Add default text encoding mode to system definition

This allows C64/C128/Pet projects to start out with PETSCII set as
the default text encoding mode.

SourceGen/DisasmProject.cs

@@ -260,6 +260,9 @@ namespace SourceGen {
             ProjectProps.IncludeUndocumentedInstr = includeUndoc;
             UpdateCpuDef();
 
+            ProjectProps.AnalysisParams.DefaultTextScanMode =
+                SystemDefaults.GetTextScanMode(sysDef);
+
             ProjectProps.EntryFlags = SystemDefaults.GetEntryFlags(sysDef);
 
             // Configure the load address.

SourceGen/RuntimeData/README.md

@@ -35,6 +35,8 @@ The currently-supported parameters are:
    opcodes.  They are disabled by default.
  * first-word-is-load-addr={true|false} - If true, the first two bytes of
    the file contain the load address.
+ * default-text-encoding=<mode> - Specify default character encoding.
+   Use "c64-petscii" for PETSCII.  The default is low/high ASCII.
 
 All of these things can be changed after the project has begun, but it's
 nice to have them configured in advance.

SourceGen/RuntimeData/SystemDefs.json

@@ -176,7 +176,8 @@
       "ExtensionScripts" : [
       ],
       "Parameters" : {
-        "first-word-is-load-addr":"true"
+        "first-word-is-load-addr":"true",
+        "default-text-encoding":"c64-petscii"
       }
     },
     {
@@ -191,6 +192,7 @@
       "ExtensionScripts" : [
       ],
       "Parameters" : {
+        "default-text-encoding":"c64-petscii"
       }
     },
     {
@@ -206,6 +208,7 @@
       "ExtensionScripts" : [
       ],
       "Parameters" : {
+        "default-text-encoding":"c64-petscii"
       }
     },
     {
@@ -219,6 +222,7 @@
       "ExtensionScripts" : [
       ],
       "Parameters" : {
+        "default-text-encoding":"c64-petscii"
       }
     },
     {

SourceGen/SystemDefaults.cs

@@ -18,6 +18,7 @@ using System.Collections.Generic;
 using System.Diagnostics;
 
 using Asm65;
+using TextScanMode = SourceGen.ProjectProperties.AnalysisParameters.TextScanMode;
 
 namespace SourceGen {
     /// <summary>
@@ -28,11 +29,14 @@ namespace SourceGen {
         private const string ENTRY_FLAGS = "entry-flags";
         private const string UNDOCUMENTED_OPCODES = "undocumented-opcodes";
         private const string FIRST_WORD_IS_LOAD_ADDR = "first-word-is-load-addr";
+        private const string DEFAULT_TEXT_ENCODING = "default-text-encoding";
 
         private const string ENTRY_FLAG_EMULATION = "emulation";
         private const string ENTRY_FLAG_NATIVE_LONG = "native-long";
         private const string ENTRY_FLAG_NATIVE_SHORT = "native-short";
 
+        private const string TEXT_ENCODING_C64_PETSCII = "c64-petscii";
+
 
         /// <summary>
         /// Gets the default load address.
@@ -105,16 +109,33 @@ namespace SourceGen {
         /// Gets the default setting for using the first two bytes of the file as the
         /// load address.
         /// 
-        /// This is primarily for C64.  Apple II DOS 3.3 binary files also put the load
-        /// address first, followed by the length, but that's typically stripped out when
+        /// This is primarily for C64 PRG files.  Apple II DOS 3.3 binary files also put the
+        /// load address first, followed by the length, but that's typically stripped out when
         /// the file is extracted.
         /// </summary>
-        /// <param name="sysDef"></param>
-        /// <returns></returns>
+        /// <param name="sysDef">SystemDef instance.</param>
+        /// <returns>True if the first word holds the load address.</returns>
         public static bool GetFirstWordIsLoadAddr(SystemDef sysDef) {
             return GetBoolParam(sysDef, FIRST_WORD_IS_LOAD_ADDR, false);
         }
 
+        /// <summary>
+        /// Gets the default setting for the text scan encoding mode.
+        /// </summary>
+        /// <param name="sysDef">SystemDef instance.</param>
+        /// <returns>Preferred text scan mode.</returns>
+        public static TextScanMode GetTextScanMode(SystemDef sysDef) {
+            Dictionary<string, string> parms = sysDef.Parameters;
+            TextScanMode mode = TextScanMode.LowHighAscii;
+
+            if (parms.TryGetValue(DEFAULT_TEXT_ENCODING, out string valueStr)) {
+                if (valueStr == TEXT_ENCODING_C64_PETSCII) {
+                    mode = TextScanMode.C64Petscii;
+                }
+            }
+            return mode;
+        }
+
         /// <summary>
         /// Looks for a parameter with a matching name and a boolean value.
         /// </summary>