Add ability to "erase" previously-defined platform symbols

While disassembling some code I found that I wanted the ROM entry
points, but the zero page usage was significantly different and the
ROM labels were distracting.  Splitting the symbol file in two was
a possibility, but I'm afraid this will lead to a very large
collection of very small files, and we'll lose any sense of relation
between the ROM entry points and the ZP addresses used to pass
arguments.

Platform symbols have the lowest priority when resolving by address,
but using that to hide the unwanted labels requires creating project
symbols or local variables for things that you might not know what
they do yet.  It's possible to hide a platform symbol by adding
another symbol with the same label and an invalid value.

This change formalizes and extends the "hiding" of platform symbols
to full erasure, so that they don't clutter up the symbol table.
This also tightens up the platform symbol parser to only accept
values in the range 0 <= value <= 0x00ffffff (24-bit positive
integers).

An "F8-ROM-nozp" symbol file is now part of the standard set.  A
project can include that to erase the zero-page definitions.

(I'm not entirely convinced this is the right approach, so I'm not
doing this treatment on other symbol files... consider this an
experiment.  Another approach would be some sort of conditional
inclusion, or perhaps erase-by-tag, but that requires some UI work
in the app to define what you want included or excluded.)

SourceGen/DisasmProject.cs

@@ -1037,9 +1037,17 @@ namespace SourceGen {
             // order, so we can just overwrite earlier symbols with matching labels.
             foreach (PlatformSymbols ps in PlatformSyms) {
                 foreach (Symbol sym in ps) {
+                    if (sym.Value == PlatformSymbols.ERASE_VALUE) {
+                        // "erase" value
+                        if (SymbolTable.TryGetValue(sym.Label, out Symbol found)) {
+                            SymbolTable.Remove(found);
+                        }
+                    } else {
+                        // overwrite
                         SymbolTable[sym.Label] = sym;
                     }
                 }
+            }
 
             // Now add project symbols, overwriting platform symbols with the same label.
             foreach (KeyValuePair<string, DefSymbol> kvp in ProjectProps.ProjectSyms) {

SourceGen/PlatformSymbols.cs

@@ -28,6 +28,8 @@ namespace SourceGen {
     /// </summary>
     public class PlatformSymbols : IEnumerable<Symbol> {
         public const string FILENAME_EXT = ".sym65";
+        private const string ERASE_VALUE_STR = "ERASE";
+        public const int ERASE_VALUE = -1;
         public static readonly string FILENAME_FILTER = Res.Strings.FILE_FILTER_SYM65;
 
         /// <summary>
@@ -165,20 +167,28 @@ namespace SourceGen {
                         } else if (typeAndDir == '>') {
                             direction = DefSymbol.DirectionFlags.Write;
                         }
+
                         string badParseMsg;
                         int value, numBase;
                         bool parseOk;
+                        string valueStr = matches[0].Groups[GROUP_VALUE].Value;
                         if (isConst) {
                             // Allow various numeric options, and preserve the value.
-                            parseOk = Asm65.Number.TryParseInt(matches[0].Groups[GROUP_VALUE].Value,
-                                out value, out numBase);
+                            parseOk = Asm65.Number.TryParseInt(valueStr, out value, out numBase);
                             badParseMsg =
                                 CommonUtil.Properties.Resources.ERR_INVALID_NUMERIC_CONSTANT;
+                        } else if (valueStr.ToUpperInvariant().Equals(ERASE_VALUE_STR)) {
+                            parseOk = true;
+                            value = ERASE_VALUE;
+                            numBase = 10;
+                            badParseMsg = CommonUtil.Properties.Resources.ERR_INVALID_ADDRESS;
                         } else {
                             // Allow things like "05/1000".  Always hex.
                             numBase = 16;
-                            parseOk = Asm65.Address.ParseAddress(matches[0].Groups[GROUP_VALUE].Value,
-                                (1 << 24) - 1, out value);
+                            parseOk = Asm65.Address.ParseAddress(valueStr, (1 << 24) - 1,
+                                out value);
+                            // limit to positive 24-bit values
+                            parseOk &= (value >= 0 && value < 0x01000000);
                             badParseMsg = CommonUtil.Properties.Resources.ERR_INVALID_ADDRESS;
                         }
 

SourceGen/RuntimeData/Apple/F8-ROM-nozp.sym65

@@ -0,0 +1,45 @@
+; Copyright 2019 faddenSoft. All Rights Reserved.
+; See the LICENSE.txt file for distribution terms (Apache 2.0).
+
+;
+; Sometimes code uses the ROM entry points but redefines the zero-page
+; addresses for its own purposes.  Having the MON_* symbols is distracting.
+; This erases them from the symbol table.
+;
+; To use: include this file, and ensure it comes after F8-ROM.sym65 in
+; the platform symbol file list.
+;
+
+*SYNOPSIS Remove monitor ROM zero-page symbols
+
+MON_WNDLEFT     @ erase
+MON_WNDWDTH     @ erase
+MON_WNDTOP      @ erase
+MON_WNDBTM      @ erase
+MON_CH          @ erase
+MON_CV          @ erase
+MON_GBASL       @ erase
+MON_GBASH       @ erase
+MON_H2          @ erase
+MON_V2          @ erase
+MON_COLOR       @ erase
+MON_INVFLAG     @ erase
+MON_PROMPT      @ erase
+MON_CSWL        @ erase
+MON_CSWH        @ erase
+MON_KSWL        @ erase
+MON_KSWH        @ erase
+MON_PCL         @ erase
+MON_PCH         @ erase
+MON_A1L         @ erase
+MON_A1H         @ erase
+MON_A2L         @ erase
+MON_A2H         @ erase
+MON_A3L         @ erase
+MON_A3H         @ erase
+MON_A4L         @ erase
+MON_A4H         @ erase
+MON_A5L         @ erase
+MON_A5H         @ erase
+MON_RNDL        @ erase
+MON_RNDH        @ erase

SourceGen/RuntimeData/Help/advanced.html

@@ -71,7 +71,8 @@ written.</p>
 <p>The VALUE is a number in decimal, hexadecimal (with a leading '$'), or
 binary (with a leading '%').  The numeric base will be recorded and used when
 formatting the symbol in generated output, so use whichever form is most
-appropriate.  Values are unsigned 24-bit numbers.</p>
+appropriate.  Values are unsigned 24-bit numbers.  The special value
+"erase" may be used to erase a symbol defined in an earlier platform file.</p>
 
 <p>The WIDTH is optional, and ignored for constants.  It must be a
 decimal or hexadecimal value between 1 and 65536, inclusive.  If omitted,