add iowrite.readbyte() lua function, to read the last value written to write-only port. it currently supports only general sound registers ($4000-4013, $4015, $4017).

src/fceu.cpp

@@ -205,6 +205,8 @@ static readfunc *AReadG;
 static writefunc *BWriteG;
 static int RWWrap=0;
 
+uint8 IOWriteLog[0x10000];
+
 //mbg merge 7/18/06 docs
 //bit0 indicates whether emulation is paused
 //bit1 indicates whether emulation is in frame step mode

src/fceu.h

@@ -54,6 +54,8 @@ uint8 FCEU_ReadRomByte(uint32 i);
 extern readfunc ARead[0x10000];
 extern writefunc BWrite[0x10000];
 
+extern uint8 IOWriteLog[0x10000];
+
 enum GI {
 	GI_RESETM2	=1,
 	GI_POWER =2,

src/lua-engine.cpp

@@ -1206,6 +1206,8 @@ static int rom_gethash(lua_State *L) {
 	else lua_pushstring(L, "");
 	return 1;
 }
+static int iowrite_readbyte(lua_State *L) {   int addr = luaL_checkinteger(L,1); lua_pushinteger(L, (addr >= 0 && addr <= 0xFFFF) ? IOWriteLog[addr] : 0); return 1; }
+static int iowrite_readbytesigned(lua_State *L) {   int addr = luaL_checkinteger(L,1); lua_pushinteger(L, (addr >= 0 && addr <= 0xFFFF) ? (signed char)IOWriteLog[addr] : 0); return 1; }
 static int memory_readbyte(lua_State *L) {   lua_pushinteger(L, FCEU_CheatGetByte(luaL_checkinteger(L,1))); return 1; }
 static int memory_writebyte(lua_State *L) {   FCEU_CheatSetByte(luaL_checkinteger(L,1), luaL_checkinteger(L,2)); return 0; }
 static int memory_readbyterange(lua_State *L) {
@@ -4699,6 +4701,16 @@ static const struct luaL_reg memorylib [] = {
 	{NULL,NULL}
 };
 
+static const struct luaL_reg iowritelib [] = {
+
+	{"readbyte", iowrite_readbyte},
+	{"readbytesigned", iowrite_readbytesigned},
+	// alternate naming scheme for unsigned
+	{"readbyteunsigned", iowrite_readbyte},
+
+	{NULL,NULL}
+};
+
 static const struct luaL_reg joypadlib[] = {
 	{"get", joypad_get},
 	{"getdown", joypad_getdown},
@@ -4921,6 +4933,7 @@ int FCEU_LoadLuaCode(const char *filename, const char *arg) {
 		luaL_register(L, "emu", emulib); // added for better cross-emulator compatibility
 		luaL_register(L, "FCEU", emulib); // kept for backward compatibility
 		luaL_register(L, "memory", memorylib);
+		luaL_register(L, "iowrite", iowritelib);
 		luaL_register(L, "rom", romlib);
 		luaL_register(L, "joypad", joypadlib);
 		luaL_register(L, "zapper", zapperlib);

src/sound.cpp

@@ -239,6 +239,7 @@ static void SQReload(int x, uint8 V)
 
 static DECLFW(Write_PSG)
 {
+ IOWriteLog[A] = V;
  A&=0x1F;
  switch(A)
  {
@@ -311,6 +312,7 @@ static DECLFW(Write_PSG)
 
 static DECLFW(Write_DMCRegs)
 {
+ IOWriteLog[A] = V;
  A&=0xF;
 
  switch(A)
@@ -342,6 +344,7 @@ static DECLFW(Write_DMCRegs)
 static DECLFW(StatusWrite)
 {
 	int x;
+	IOWriteLog[A] = V;
 
         DoSQ1();
         DoSQ2();
@@ -1005,6 +1008,7 @@ static void RDoNoise(void)
 
 DECLFW(Write_IRQFM)
 {
+ IOWriteLog[A] = V;
  V=(V&0xC0)>>6;
  fcnt=0;
  if(V&0x2)