6502bench/SourceGen/RuntimeData/Help/tutorials.html

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<h1>6502bench SourceGen: Tutorials</h1>
<p><a href="index.html">Back to index</a></p>

<p>The tutorials introduce SourceGen and cover some of the basic
features.  They skim lightly over some important concepts, like the
difference between numeric and symbolic references, so reading the
manual is recommended.</p>

<ul>
  <li><a href="#basic-features">Basic Features</a></li>
  <li><a href="#advanced-features">Advanced Features</a></li>
</ul>


<h2><a name="basic-features">Tutorial #1: Basic Features</a></h2>

<p>Start by launching SourceGen.  The initial screen has a large
center area with some buttons, and some mostly-empty windows on the sides.
The buttons are shortcuts for menu items in the File menu.</p>


<h3>Create the project</h3>

<p>Click "Start new project".</p>

<p>The New Project window has three parts.  The top-left window has a
tree of known platforms, arranged by manufacturer.  The top-right window
provides some details on whichever platform is selected.  The bottom
window will have some information about the data file, once we choose one.</p>
<p>Scroll down in the list, and select "Generic 6502".  Then click
"Select File...", navigate to the SourceGen installation directory,
open the Examples folder, then open the "Tutorial" folder.  Select the
file named "Tutorial1", and click "Open".</p>
<p>The filename now appears in the bottom window, along with an indication
of the file's size.</p>
<p>Click OK to create the project.</p>


<h3>Getting Around</h3>

<p>The first thing we'll do is save the project.  Some features create or
load files from the directory where the project lives, so we want to
establish that.</p>
<p>Select File &gt; Save, which will bring up a standard save-file dialog.
Make sure you're in still in the Examples/Tutorial folder.  The default
project file name is "Tutorial1.dis65", which is what we want, so just
click "Save".</p>

<p>The display is divided into rows, one per line of disassembled code
or data.  This is a standard Windows "list view", so you can select a row
by left-clicking anywhere in it.  Use Ctrl+Click to toggle the selection
on individual lines, and Shift+Click to select a range of lines.  You can
move the selection around with the up/down arrow keys and PgUp/PgDn.  Scroll
the window with the mouse wheel or by grabbing the scroll bar.</p>

<p>Each row is divided into nine columns.  You can adjust the column
widths by clicking and dragging the column dividers in the header.  The
columns on the right side of the screen are similar to what you'd find
in assembly source code: label, opcode, operand, comment.  The columns
on the left are what you'd find in a disassembly (file offset, address,
raw bytes), plus some information about processor status flags and line
attributes that may or may not be useful to you.  If you find any of
these distracting, collapse the column.</p>

<p>Click on the fourth line down, which has address 1002.  The line has
a label, "L1002", and is performing an indexed load from L1017.  Both
of these labels were automatically generated, and are named for the
address at which they appear.  When you clicked on the line, a few
things happened:</p>
<ul>
  <li>The line was highlighted in the system selection color (usually
    blue).</li>
  <li>Address 1017 and label L1017 were highlighted.  When you select
    a line with an operand that targets an in-file address, the target
    address is highlighted.</li>
  <li>An entry appeared in the References window.  This tells you that the
    only reference to L1002 is a branch from address $100B.</li>
  <li>The Info window filled with a bunch of text that describes the
    line format and some details about the LDA instruction.</li>
</ul>

<p>Click some other lines, such as address $100B and $1014.  Note how the
highlights and contents of other windows change.</p>
<p>Click on L1002 again, then double-click on the opcode ("LDA").  The
selection jumps to L1017.  When an operand references an in-file address,
double-clicking on the opcode will take you to it.  (Double-clicking on
the operand itself opens a format editor; more on that later.)</p>
<p>With L1017 highlighted, double-click on the line that appears in the
References window.  Note the selection jumps to L1002.  You can immediately
jump to any reference.</p>
<p>At the top of the Symbols window on the right side of the screen is a
row of buttons.  Make sure "Auto" is selected.  You should see three
labels in the window (L1002, L1014, L1017).  Double-click on L1014.  The
selection jumps to the appropriate line.</p>

