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<h2>Labels &amp; Symbols</h2>
<div class="grid-container">
<div class="grid-item-text">
<p>Suppose you want to call some code at address $1000. CPUs
fundamentally deal with numeric values, so the machine code to
call it would be <code>JSR $1000</code>. Humans tend to work better with
words, so associating a meaningful symbol with address $1000
can greatly improve the readability of the code: something like
<code>JSR DrawSprite</code> is far more helpful for human readers.
Further, once the code has been disassembled to source code, using symbols
instead of fixed addresses makes it easier to alter the program or re-use
the code.</p>
<p>When the target address of instructions like <code>JSR</code> and
<code>LDA</code> falls within the scope of the data file, SourceGen classifies
the reference as <i>internal</i>, and automatically adds a generic
symbolic label (e.g. <code>L1000</code>). This can be edited if desired.</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-image">
<img src="images/t1-edit-label.png" alt="t1-edit-label"/>
</div>
<div class="grid-item-text">
<p>On the line at address $2000, select
<samp>Actions &gt; Edit Label</samp>, or double-click on the label
"<code>L2000</code>". Change the label to "<kbd>MAIN</kbd>", and hit
<kbd class="key">Enter</kbd>. The label changes on that line,
and on the two lines that refer to address $2000.
(If you're not sure which lines refer to address $2000,
select line $2000 and check the list in the References window.)</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-text">
<p>Sometimes the target address falls outside the data file. Examples
include calls to ROM routines, use of zero-page storage, and access to
memory-mapped I/O locations. SourceGen classifies these as <i>external</i>,
and does not generate a symbol. In an assembler source file, symbols
for these would be expressed as equates (e.g. <code>FOO = $8000</code>),
usually at the top of the file or in an "include file". SourceGen
allows you to specify symbols for addresses and numeric constants within
the project ("<i>project symbols</i>"), or in a symbol file that can be
included in multiple projects ("<i>platform symbols</i>"). The SourceGen
distribution includes platform symbol files with ROM addresses for
several common systems.</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-image">
<img src="images/t1-pre-sym-2000.png" alt="t1-pre-sym-2000"/>
</div>
<div class="grid-item-text">
<p>For an example, consider the code at address $2000, which is
<code>LDA $3000</code>. We want to assign the symbol "INPUT" to address
$3000, but we can't do that by editing a label because it's not inside
the file bounds. We can open the project symbol editor from the project
properties editor, or we can use a shortcut.</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-image">
<img src="images/t1-edit-sym-2000.png" alt="t1-edit-sym-2000"/>
</div>
<div class="grid-item-text">
<p>With the line at $2000 selected, use <samp>Actions &gt; Edit Operand</samp>,
or double-click on the value in the <samp>Operand</samp> column
("<code>$3000</code>"). This opens the
Edit Instruction Operand dialog. In the bottom left, click
<samp>Create Project Symbol</samp>. Set the <samp>Label</samp> field to
"<kbd>INPUT</kbd>", and
click <samp>OK</samp>, then <samp>OK</samp> in the operand editor.</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-image">
<img src="images/t1-edit-2000-done.png" alt="t1-edit-2000-done"/>
</div>
<div class="grid-item-text">
<p>The instruction at $2000 now uses the symbol "<samp>INPUT</samp>"
as its operand. If you scroll to the top of the file, you will see a
"<code>.EQ</code>" line for the symbol.</p>
</div>
</div>
<hr style="width:80%;"/>
<h4>Numeric v. Symbolic</h4>
<div class="grid-container">
<div class="grid-item-text">
<p>When SourceGen sees a reference to an address, such as the operand of an
absolute <code>JSR</code> or <code>LDA</code>, it recognizes it
as a <i>numeric reference</i>. You can edit the instruction's operand
to use a symbol instead, changing to a <i>symbolic reference</i>.
Sometimes the way these are handled can be confusing.</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-image">
<img src="images/t1-sym-2005-before.png" alt="t1-sym-2005-before"/>
</div>
<div class="grid-item-text">
<p>Let's use the branch statement at $2005 to illustrate the difference.
It performs a branch to $2009, which was automatically assigned the
label "<samp>L2009</samp>".</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-image">
<img src="images/t1-sym-2005-labeled.png" alt="t1-sym-2005-labeled"/>
</div>
<div class="grid-item-text">
<p>Edit the label at $2009 (double-click on "<samp>L2009</samp>" there),
and change it to "<kbd>IN_RANGE</kbd>". Line $2005 changes to match.
This works because SourceGen
is auto-formatting line $2005's operand based on the label it finds when it
chases the numeric reference to $2009.
The Info window shows this as <code>Format (auto): symbol "IN_RANGE"</code>.</p>
<p>Use <samp>Edit &gt; Undo</samp> to revert the label change.</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-image">
<img src="images/t1-sym-2005-edit.png" alt="t1-sym-2005-edit"/>
</div>
<div class="grid-item-text">
<p>Edit the instruction operand at $2005 (double-click on
"<samp>L2009</samp>" there). Change the format to <samp>Symbol</samp>,
and type "<kbd>IN_RANGE</kbd>" in the symbol box.
