Progress toward 64tass support

Most tests pass, but 2007-labels-and-symbols fails because the
expressions recognized by 64tass don't match up with either of the
other assemblers.

This is currently using a workaround for the local label syntax.
64tass uses '_' as the prefix, which is unfortunate since SourceGen
explicitly allowed underscores in labels.  (So does 64tass for that
matter, but it treats labels specially when the '_' comes first.)
We will need to rename any non-local user labels that start with '_'.

(issue #16)

SourceGen/RuntimeData/Help/codegen.html

@@ -24,6 +24,7 @@ for each.</p>
 
 <p>SourceGen currently supports the following cross-assemblers:</p>
 <ul>
+  <li><a href="https://sourceforge.net/projects/tass64/">64tass</a> v1.53.1515 or later</li>
   <li><a href="https://cc65.github.io/">cc65</a> v2.17 or later</li>
   <li><a href="https://www.brutaldeluxe.fr/products/crossdevtools/merlin/">Merlin 32</a> v1.0.0 or later</li>
 </ul>
@@ -105,6 +106,104 @@ and report any differences.</p>
 SourceGen.  However, SourceGen can generally work around assembler bugs,
 so any failure is an opportunity for improvement.</p>
 
+
+<h2><a name="quirks">Assembler-Specific Bugs &amp; Quirks</a></h2>
+
+<p>This is a list of bugs and quirky behavior in cross-assemblers that
+SourceGen works around when generating code.</p>
+<p>Every assembler seems to have a different way of dealing with expressions.
+Most of them will let you group expressions with parenthesis, but that
+doesn't always help.  For example, <code>PEA label >> 8 + 1</code> is
+perfectly valid, but writing <code>PEA (label >> 8) + 1</code> will cause
+most assemblers to assume you're trying to use an alterate form of PEA
+with indirect addressing (which doesn't exist).  The code generator needs
+to understand expression syntax and operator precedence to generate correct
+code, but also needs to know how to handle the corner cases.</p>
+
+
+<h3><a name="64tass">64tass</a></h3>
+
+<p>Code is generated for 64tass v1.53.1515.</p>
+
+<p>Bugs:</p>
+<ul>
+  <li>Undocumented opcodes: <code>SHA (ZP),Y</code> ($93) is not supported;
+    the assembler appears to be expecting <code>SHA ABS,X</code> instead.</li>
+  <li>BRK, COP, and WDM are not allowed to have operands.</li>
+</ul>
+
+<p>Quirks:</p>
+<ul>
+  <li>The underscore character ('_') is allowed as a character in labels,
+    but when used as the first character in a label it indicates the
+    label is local.  If you create labels with leading underscores that
+    are not local, the labels must be altered to start with some other
+    character, and made unique.</li>
+  <li>By default, 64tass sets the first two bytes of the output file to
+    the load address.  The <code>--nostart</code> flag is used to
+    suppress this.</li>
+  <li>By default, 64tass is case-insensitive, but SourceGen treats labels
+    as case-sensitive.  The <code>--case-sensitive</code> must be passed to
+    the assembler.</li>
+  <li>If you set the <code>--case-sensitive</code> flag, <b>all</b> opcodes
+    and operands must be lower-case.  Most of the flags used to show
+    things in upper case must be disabled.</li>
+  <li>For 65816, selecting the bank byte is done with the back-quote ('`')
+    rather than the caret ('^').  (There's a note in the docs to the effect
+    that they plan to move to carets.)</li>
+</ul>
+
+
+<h3><a name="cc65">cc65</a></h3>
+
+<p>Code is generated for cc65 v2.27.</p>
+
+<p>Bugs:</p>
+<ul>
+  <li>The arguments to MVN/MVP are reversed.</li>
+  <li>PC relative branches don't wrap around at bank boundaries.</li>
+  <li>BRK &lt;arg&gt; is assembled to opcode $05 rather than $00.</li>
+  <li>WDM is not supported.</li>
+</ul>
+
+<p>Quirks:</p>
+<ul>
+  <li>Operator precedence is unusual.  Consider <code>label >> 8 - 16</code>.
+    cc65 puts shift higher than subtraction, whereas languages like C
+    and assemblers like 64tass do it the other way around.  So cc65
+    regards the expression as <code>(label >> 8) - 16</code>, while the
+    more common interpretation would be <code>label >> (8 - 16)</code>.
+    (This is actually somewhat convenient, since many common expressions
+    don't require parenthesis.)</li>
+  <li>Undocumented opcodes: SBX ($cb) uses the mnemonic AXS.  All other
+    opcodes match up with the "unintended opcodes" document.</li>
+</ul>
+
+
+<h3><a name="merlin32">Merlin 32</a></h3>
+
+<p>Code is generated for Merlin 32 v1.0.</p>
+
+<p>Bugs:</p>
+<ul>
+  <li>PC relative branches don't wrap around at bank boundaries.</li>
+  <li>For some failures, an exit code of zero is returned.</li>
+</ul>
+
+<p>Quirks:</p>
+<ul>
+  <li>Operator precedence is unusual.  Expressions are processed from
+    left to right, with no operator precedence.</li>
+  <li>The byte selection operators  ('&lt;', '&gt;', '^') are actually
+    word-selection operators, yielding 16-bit values when wide registers
+    are enabled on the 65816.</p>
+  <li>The assembler tracks register widths when it sees SEP/REP instructions,
+    but doesn't attempt to track the emulation flag.  So if the registers
+    are long when you switch to emulation, incorrect code is generated.
+    (Really I just want to be able to turn the auto-tracking off.)</li>
+  <li>Non-unique local labels don't cause an error.</li>
+</ul>
+
 </div>
 
 <div id="footer">

SourceGen/RuntimeData/Help/index.html

@@ -85,6 +85,12 @@ and 65816 code.  The official web site is
       <li><a href="codegen.html#localizer">Label Localizer</a></li>
     </ul></li>
     <li><a href="codegen.html#assemble">Cross-Assembling Generated Code</a></li>
+    <li><a href="codegen.html#quirks">Assembler-Specific Bugs &amp; Quirks</a>
+    <ul>
+      <li><a href="codegen.html#64tass">64tass</a></li>
+      <li><a href="codegen.html#cc65">cc65</a></li>
+      <li><a href="codegen.html#merlin32">Merlin 32</a></li>
+    </ul></li>
   </ul></li>
 
   <li><a href="settings.html">Properties &amp; Settings</a>