signal generator frequency */
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/* Output calculation variables */
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int ToneFreqVals[4]; /* Frequency register values (counters) */
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int ToneFreqPos[4]; /* Frequency channel flip-flops */
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int Channel[4][2]; /* current amplitude of each (stereo) channel */
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int ChanOut[4][2]; /* current output value of each (stereo) channel */
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/* Internal M-clock counter */
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unsigned long clocks;
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} SN76489_Context;
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static const uint16 PSGVolumeValues[16] =
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{
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/* These values are taken from a real SMS2's output */
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/*{892,892,892,760,623,497,404,323,257,198,159,123,96,75,60,0}, */
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/* I can't remember why 892... :P some scaling I did at some point */
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/* these values are true volumes for 2dB drops at each step (multiply previous by 10^-0.1) */
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1516,1205,957,760,603,479,381,303,240,191,152,120,96,76,60,0
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};
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static SN76489_Context SN76489;
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static blip_t* blip[2];
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void SN76489_Init(blip_t* left, blip_t* right, int type)
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{
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int i;
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blip[0] = left;
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blip[1] = right;
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for (i=0; i<4; i++)
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{
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SN76489.PreAmp[i][0] = 100;
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SN76489.PreAmp[i][1] = 100;
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}
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if (type == SN_DISCRETE)
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{
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SN76489.NoiseFeedback = FB_DISCRETE;
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SN76489.SRWidth = SRW_DISCRETE;
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}
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else
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{
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SN76489.NoiseFeedback = FB_SEGAVDP;
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SN76489.SRWidth = SRW_SEGAVDP;
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}
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}
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void SN76489_Reset()
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{
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int i;
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for(i = 0; i <= 3; i++)
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{
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/* Initialise PSG state */
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SN76489.Registers[2*i] = 1; /* tone freq=1 */
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SN76489.Registers[2*i+1] = 0xf; /* vol=off */
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/* Set counters to 0 */
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SN76489.ToneFreqVals[i] = 0;
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/* Set flip-flops to 1 */
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SN76489.ToneFreqPos[i] = 1;
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/* Clear stereo channels amplitude */
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SN76489.Channel[i][0] = 0;
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SN76489.Channel[i][1] = 0;
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/* Clear stereo channel outputs in delta buffer */
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SN76489.ChanOut[i][0] = 0;
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SN76489.ChanOut[i][1] = 0;
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}
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/* Initialise latched register index */
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SN76489.LatchedRegister = 0;
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/* Initialise noise generator */
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SN76489.NoiseShiftRegister=NoiseInitialState;
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SN76489.NoiseFreq = 0x10;
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/* Reset internal M-cycle counter */
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SN76489.clocks = 0;
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}
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void *SN76489_GetContextPtr(void)
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{
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return (uint8 *)&SN76489;
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}
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int SN76489_GetContextSize(void)
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{
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return sizeof(SN76489_Context);
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}
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/* Updates tone amplitude in delta buffer. Call whenever amplitude might have changed. */
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INLINE void UpdateToneAmplitude(int i, int time)
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{
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int delta;
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/* left output */
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delta = (SN76489.Channel[i][0] * SN76489.ToneFreqPos[i]) - SN76489.ChanOut[i][0];
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if (delta != 0)
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{
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SN76489.ChanOut[i][0] += delta;
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blip_add_delta_fast(blip[0], time, delta);
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}
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/* right output */
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delta = (SN76489.Channel[i][1] * SN76489.ToneFreqPos[i]) - SN76489.ChanOut[i][1];
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if (delta != 0)
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{
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SN76489.ChanOut[i][1] += delta;
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blip_add_delta_fast(blip[1], time, delta);
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}
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}
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/* Updates noise amplitude in delta buffer. Call whenever amplitude might have changed. */
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INLINE void UpdateNoiseAmplitude(int time)
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{
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int delta;
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/* left output */
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delta = (SN76489.Channel[3][0] * ( SN76489.NoiseShiftRegister & 0x1 )) - SN76489.ChanOut[3][0];
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if (delta != 0)
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{
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SN76489.ChanOut[3][0] += delta;
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blip_add_delta_fast(blip[0], time, delta);
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}
