Fix -di command line option for template images causing 'Problem reading from disc image.'

src/asmexception.h

@@ -58,7 +58,14 @@ class AsmException_FileError : public AsmException
 public:
 
 	explicit AsmException_FileError( const std::string& filename )
-		:	m_filename( filename )
+		:	m_filename( filename ),
+			m_detail()
+	{
+	}
+
+	AsmException_FileError( const std::string& filename, const std::string& detail )
+		:	m_filename( filename ),
+			m_detail( detail )
 	{
 	}
 
@@ -68,12 +75,17 @@ class AsmException_FileError : public AsmException
 
 	virtual const char* Message() const
 	{
+		if ( !m_detail.empty() )
+		{
+			return m_detail.c_str();
+		}
 		return "Unspecified file error.";
 	}
 
 protected:
 
 	std::string			m_filename;
+	std::string			m_detail;
 };
 
 
@@ -84,9 +96,19 @@ public:																		\
 	explicit AsmException_FileError_##a( const std::string& filename )		\
 		:	AsmException_FileError( filename ) {}							\
 																			\
+	AsmException_FileError_##a( const std::string& filename, const std::string& detail )	\
+		:	AsmException_FileError( filename, detail ) {}					\
+																			\
 	virtual ~AsmException_FileError_##a() {}								\
 																			\
-	virtual const char* Message() const { return msg; }						\
+	virtual const char* Message() const										\
+	{																		\
+		if ( !m_detail.empty() )											\
+		{																	\
+			return m_detail.c_str();										\
+		}																	\
+		return msg;															\
+	}																		\
 }
 
 

src/discimage.cpp

@@ -31,6 +31,55 @@
 using namespace std;
 
 
+/*************************************************************************************************/
+/**
+	BuildInputFileErrorMessage()
+
+	Helper function to build detailed diagnostic message for input file read failures.
+	Examines stream state to provide actionable information about what went wrong.
+
+	@param		description			What was being read (e.g., "catalogue", "sector 5")
+	@param		expectedBytes		Number of bytes attempted to read
+	@param		inputStream			The input stream that failed
+	@return		A detailed error message string
+*/
+/*************************************************************************************************/
+static string BuildInputFileErrorMessage(
+	const string& description,
+	size_t expectedBytes,
+	ifstream& inputStream )
+{
+	ostringstream msg;
+
+	streamsize actualBytes = inputStream.gcount();
+	bool hitEof = inputStream.eof();
+	streampos currentPos = inputStream.tellg();
+
+	msg << "Problem reading from disc image.";
+
+	// Add specific context about what was being read
+	msg << " Failed to read " << description;
+	msg << ": attempted " << expectedBytes << " bytes";
+	msg << ", got " << actualBytes;
+
+	// Add EOF information if applicable
+	if ( hitEof )
+	{
+		msg << " (unexpected end of file)";
+	}
+
+	// Add current position if available
+	if ( currentPos != static_cast<streampos>(-1) )
+	{
+		msg << ". Position: " << currentPos;
+	}
+
+	msg << ".";
+
+	return msg.str();
+}
+
+
 /*************************************************************************************************/
 /**
 	DiscImage::DiscImage()
@@ -61,6 +110,7 @@ DiscImage::DiscImage( const char* pOutput, const char* pInput )
 			throw AsmException_FileError_OpenDiscSource( pInput );
 		}
 
+		// Read the catalogue
 		if ( !m_inputFile.read( reinterpret_cast< char* >( m_aCatalog ), 0x200 ) )
 		{
 			throw AsmException_FileError_ReadDiscSource( pInput );
@@ -94,13 +144,30 @@ DiscImage::DiscImage( const char* pOutput, const char* pInput )
 		assert( length >= endSectorAddr * 0x100 );
 #endif
 
+		// Write the catalogue (sectors 0-1) directly from the in-memory buffer.
+		// This avoids redundant I/O and is consistent with the destructor
+		// which also writes the catalogue from m_aCatalog.
+
+		if ( !m_outputFile.write( reinterpret_cast< char* >( m_aCatalog ), 0x200 ) )
+		{
+			throw AsmException_FileError_WriteDiscDest( pOutput );
+		}
+
+		// Copy the remaining file data sectors (2 onwards) from the input disc image.
+		// The input file pointer is correctly positioned at offset 0x200 (start of sector 2)
+		// after the initial catalogue read
+
 		char sector[ 0x100 ];
 
-		for ( int sect = 0; sect < endSectorAddr; sect++ )
+		for ( int sect = 2; sect < endSectorAddr; sect++ )
 		{
 			if ( !m_inputFile.read( sector, 0x100 ) )
 			{
-				throw AsmException_FileError_ReadDiscSource( pInput );
+				// Build detailed diagnostic message
+				ostringstream desc;
+				desc << "sector " << sect;
+				string errorMsg = BuildInputFileErrorMessage( desc.str(), 0x100, m_inputFile );
+				throw AsmException_FileError_ReadDiscSource( pInput, errorMsg );
 			}
 
 			if ( !m_outputFile.write( sector, 0x100 ) )

test/3-directives/template/usetemplateboot.6502

@@ -0,0 +1,11 @@
+\ beebasm -di template.ssd -do test.ssd
+
+\ Test using -di option with a template disc image that has a !BOOT file.
+
+ORG &8000
+
+.start
+    RTS
+.end
+
+SAVE "tester", start, end