<p>Select Edit &gt; Find.  Type "hello", and hit Enter.  The selection will
move to address $100E, which is a string that says "hello!".  You can use
Edit &gt; Find Next to try to find the next occurrence (there isn't one).  You
can search for any text that appears in the rightmost columns (label, opcode,
operand, comment).</p>
<p>Select Edit &gt; Go To.  You can enter a label, address, or file offset.
Enter "100b" to set the selection to $100B.</p>

<p>Near the top-left of the SourceGen window is a set of toolbar icons.
Click the curly left-pointing arrow, and watch the selection move.  Click
it again.  Then click the curly right-arrow a couple of times.  Whenever
you jump around in the file by using the Go To feature, or by double-clicking
on opcodes or lines in the side windows, the locations are added to a
navigation history.  The arrows let you move forward and backward
through it.</p>


<h3>Editing</h3>

<p>Click the very first line of the file, which is a comment that says
something like "6502bench SourceGen vX.Y.Z".  There are three ways to
open the comment editor:</p>
<ol>
  <li>Select Actions &gt; Edit Long Comment from the menu bar.</li>
  <li>Right click, and select Edit Long Comment from the
    pop-up menu.  (This menu is exactly the same as the Actions menu.)</li>
  <li>Double-click the comment</li>
</ol>
<p>Most things in the code list will respond to a double-click.
Double-clicking on addresses, flags, labels, operands, and comments will
open editors for those things.  Double-clicking on a value in the "bytes"
column will open a floating hex dump viewer.  This is usually the most
convenient way to edit something: point and click.</p>
<p>Double-click the comment to open the editor.  Type some words into the
upper window, and note that a formatted version appears in the bottom
window.  Experiment with the maximum line width and "render in box"
settings to see what they do.  You can hit Enter to create line breaks,
or let SourceGen wrap lines for you.  When you're done, click OK.  (Or
hit Ctrl+Enter.)</p>
<p>When the dialog closes, you'll see your new comment in place at the
top of the file.  If you typed enough words, your comment will span
multiple lines.  You can select the comment by selecting any line in it.</p>

<p>Click on the comment, then shift-click on L1014.  Right-click, and look
at the menu.  Nearly all of the menu items are disabled.  Most editors are
only enabled when a single instance of a relevant item is selected, so
for example Edit Long Comment won't be enabled if you have an instruction
selected.</p>

<p>Let's add a note.  Click on $100E (the line with "hello!"), then
select Actions &gt; Edit Note.  Type a few words, pick a color, and click "OK"
(or hit Ctrl+Enter).  Your note appears in the code, and also in the
window on the bottom left.  Notes are like long comments, with three key
differences:</p>
<ol>
  <li>You can't pick their line width, but you can pick their color.</li>
  <li>They don't appear in generated assembly sources, making them
    useful for leaving notes to yourself as you work.</li>
  <li>They're listed in the Notes window.  Double-clicking them jumps
    the selection to the note, making them useful as bookmarks.</li>
</ol>

<p>It's time to do something with the code.  If you look at what the code
does you'll see that it's copying several dozen bytes from $1017
to $2000, then jumping to $2000.  It appears to be relocating the next
part of the code before
executing it.  We want to let the disassembler know what's going on, so
select the line at address $1017 and then
Edit &gt; Set Address.  (Or double-click the "1017" in the Addr column.)
In the Set Address dialog, type "2000", and hit Enter.)</p>

<p>Note the way the code list has changed.  When you changed the address,
the "JMP $2000" at L1014 found a home inside the bounds of the file, so
the code tracer was able to find the instructions there.</p>
<p>From the menu, select Edit &gt; Undo.  Notice how everything reverts to
the way it was.  Now, select Edit &gt; Redo.  You can undo any change you
make to the project.  (The undo history is <strong>not</strong> saved in
the project file, though, so when you exit the program the history is
lost.)</p>
<p>Notice that, while the address column has changed, the offset column
has not.  File offsets never change, which is why they're shown here and
in the References and Notes windows.  (They can, however, be distracting,
so you'll be forgiven if you reduce the offset column width to zero.)</p>
<p>On the line at address $2000, select Actions &gt; Edit Label, or
double-click on the label "L2000".  Change the label to "MAIN", and hit
Enter.  The label changes on that line, and on the two lines that refer
to address $2000.  (If you're not sure what refers to address $2000, select
that line and check the References window.)</p>
<p>On that same line, select Actions &gt; Edit Comment.  Type a short
comment, and hit Enter.  Your comment appears in the "comment" column.</p>


