Gh1267 update paging (#1326)

MemGetBankPtr(): simplify with a default arg. (#1262, PR #1326)
. UpdatePaging(): improve comment for page0 & page1 and memdirty

source/Debugger/Debug.cpp

@@ -4493,7 +4493,7 @@ Update_t CmdMemoryLoad (int nArgs)
 	}
 	const std::string sLoadSaveFilePath = g_sCurrentDir + g_sMemoryLoadSaveFileName; // TODO: g_sDebugDir
 	
-	BYTE * const pMemBankBase = bBankSpecified ? MemGetBankPtr(nBank, true) : mem;
+	BYTE * const pMemBankBase = bBankSpecified ? MemGetBankPtr(nBank) : mem;
 	if (!pMemBankBase)
 	{
 		ConsoleBufferPush( TEXT( "Error: Bank out of range." ) );
@@ -4832,7 +4832,7 @@ Update_t CmdMemorySave (int nArgs)
 			}
 			sLoadSaveFilePath += g_sMemoryLoadSaveFileName;
 
-			const BYTE * const pMemBankBase = bBankSpecified ? MemGetBankPtr(nBank, true) : mem;
+			const BYTE * const pMemBankBase = bBankSpecified ? MemGetBankPtr(nBank) : mem;
 			if (!pMemBankBase)
 			{
 				ConsoleBufferPush( TEXT( "Error: Bank out of range." ) );

source/Memory.cpp

@@ -1269,8 +1269,11 @@ static void UpdatePaging(BOOL initialize)
 	// PAGING SHADOW TABLE
 	//
 	// NB. the condition 'loop <= 1' is there because:
-	// . Page0 (ZP)    : memdirty[0] is set when the 6502 CPU does a ZP-write, but perhaps older versions didn't set this flag (eg. the asm version?).
+	// . Page0 (ZP) and Page1 (stack) are written to so often that it's almost certain that they'll be dirty every time this function is called.
-	// . Page1 (stack) : memdirty[1] is NOT set when the 6502 CPU writes to this page with JSR, etc.
+	// Note also that:
+	// . Page0 (ZP)    : memdirty[0] is set when the 6502 CPU writes to ZP.
+	// . Page1 (stack) : memdirty[1] is NOT set when the 6502 CPU writes to this page with JSR, PHA, etc.
+	// Ultimately this is an optimisation (due to Page1 writes not setting memdirty[1]) and Page0 could be optimised to also not set memdirty[0].
 
 	for (loop = 0x00; loop < 0x100; loop++)
 	{
@@ -1347,19 +1350,6 @@ bool MemCheckINTCXROM()
 
 //===========================================================================
 
-static void BackMainImage(void)
-{
-	for (UINT loop = 0; loop < 256; loop++)
-	{
-		if (memshadow[loop] && ((*(memdirty+loop) & 1) || (loop <= 1)))
-			memcpy(memshadow[loop], mem+(loop << 8), 256);
-
-		*(memdirty+loop) &= ~1;
-	}
-}
-
-//===========================================================================
-
 static LPBYTE MemGetPtrBANK1(const WORD offset, const LPBYTE pMemBase)
 {
 	if ((offset & 0xF000) != 0xC000)	// Requesting RAM at physical addr $Cxxx (ie. 4K RAM BANK1)
@@ -1445,14 +1435,27 @@ LPBYTE MemGetMainPtr(const WORD offset)
 
 //===========================================================================
 
+static void BackMainImage(void)
+{
+	for (UINT loop = 0; loop < 256; loop++)
+	{
+		if (memshadow[loop] && ((*(memdirty + loop) & 1) || (loop <= 1)))
+			memcpy(memshadow[loop], mem + (loop << 8), 256);
+
+		*(memdirty + loop) &= ~1;
+	}
+}
+
+//-------------------------------------
+
 // Used by:
 // . Savestate: MemSaveSnapshotMemory(), MemLoadSnapshotAux()
 // . VidHD    : SaveSnapshot(), LoadSnapshot()
 // . Debugger : CmdMemorySave(), CmdMemoryLoad()
-LPBYTE MemGetBankPtr(const UINT nBank, const bool isSaveSnapshotOrDebugging)
+LPBYTE MemGetBankPtr(const UINT nBank, const bool isSaveSnapshotOrDebugging/*=true*/)
 {
 	// Only call BackMainImage() when a consistent 64K bank is needed, eg. for saving snapshot or debugging
-	// - for snapshot loads it's pointless, and worse it can corrupt pages 0 & 1 for aux banks (GH#1262)
+	// - for snapshot *loads* it's redundant, and worse it can corrupt pages 0 & 1 for aux banks, so must be avoided (GH#1262)
 	if (isSaveSnapshotOrDebugging)
 		BackMainImage();	// Flush any dirty pages to back-buffer
 
@@ -2412,7 +2415,7 @@ static const std::string& MemGetSnapshotAuxMemStructName(void)
 
 static void MemSaveSnapshotMemory(YamlSaveHelper& yamlSaveHelper, bool bIsMainMem, UINT bank=0, UINT size=64*1024)
 {
-	LPBYTE pMemBase = MemGetBankPtr(bank, true);
+	LPBYTE pMemBase = MemGetBankPtr(bank);
 
 	if (bIsMainMem)
 	{

source/Memory.h

@@ -57,7 +57,7 @@ bool	MemCheckSLOTC3ROM();
 bool	MemCheckINTCXROM();
 LPBYTE  MemGetAuxPtr(const WORD);
 LPBYTE  MemGetMainPtr(const WORD);
-LPBYTE  MemGetBankPtr(const UINT nBank, const bool isSaveSnapshotOrDebugging);
+LPBYTE  MemGetBankPtr(const UINT nBank, const bool isSaveSnapshotOrDebugging = true);
 LPBYTE  MemGetCxRomPeripheral();
 DWORD   GetMemMode(void);
 void    SetMemMode(DWORD memmode);

source/VidHD.cpp

@@ -210,7 +210,7 @@ void VidHDCard::SaveSnapshot(YamlSaveHelper& yamlSaveHelper)
 		// Save [$400-$9FFF]
 		YamlSaveHelper::Label state(yamlSaveHelper, "%s:\n", MemGetSnapshotAuxMemStructName().c_str());
 
-		LPBYTE pMemBase = MemGetBankPtr(1, true);
+		LPBYTE pMemBase = MemGetBankPtr(1);
 		yamlSaveHelper.SaveMemory(pMemBase, (SHR_MEMORY_END + 1) - TEXT_PAGE1_BEGIN, TEXT_PAGE1_BEGIN);
 	}
 }