diff --git a/src/assemble.cpp b/src/assemble.cpp
index e7f7801..a5ca587 100644
--- a/src/assemble.cpp
+++ b/src/assemble.cpp
@@ -32,6 +32,7 @@
 #include "objectcode.h"
 #include "asmexception.h"
 #include "sourcecode.h"
+#include "stringutils.h"
 
 
 using namespace std;
@@ -165,7 +166,7 @@ int LineParser::GetInstructionAndAdvanceColumn(bool requireDistinctOpcodes)
 		bool bMatch = true;
 		for ( unsigned int j = 0; j < len; j++ )
 		{
-			if ( token[ j ] != toupper( m_line[ m_column + j ] ) )
+			if ( token[ j ] != Ascii::ToUpper( m_line[ m_column + j ] ) )
 			{
 				bMatch = false;
 				break;
@@ -480,7 +481,7 @@ void LineParser::HandleAssembler( int instruction )
 
 	// see if it's accumulator mode
 
-	if ( m_column < m_line.length() && toupper( m_line[ m_column ] ) == 'A' && HasAddressingMode( instruction, ACC ) )
+	if ( m_column < m_line.length() && Ascii::ToUpper( m_line[ m_column ] ) == 'A' && HasAddressingMode( instruction, ACC ) )
 	{
 		// might be... but only if the next character is a separator or whitespace
 		// otherwise, we must assume a label beginning with A
@@ -591,7 +592,7 @@ void LineParser::HandleAssembler( int instruction )
 					throw AsmException_SyntaxError_BadIndirect( m_line, m_column );
 				}
 
-				if ( toupper( m_line[ m_column ] ) != 'Y' )
+				if ( Ascii::ToUpper( m_line[ m_column ] ) != 'Y' )
 				{
 					// We were expecting an Y
 					throw AsmException_SyntaxError_BadIndirect( m_line, m_column );
@@ -648,7 +649,7 @@ void LineParser::HandleAssembler( int instruction )
 				throw AsmException_SyntaxError_BadIndirect( m_line, m_column );
 			}
 
-			if ( toupper( m_line[ m_column ] ) != 'X' )
+			if ( Ascii::ToUpper( m_line[ m_column ] ) != 'X' )
 			{
 				// We were expecting an X
 				throw AsmException_SyntaxError_BadIndirect( m_line, m_column );
@@ -826,7 +827,7 @@ void LineParser::HandleAssembler( int instruction )
 		throw AsmException_SyntaxError_BadAbsolute( m_line, m_column );
 	}
 
-	if ( m_column < m_line.length() && toupper( m_line[ m_column ] ) == 'X' )
+	if ( m_column < m_line.length() && Ascii::ToUpper( m_line[ m_column ] ) == 'X' )
 	{
 		m_column++;
 
@@ -857,7 +858,7 @@ void LineParser::HandleAssembler( int instruction )
 		}
 	}
 
-	if ( m_column < m_line.length() && toupper( m_line[ m_column ] ) == 'Y' )
+	if ( m_column < m_line.length() && Ascii::ToUpper( m_line[ m_column ] ) == 'Y' )
 	{
 		m_column++;
 
diff --git a/src/commands.cpp b/src/commands.cpp
index 08fbece..bb3bd0f 100644
--- a/src/commands.cpp
+++ b/src/commands.cpp
@@ -123,7 +123,7 @@ int LineParser::GetTokenAndAdvanceColumn()
 			bool bMatch = true;
 			for ( unsigned int j = 0; j < len; j++ )
 			{
-				if ( token[ j ] != toupper( m_line[ m_column + j ] ) )
+				if ( token[ j ] != Ascii::ToUpper( m_line[ m_column + j ] ) )
 				{
 					bMatch = false;
 					break;
@@ -493,7 +493,7 @@ void LineParser::HandleDefineLabel()
                 }
         }
 
-	if ( ( m_column < m_line.length() ) && ( isalpha( m_line[ m_column ] ) || m_line[ m_column ] == '_' ) )
+	if ( ( m_column < m_line.length() ) && ( Ascii::IsAlpha( m_line[ m_column ] ) || m_line[ m_column ] == '_' ) )
 	{
 		// Symbol starts with a valid character
 
@@ -1252,7 +1252,7 @@ void LineParser::HandleFor()
 