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/* right output */
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delta = (SN76489.Channel[3][1] * ( SN76489.NoiseShiftRegister & 0x1 )) - SN76489.ChanOut[3][1];
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if (delta != 0)
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{
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SN76489.ChanOut[3][1] += delta;
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blip_add_delta_fast(blip[1], time, delta);
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}
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}
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/* Runs tone channel for clock_length clocks */
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static void RunTone(int i, int clocks)
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{
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int time;
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/* Update in case a register changed etc. */
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UpdateToneAmplitude(i, SN76489.clocks);
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/* Time of next transition */
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time = SN76489.ToneFreqVals[i];
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/* Process any transitions that occur within clocks we're running */
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while (time < clocks)
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{
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if (SN76489.Registers[i*2]>PSG_CUTOFF) {
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/* Flip the flip-flop */
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SN76489.ToneFreqPos[i] = -SN76489.ToneFreqPos[i];
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} else {
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/* stuck value */
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SN76489.ToneFreqPos[i] = 1;
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}
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UpdateToneAmplitude(i, time);
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/* Advance to time of next transition */
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time += SN76489.Registers[i*2] * PSG_MCYCLES_RATIO;
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}
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/* Update channel tone counter */
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SN76489.ToneFreqVals[i] = time;
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}
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/* Runs noise channel for clock_length clocks */
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static void RunNoise(int clocks)
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{
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int time;
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/* Noise channel: match to tone2 if in slave mode */
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int NoiseFreq = SN76489.NoiseFreq;
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if (NoiseFreq == 0x80)
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{
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NoiseFreq = SN76489.Registers[2*2];
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SN76489.ToneFreqVals[3] = SN76489.ToneFreqVals[2];
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}
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/* Update in case a register changed etc. */
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UpdateNoiseAmplitude(SN76489.clocks);
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/* Time of next transition */
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time = SN76489.ToneFreqVals[3];
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/* Process any transitions that occur within clocks we're running */
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while (time < clocks)
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{
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/* Flip the flip-flop */
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SN76489.ToneFreqPos[3] = -SN76489.ToneFreqPos[3];
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if (SN76489.ToneFreqPos[3] == 1)
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{
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/* On the positive edge of the square wave (only once per cycle) */
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int Feedback = SN76489.NoiseShiftRegister;
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if ( SN76489.Registers[6] & 0x4 )
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{
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/* White noise */
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/* Calculate parity of fed-back bits for feedback */
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/* Do some optimised calculations for common (known) feedback values */
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/* If two bits fed back, I can do Feedback=(nsr & fb) && (nsr & fb ^ fb) */
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/* since that's (one or more bits set) && (not all bits set) */
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Feedback = ((Feedback & SN76489.NoiseFeedback) && ((Feedback & SN76489.NoiseFeedback) ^ SN76489.NoiseFeedback));
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}
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else /* Periodic noise */
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Feedback = Feedback & 1;
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SN76489.NoiseShiftRegister = (SN76489.NoiseShiftRegister >> 1) | (Feedback << (SN76489.SRWidth - 1));
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UpdateNoiseAmplitude(time);
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}
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/* Advance to time of next transition */
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time += NoiseFreq * PSG_MCYCLES_RATIO;
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}
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/* Update channel tone counter */
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SN76489.ToneFreqVals[3] = time;
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}
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static void SN76489_RunUntil(unsigned int clocks)
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{
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int i;
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/* Run noise first, since it might use current value of third tone frequency counter */
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RunNoise(clocks);
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/* Run tone channels */
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for (i=0; i<3; ++i)
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{
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RunTone(i, clocks);
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}
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}
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void SN76489_Config(unsigned int clocks, int preAmp, int boostNoise, int stereo)
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{
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int i;
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/* cycle-accurate Game Gear stereo */
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if (clocks > SN76489.clocks)
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{
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/* Run chip until current timestamp */
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SN76489_RunUntil(clocks);
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/* Update internal M-cycle counter */
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SN76489.clocks += ((clocks - SN76489.clocks + PSG_MCYCLES_RATIO - 1) / PSG_MCYCLES_RATIO) * PSG_MCYCLES_RATIO;
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}
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for (i=0; i<4; i++)
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{
