Pirate Gemlins 2 RE

CRC32s:
    Original   Patched
PRG 11fc8686   80006759
CHR a13b9d3a   fc5f907d

#include "mapinc.h"

static int prg[2];
static int mirr;
static int chr[8];
static int counter;
static int counting_enabled;
static int m2_prescaler;
static int first_phase;
static int counterchanged;

static SFORMAT StateRegs[] =
{
   { 0 }
};


//shifts bit from position `bit` into position `pos` of expression `exp`
#define shi(exp, bit, pos) \
   ((((exp) & (1 << (bit))) >> (bit)) << (pos))

static int vrc_addr_mix(int A) {
   //this game wires A0 to VRC_A0 and A1 to VRC_A1
   return (A & 0xf000) | shi(A, 2, 0) | shi(A, 3, 1);
}

static void M273IRQHook(int a)  {
   if (counting_enabled) {
      m2_prescaler += a;
      if (m2_prescaler >= (first_phase ? 128 : 256)) {
         first_phase = 0;
         m2_prescaler -= (first_phase ? 128 : 256);
         counter++;
         counterchanged=1;
      }
      else {
         counterchanged=0;
      }
      
      if (counterchanged) {
         if (counter >= 256) {
            counter -= 256;
            X6502_IRQBegin(FCEU_IQEXT);
         }
         else {
            X6502_IRQEnd(FCEU_IQEXT);
         }
      }
      
   }
}

static void Sync(void) {
   setprg8(0x8000, prg[0]);
   setprg8(0xa000, prg[1]);
   setprg16(0xc000, -1);   
   for (int i = 0; i < 8; ++i) {
      setchr1(0x400 * i, chr[i]);
   }
   
   
   switch (mirr) {
   case 0: setmirror(MI_V); break;
   case 1: setmirror(MI_H); break;
   }
   
}

static DECLFW(M273Write) {
   //writes to VRC chip
   switch (vrc_addr_mix(A)) {
   case 0x8000:
   case 0x8001:
   case 0x8002:
   case 0x8003:
      prg[0] = V;
      break;
   case 0xA000:
   case 0xA001:
   case 0xA002:
   case 0xA003:
      prg[1] = V;
      break;
   case 0x9000:
   case 0x9001:
   case 0x9002:
   case 0x9003:
      mirr = V & 1;
      break;
   case 0xb000: chr[0] = (chr[0] & 0xF0) | (V & 0xF); break;
   case 0xb001: chr[0] = (chr[0] & 0xF) | ((V & 0xF) << 4); break;
   case 0xb002: chr[1] = (chr[1] & 0xF0) | (V & 0xF); break;
   case 0xb003: chr[1] = (chr[1] & 0xF) | ((V & 0xF) << 4); break;
   case 0xc000: chr[2] = (chr[2] & 0xF0) | (V & 0xF); break;
   case 0xc001: chr[2] = (chr[2] & 0xF) | ((V & 0xF) << 4); break;
   case 0xc002: chr[3] = (chr[3] & 0xF0) | (V & 0xF); break;
   case 0xc003: chr[3] = (chr[3] & 0xF) | ((V & 0xF) << 4); break;
   case 0xd000: chr[4] = (chr[4] & 0xF0) | (V & 0xF); break;
   case 0xd001: chr[4] = (chr[4] & 0xF) | ((V & 0xF) << 4); break;
   case 0xd002: chr[5] = (chr[5] & 0xF0) | (V & 0xF); break;
   case 0xd003: chr[5] = (chr[5] & 0xF) | ((V & 0xF) << 4); break;
   case 0xe000: chr[6] = (chr[6] & 0xF0) | (V & 0xF); break;
   case 0xe001: chr[6] = (chr[6] & 0xF) | ((V & 0xF) << 4); break;
   case 0xe002: chr[7] = (chr[7] & 0xF0) | (V & 0xF); break;
   case 0xe003: chr[7] = (chr[7] & 0xF) | ((V & 0xF) << 4); break;
      
   default:
      break;
   }
   
   switch (A & 0xf00c) {
   case 0xf000: counter = V; X6502_IRQEnd(FCEU_IQEXT); break;
   case 0xf008: if ((V & 1) == 0) {counting_enabled = 0; m2_prescaler = 0; first_phase = 1; X6502_IRQEnd(FCEU_IQEXT);}
               else            {counting_enabled = 1; }
            break;
   }

   Sync();
}

static void M273Power(void) {
   SetWriteHandler(0x8000, 0xFFFF, M273Write);
   SetReadHandler(0x8000, 0xFFFF, CartBR);
   Sync();
   
   counter = 0;
}

static void M273Reset(void) {
   counter = 0;
   Sync();
}

void Mapper273_Init(CartInfo *info) {
   info->Power = M273Power;
   info->Reset = M273Reset;
   MapIRQHook = M273IRQHook;

   AddExState(&StateRegs, ~0, 0, 0);
}