 	// first look for the variable name
 
-	if ( !isalpha( m_line[ m_column ] ) && m_line[ m_column ] != '_' )
+	if ( !Ascii::IsAlpha( m_line[ m_column ] ) && m_line[ m_column ] != '_' )
 	{
 		throw AsmException_SyntaxError_InvalidSymbolName( m_line, m_column );
 	}
@@ -1677,7 +1677,7 @@ void LineParser::HandleMacro()
 
 	string macroName;
 
-	if ( isalpha( m_line[ m_column ] ) || m_line[ m_column ] == '_' )
+	if ( Ascii::IsAlpha( m_line[ m_column ] ) || m_line[ m_column ] == '_' )
 	{
 		macroName = GetSymbolName();
 
@@ -1713,7 +1713,7 @@ void LineParser::HandleMacro()
 				throw AsmException_SyntaxError_MissingComma( m_line, m_column );
 			}
 		}
-		else if ( isalpha( m_line[ m_column ] ) || m_line[ m_column ] == '_' )
+		else if ( Ascii::IsAlpha( m_line[ m_column ] ) || m_line[ m_column ] == '_' )
 		{
 			string param = GetSymbolName();
 
diff --git a/src/discimage.cpp b/src/discimage.cpp
index 7fff2e4..7680cbf 100644
--- a/src/discimage.cpp
+++ b/src/discimage.cpp
@@ -26,6 +26,7 @@
 #include "discimage.h"
 #include "asmexception.h"
 #include "globaldata.h"
+#include "stringutils.h"
 
 using namespace std;
 
@@ -208,14 +209,14 @@ void DiscImage::AddFile( const char* pName, const unsigned char* pAddr, int load
 
 		for ( size_t j = 0; j < 7; j++ )
 		{
-			if ( toupper( pPaddedName[ j ] ) != toupper( m_aCatalog[ i + j ] ) )
+			if ( Ascii::ToUpper( pPaddedName[ j ] ) != Ascii::ToUpper( m_aCatalog[ i + j ] ) )
 			{
 				bTheSame = false;
 				break;
 			}
 		}
 
-		if ( bTheSame && ( toupper( m_aCatalog[ i + 7 ] & 0x7F ) ) == toupper( dirName ) )
+		if ( bTheSame && ( Ascii::ToUpper( m_aCatalog[ i + 7 ] & 0x7F ) ) == Ascii::ToUpper( dirName ) )
 		{
 			// File already exists
 			throw AsmException_FileError_FileExists( m_outputFilename );
diff --git a/src/expression.cpp b/src/expression.cpp
index d519303..8aa339a 100644
--- a/src/expression.cpp
+++ b/src/expression.cpp
@@ -204,7 +204,7 @@ Value LineParser::GetValue()
 		}
 		value = String(text.data(), text.size());
 	}
-	else if ( m_column < m_line.length() && ( isalpha( m_line[ m_column ] ) || m_line[ m_column ] == '_' ) )
+	else if ( m_column < m_line.length() && ( Ascii::IsAlpha( m_line[ m_column ] ) || m_line[ m_column ] == '_' ) )
 	{
 		// get a symbol
 
@@ -284,7 +284,7 @@ Value LineParser::EvaluateExpression( bool bAllowOneMismatchedCloseBracket )
 				bool bMatch = true;
 				for ( unsigned int j = 0; j < len; j++ )
 				{
-					if ( ( m_column + j >= m_line.length() ) || ( token[ j ] != toupper( m_line[ m_column + j ] ) ) )
+					if ( ( m_column + j >= m_line.length() ) || ( token[ j ] != Ascii::ToUpper( m_line[ m_column + j ] ) ) )
 					{
 						bMatch = false;
 						break;
@@ -400,7 +400,7 @@ Value LineParser::EvaluateExpression( bool bAllowOneMismatchedCloseBracket )
 				bool bMatch = true;
 				for ( unsigned int j = 0; j < len; j++ )
 				{
-					if ( ( m_column + j >= m_line.length() ) || ( token[ j ] != toupper( m_line[ m_column + j ] ) ) )
+					if ( ( m_column + j >= m_line.length() ) || ( token[ j ] != Ascii::ToUpper( m_line[ m_column + j ] ) ) )
 					{
 						bMatch = false;
 						break;
diff --git a/src/lineparser.cpp b/src/lineparser.cpp
index a43bb3f..59a7859 100644
--- a/src/lineparser.cpp
+++ b/src/lineparser.cpp
@@ -96,15 +96,15 @@ void LineParser::Process( string line )
 
 		bool bIsSymbolAssignment = false;
 