<h3>Editing Instruction Operands</h3>

<p>The operand in the LDA instruction at line $2000 refers to an address
($3000) that isn't part of the file.  We want to create an equate directive to
give it a name.  With the line at $2000 selected, use Actions &gt; Edit Operand,
or double-click on "$3000".  Select the "Symbol" radio button, then type
"INPUT" in the text box.  Click "OK".</p>
<p>Disappointed?  Nothing seems to have happened.  The problem is that we
updated the operand to reference a symbol that doesn't exist.  Open the
operand editor again, but this time click on "Set operand AND create project
symbol".  Click "OK".</p>
<p>That's better.  If you scroll up to the top of the project, you'll see
that there's now a ".EQ" line for the symbol.</p>
<p>Operands that refer to in-file locations behave similarly.  Select the
line two down, at address $2005, and Actions &gt; Edit Operand.  Enter the
symbol "IS_OK".  (Note you don't actually have to click Symbol first -- if
you just start typing as soon as the dialog opens, it'll select Symbol
for you automatically.)  Click "OK".</p>
<p>As before, nothing appears to have happened, but if you were watching
carefully you would have noticed that the label at $2009 ("L2009") has
disappeared.  This happened because the code at $2005 used to have a
<i>numeric</i> reference to $2009, and SourceGen automatically created a
label.  However, you changed the code at $2005 to have a <i>symbolic</i>
reference to a symbol called "IS_OK", and there were no other numeric
references to $2009, so the auto-label was no longer
needed.  Because IS_OK doesn't exist, the operand at $2005 is just formatted
as a hexadecimal value.</p>
<p>Let's fix this.  Select the line at address $2009, then
Actions &gt; Edit Label.  Enter "IS_OK", and hit Enter.  (NOTE: labels are
case-sensitive, so it needs to match the operand at $2005 exactly.)  You'll
see the new label appear, and the operand at line $2005 will use it.</p>
<p>There's an easier way.  Use Edit &gt; Undo twice, to get back to the time
where line $2005 is using "L2009" as its operand.  Select that line and
Actions &gt; Edit Operand.  Enter "IS_OK", then select "Create label at target
address instead".  Hit "OK".</p>
<p>You should now see that both the operand at $2005 and the label at
$2009 have changed to IS_OK, accomplishing what we wanted to do in a
single step.  (There's actually a subtle difference compared to the two-step
process: the operand at $2005 is still a numeric reference.  It was
automatically changed to match IS_OK in the same way that the references
to MAIN were when we renamed "L2000" earlier.  If you actually do want the
symbolic reference, there's another option in the Edit Operand dialog that
does it.  The difference can be noted in the Info window.)</p>