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/* stereo channel pre-amplification */
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SN76489.PreAmp[i][0] = preAmp * ((stereo >> (i + 4)) & 1);
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SN76489.PreAmp[i][1] = preAmp * ((stereo >> (i + 0)) & 1);
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/* noise channel boost */
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if (i == 3)
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{
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SN76489.PreAmp[3][0] = SN76489.PreAmp[3][0] << boostNoise;
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SN76489.PreAmp[3][1] = SN76489.PreAmp[3][1] << boostNoise;
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}
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/* update stereo channel amplitude */
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SN76489.Channel[i][0]= (PSGVolumeValues[SN76489.Registers[i*2 + 1]] * SN76489.PreAmp[i][0]) / 100;
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SN76489.Channel[i][1]= (PSGVolumeValues[SN76489.Registers[i*2 + 1]] * SN76489.PreAmp[i][1]) / 100;
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}
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}
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void SN76489_Update(unsigned int clocks)
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{
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int i;
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if (clocks > SN76489.clocks)
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{
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/* Run chip until current timestamp */
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SN76489_RunUntil(clocks);
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/* Update internal M-cycle counter */
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SN76489.clocks += ((clocks - SN76489.clocks + PSG_MCYCLES_RATIO - 1) / PSG_MCYCLES_RATIO) * PSG_MCYCLES_RATIO;
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}
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/* Adjust internal M-cycle counter for next frame */
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SN76489.clocks -= clocks;
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/* Adjust channel time counters for new frame */
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for (i=0; i<4; ++i)
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{
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SN76489.ToneFreqVals[i] -= clocks;
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}
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}
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void SN76489_Write(unsigned int clocks, unsigned int data)
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{
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unsigned int index;
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if (clocks > SN76489.clocks)
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{
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/* run chip until current timestamp */
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SN76489_RunUntil(clocks);
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/* update internal M-cycle counter */
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SN76489.clocks += ((clocks - SN76489.clocks + PSG_MCYCLES_RATIO - 1) / PSG_MCYCLES_RATIO) * PSG_MCYCLES_RATIO;
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}
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if (data & 0x80)
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{
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/* latch byte %1 cc t dddd */
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SN76489.LatchedRegister = index = (data >> 4) & 0x07;
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}
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else
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{
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/* restore latched register index */
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index = SN76489.LatchedRegister;
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}
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switch (index)
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{
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case 0:
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case 2:
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case 4: /* Tone Channels frequency */
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{
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if (data & 0x80)
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{
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/* Data byte %1 cc t dddd */
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SN76489.Registers[index] = (SN76489.Registers[index] & 0x3f0) | (data & 0xf);
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}
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else
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{
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/* Data byte %0 - dddddd */
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SN76489.Registers[index] = (SN76489.Registers[index] & 0x00f) | ((data & 0x3f) << 4);
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}
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/* zero frequency behaves the same as a value of 1 */
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if (SN76489.Registers[index] == 0)
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{
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SN76489.Registers[index] = 1;
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}
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break;
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}
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case 1:
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case 3:
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case 5: /* Tone Channels attenuation */
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{
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data &= 0x0f;
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SN76489.Registers[index] = data;
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data = PSGVolumeValues[data];
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index >>= 1;
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SN76489.Channel[index][0] = (data * SN76489.PreAmp[index][0]) / 100;
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SN76489.Channel[index][1] = (data * SN76489.PreAmp[index][1]) / 100;
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break;
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}
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case 6: /* Noise control */
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{
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SN76489.Registers[6] = data & 0x0f;
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/* reset shift register */
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SN76489.NoiseShiftRegister = NoiseInitialState;
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/* set noise signal generator frequency */
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SN76489.NoiseFreq = 0x10 << (data&0x3);
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break;
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}
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case 7: /* Noise attenuation */
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{
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data &= 0x0f;
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SN76489.Registers[7] = data;
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data = PSGVolumeValues[data];
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SN76489.Channel[3][0] = (data * SN76489.PreAmp[3][0]) / 100;
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SN76489.Channel[3][1] = (data * SN76489.PreAmp[3][1]) / 100;
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break;
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}
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}
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}
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