-		if ( isalpha( m_line[ m_column ] ) || m_line[ m_column ] == '_' )
+		if ( Ascii::IsAlpha( m_line[ m_column ] ) || m_line[ m_column ] == '_' )
 		{
 			do
 			{
 				m_column++;
 
 			} while ( m_column < m_line.length() &&
-					  ( isalpha( m_line[ m_column ] ) ||
-						isdigit( m_line[ m_column ] ) ||
+					  ( Ascii::IsAlpha( m_line[ m_column ] ) ||
+						Ascii::IsDigit( m_line[ m_column ] ) ||
 						m_line[ m_column ] == '_' ||
 						m_line[ m_column ] == '%' ||
 						m_line[ m_column ] == '$' ) &&
@@ -216,7 +216,7 @@ void LineParser::Process( string line )
 
 		// Check macro matches
 
-		if ( isalpha( m_line[ m_column ] ) || m_line[ m_column ] == '_' )
+		if ( Ascii::IsAlpha( m_line[ m_column ] ) || m_line[ m_column ] == '_' )
 		{
 			string macroName = GetSymbolName();
 			const Macro* macro = MacroTable::Instance().Get( macroName );
@@ -547,7 +547,7 @@ bool LineParser::AdvanceAndCheckEndOfSubStatement(bool includeComma)
 /*************************************************************************************************/
 string LineParser::GetSymbolName()
 {
-	assert( isalpha( m_line[ m_column ] ) || m_line[ m_column ] == '_' );
+	assert( Ascii::IsAlpha( m_line[ m_column ] ) || m_line[ m_column ] == '_' );
 
 	string symbolName;
 
@@ -556,8 +556,8 @@ string LineParser::GetSymbolName()
 		symbolName += m_line[ m_column++ ];
 
 	} while ( m_column < m_line.length() &&
-			  ( isalpha( m_line[ m_column ] ) ||
-				isdigit( m_line[ m_column ] ) ||
+			  ( Ascii::IsAlpha( m_line[ m_column ] ) ||
+				Ascii::IsDigit( m_line[ m_column ] ) ||
 				m_line[ m_column ] == '_' ||
 				m_line[ m_column ] == '%' ||
 				m_line[ m_column ] == '$' ) &&
diff --git a/src/literals.cpp b/src/literals.cpp
index 113a676..1f67847 100644
--- a/src/literals.cpp
+++ b/src/literals.cpp
@@ -25,12 +25,7 @@
 
 #include "asmexception.h"
 #include "literals.h"
-
-// Don't use isdigit because compilers can't agree on its locale dependence
-static bool is_decimal_digit(char c)
-{
-	return '0' <= c && c <= '9';
-}
+#include "stringutils.h"
 
 static int hex_digit_value(char c)
 {
@@ -119,7 +114,7 @@ static bool CopyDigitsSkippingUnderscores(const std::string& line, size_t& index
 	}
 
 	size_t start_index = index;
-	while ( index < line.length() && (is_decimal_digit(line[index]) || line[index] == '_') )
+	while ( index < line.length() && (Ascii::IsDigit(line[index]) || line[index] == '_') )
 	{
 		if (line[index] == '_')
 		{
@@ -165,7 +160,7 @@ bool Literals::ParseNumeric(const std::string& line, size_t& index, double& resu
 		return false;
 	}
 