The preview shows <samp>BCC IN_RANGE (?)</samp>, which hints at a
problem. Click <samp>OK</samp>.</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-image">
<img src="images/t1-sym-2005-nosym.png" alt="t1-sym-2005-nosym"/>
</div>
<div class="grid-item-text">
<p>Some things changed, but not the things we wanted. Line $2005 now
says <code>BCC $2009</code>, instead of <code>BCC L2009</code>, and the
label at $2009 has disappeared entirely. What went wrong?</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-text">
<p>The problem is that we edited the operand to use a symbol that isn't
defined anywhere. Because "IN_RANGE" isn't defined, the operand was
given the default format, and displayed as a hex value.
The numeric reference to $2009 was replaced by the symbol, and nothing
else refers to that address,
so SourceGen no longer had any reason to put an auto-generated label
on line $2009, which is why that disappeared.</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-image">
<img src="images/t1-sym-2005-msg-window.png" alt="t1-sym-2005-msg-window"/>
</div>
<div class="grid-item-text">
<p>The missing symbol is called out in a message window that popped up
at the bottom of the code list window. The message window only appears
when there are messages to read. You can hide the window with the
<samp>Hide</samp> button, and make it re-appear with the button in the
bottom right of the main window that currently says <samp>1 message</samp>.</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-image">
<img src="images/t1-sym-2005-explicit.png" alt="t1-sym-2005-explicit"/>
</div>
<div class="grid-item-text">
<p>We can resolve this issue by providing the desired symbol. As you
did earlier, edit the label on line $2009 (double-click in the label column)
and set it to "<kbd>IN_RANGE</kbd>". When you do, the operand on line $2005
is updated appropriately.
If you select line $2005, the Info window shows the format as
<samp>Format: symbol "IN_RANGE"</samp>, indicating that the symbol
was set explicitly rather than automatically.</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-image">
<img src="images/t1-sym-2005-adjust.png" alt="t1-sym-2005-adjust"/>
</div>
<div class="grid-item-text">
<p>Symbolic references always link to the symbol, even when the symbol
doesn't match the numeric reference. To see this, remove the label from
line $2009 by undoing that change with <samp>Edit &gt; Undo</samp>,
so the symbol is again undefined. Now set the label on the following line,
$200A, to "<kbd>IN_RANGE</kbd>".</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-text">
<p>Line $2005 now says "<code>BCC IN_RANGE-1</code>". Earlier you set
the operand to be a symbolic reference to "<samp>IN_RANGE</samp>", but the symbol
doesn't quite match, so SourceGen automatically adjusted the operand by
one byte to point to the correct address. Generally speaking, SourceGen
will do its best to use the symbols that you tell it to, and will adjust the
symbolic references so that the code assembles correctly.</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-text">
<p>Edit the label on line $200A, and change it to "<kbd>NIFTY</kbd>".
Note how the reference on line $2005 also changed. This is an example
of a "refactoring rename": when you changed the label, SourceGen
automatically found everything that referred to it and updated it.
If you edit the operand on line $2005, you can confirm that the
symbol has changed.</p>
<p>(If you want to clean this up before continuing on to the next
section, put the label back on line $2009.)</p>
</div>
</div>
<hr style="width:80%;"/>
2021-08-03 21:48:40 +00:00
<h4>Non-Unique Labels</h4>
<div class="grid-container">
<div class="grid-item-text">
<p>Most assemblers have a notion of "local" labels, which go out of
scope when a non-local (global) label is encountered. The actual
definition of "local" is assembler-specific, but SourceGen allows you
to create labels that serve the same purpose.</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-image">
<img src="images/t1-local-loop-edit.png" alt="t1-local-loop-edit"/>
</div>
<div class="grid-item-text">
<p>By default, newly-created labels have global scope and must be
unique. You can change these attributes when you edit the label. Up near the
top of the file, at address $1002, double-click on the label ("L1002").
Change the label to "<kbd>LOOP</kbd>" and click the "non-unique local"
radio button.
Click <samp>OK</samp>.</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-image">
<img src="images/t1-local-loop1.png" alt="t1-local-loop1"/>
</div>
<div class="grid-item-text">
<p>The label at line $1002 (and the operand on line $100B) should now
be "<samp>@LOOP</samp>". By default, '@' is used to indicate non-unique labels,
though you can change it to a different character in the application
settings.</p>
</div>
</div>
<div class="grid-container">
<div class="grid-item-image">
<img src="images/t1-local-loop2.png" alt="t1-local-loop2"/>
</div>
<div class="grid-item-text">
<p>At address $2019, double-click to edit the label ("<samp>L2019</samp>"). If
you type "<kbd>MAIN</kbd>" or "<kbd>IS_OK</kbd>" with Global selected you'll
get an error, but if you type "<kbd>@LOOP</kbd>" it will be accepted. Note
the "non-unique local" radio
button is selected automatically if you start a label with '@' (or
whatever character you have configured). Click <samp>OK</samp>.</p>
<p>You now have two lines with the same label. In some cases the
assembly source generator need to may "promote" them to globals, or
rename them to make them unique, depending on what your preferred assembler
allows.</p>
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