-	if ( is_decimal_digit(line[index]) || line[index] == '.' || line[index] == '-' )
+	if ( Ascii::IsDigit(line[index]) || line[index] == '.' || line[index] == '-' )
 	{
 		// Copy the number without underscores to this buffer
 		std::string buffer;
@@ -198,7 +193,7 @@ bool Literals::ParseNumeric(const std::string& line, size_t& index, double& resu
 		// Copy exponent if it's followed by a sign or digit
 		if ( index + 1 < line.length() &&
 			( line[index] == 'e' || line[index] == 'E' ) &&
-			( line[index + 1] == '+' || line[index + 1] == '-' || is_decimal_digit(line[index + 1]) ) )
+			( line[index + 1] == '+' || line[index + 1] == '-' || Ascii::IsDigit(line[index + 1]) ) )
 		{
 			buffer.push_back('e');
 			index++;
diff --git a/src/stringutils.h b/src/stringutils.h
index 62fcaa1..f952964 100644
--- a/src/stringutils.h
+++ b/src/stringutils.h
@@ -34,4 +34,47 @@ namespace StringUtils
 }
 
 
+// Built-in char functions like isdigit, toupper, etc. are locale dependent and quite slow.
+struct Ascii
+{
+	static bool IsAlpha(char c)
+	{
+		unsigned int uc = static_cast<unsigned char>(c);
+		return (uc | 0x20) - 'a' < 26;
+	}
+
+	static bool IsDigit(char c)
+	{
+		unsigned int uc = static_cast<unsigned char>(c);
+		return (uc - '0') < 10;
+	}
+
+	static char ToLower(char c)
+	{
+		unsigned int uc = static_cast<unsigned char>(c);
+		if (uc - 'A' < 26)
+		{
+			return c | 0x20;
+		}
+		else
+		{
+			return c;
+		}
+	}
+
+	static char ToUpper(char c)
+	{
+		unsigned int uc = static_cast<unsigned char>(c);
+		if (uc - 'a' < 26)
+		{
+			return c & 0xDF;
+		}
+		else
+		{
+			return c;
+		}
+	}
+};
+
+
 #endif // STRINGUTILS_H_
diff --git a/src/symboltable.cpp b/src/symboltable.cpp
index 85211f0..67cb30b 100644
--- a/src/symboltable.cpp
+++ b/src/symboltable.cpp
@@ -32,6 +32,7 @@
 #include "constants.h"
 #include "asmexception.h"
 #include "literals.h"
+#include "stringutils.h"
 
 
 using namespace std;
@@ -189,8 +190,8 @@ bool SymbolTable::AddCommandLineSymbol( const std::string& expr )
 	}
 	for ( std::string::size_type i = 0; i < symbol.length(); ++i )
 	{
-		bool valid = ( isalpha( symbol[ i ] ) || ( symbol[ i ] == '_' ) );
-		valid = valid || ( ( i > 0 ) && isdigit( symbol[ i ] ) );
+		bool valid = ( Ascii::IsAlpha( symbol[ i ] ) || ( symbol[ i ] == '_' ) );
+		valid = valid || ( ( i > 0 ) && Ascii::IsDigit( symbol[ i ] ) );
 		if ( !valid )
 		{
 			return false;
diff --git a/src/value.h b/src/value.h
index ce3e1fa..80891e5 100644
--- a/src/value.h
+++ b/src/value.h
@@ -27,6 +27,8 @@
 #include <string.h>
 #include <cstdlib>
 
+#include "stringutils.h"
+
 // A simple immutable string buffer with a length and a reference count.
 // This doesn't have constructors, etc. so it can be stuffed into a union.
 struct StringHeader
@@ -133,7 +135,7 @@ struct StringHeader
 			char* pdata = StringBuffer(result);
 			for (unsigned int i = 0; i != length; ++i)
 			{
-				*pdata = ascii_upper(*pdata);
+				*pdata = Ascii::ToUpper(*pdata);
 				++pdata;
 			}
 		}
@@ -149,7 +151,7 @@ struct StringHeader
 			char* pdata = StringBuffer(result);
 			for (unsigned int i = 0; i != length; ++i)
 			{
-				*pdata = ascii_lower(*pdata);
+				*pdata = Ascii::ToLower(*pdata);
 				++pdata;
 			}
 		}
@@ -191,24 +193,6 @@ struct StringHeader
 			return 1;
 	}
 
-	static char ascii_lower(char c)
-	{
-		if (('A' <= c) && (c <= 'Z'))
-		{
-			c += 'a' - 'A';
-		}
-		return c;
-	}
-
-	static char ascii_upper(char c)
-	{
-		if (('a' <= c) && (c <= 'z'))
-		{
-			c -= 'a' - 'A';
-		}
-		return c;
-	}
-
 };
 
 // A simple immutable string.