<h3>Editing Data Operands</h3>

<p>There's some string and numeric data down at the bottom of the file.  The
final string appears to be multiple strings stuck together.  Notice that
the opcode for the very last line is '+', which means it's a continuation
of the previous line.  Long data items can span multiple lines, split
every 64 characters (including delimiters), but they are still single
items: selecting any part selects the whole.</p>
<p>Select the last line, then Actions &gt; Edit Operand.  You'll notice
that this dialog is much different from the one you got when editing the
operand of an instruction.  At the top it will say "65 bytes selected".  You
can format this
as a single 65-byte string, as 65 individual items, or various things
in between.  For now, select "Single bytes", and then on the right,
select "ASCII".  Click "OK".</p>
<p>Each character is now on its own line.  The selection still spans the
same set of addresses.</p>
<p>Select address $203D on its own, then Actions &gt; Edit Label.  Set the
label to "STR1".  Move up a bit and select address $2030, then scroll to
the bottom and shift-click address $2070.  Select Actions &gt; Edit Operand.
At the top it should now say, "65 bytes selected in 2 groups".
There are two groups because the presence of a label split the data into
two separate regions.  Select "Low or High ASCII" encoding, select the
"mixed character and non-character" string type, then click "OK".</p>
<p>We now have two ".STR" lines, one for "string zero  ", and one with the
STR1 label and the rest of the string data.  This is okay, but it's not
really what we want.  The code at $200B appears to be loading a 16-bit
address from data at $2025, so we want to use that if we can.</p>
<p>Select Edit &gt; Undo three times.  You should be back to the state where
there's a single ".STR" line at the bottom of the file, split across two
lines with a '+'.</p>
<p>Select the line at $2026.  This is currently formatted as a string,
but that appears to be incorrect, so let's format it as individual bytes
instead.  There's an easy way to do that: use Actions &gt; Toggle Single-Byte
Format (or hit Ctrl+B).</p>
<p>The data starting at $2025 appears to be 16-bit addresses that point
into the table of strings, so let's format them appropriately.</p>
<p>Double-click the operand column on line $2025 ("$30") to open
the operand data format editor.  Because you only have one byte selected,
most of the options are disabled.  This won't do what we want, so
click "Cancel".</p>
<p>Select the line at $2025, then shift-click the line at $202E.  Right-click
and select Edit Operand.  If you selected the correct set of bytes,
the top line in the dialog should now say, "10 bytes selected".  Because
10 is a multiple of two, the 16-bit formats are enabled.  It's not a multiple
of 3 or 4, so the 24-bit and 32-bit options are not enabled.  Click the
"16-bit words, little-endian" radio button, then over to the right, click
the "Address" radio button.  Click "OK".</p>
<p>We just told SourceGen that those 10 bytes are actually five 16-bit numeric
references.  SourceGen determined that the addresses are contained in the
file, and created labels for each of them.  Labels only work if they're
on their own line, so the long string was automatically split into five
separate ".STR" statements.</p>

<p>Use File &gt; Save (or hit Ctrl+S) to save your work.</p>


<h3>Generating Assembly Code</h3>

<p>You can generate assembly source code from the disassembled data.
Select File &gt; Assembler (or hit Ctrl+Shift+A) to open the source generation
and assembly dialog.</p>
<p>Pick your favorite assembler from the drop list at the top right,
then click "Generate".  An assembly source file will be generated in the
directory where your project files lives, named after a combination of the
project name and the assembler name.  A preview of the assembled code
appears in the top window.  (It's a "preview" because it has line numbers
added and is cut off after a certain limit.)</p>

<p>If you have a cross-assembler installed and configured, you can run
it by clicking "Run Assembler".  The output from the assembler will appear
in the lower window, along with an indication of whether the assembled
file matches the original.  (Barring bugs in SourceGen or the assembler,
it should always match exactly.)</p>

<p>Click "Close" to close the window.</p>


<h3>End of Part One</h3>

<p>At this point you know enough to work with a SourceGen project.  Continue
on to the next tutorial to learn more.</p>


<h2><a name="advanced-features">Tutorial #2: Advanced Features</a></h2>

<p>This tutorial will walk you through some of the fancier things SourceGen
can do.  We assume you've already finished the Basic Features tutorial.</p>


<h3>Split-Address Table Formatting</h3>

<p>Start a new project.  Select the Apple //e platform, click Select File
and navigate to the Examples directory.  In A2-Amper-fdraw, select
<code>AMPERFDRAW#061d60</code>.  Click OK to create the project.</p>
<p>Not a lot to see here -- just half a dozen lines of loads and stores.
This particular program interfaces with Applesoft BASIC, so we can make it
a bit more meaningful by loading an additional platform
symbol file.  Select Edit &gt; Project Properties, then the Symbol Files
tab.  Click Add Symbol Files.  The file browser starts in the RuntimeData
directory.  In the Apple folder, select <code>Applesoft.sym65</code>, and
click Open.  Click OK to close the project properties window.</p>
<p>The STA instructions now reference <code>AMPERV</code>, which is noted
as a call vector.  We can see the code setting up a jump to $1d70.  As it
happens, the start address of the code is $1d60 -- the last four digits of
the filename -- so let's make that change.  Double-click the initial .ORG
statement, and change it from $2000 to $1d60.  We can now see that $1d70
starts right after this initial chunk of code.</p>

<p>Select the line with address $1d70, then Actions &gt; Hint As Code Entry Point.
More code appears, but not much -- if you scroll down you'll see that most
of the file is still data.  The code at $1d70 searches through a table at
$1d88 for a match with the contents of the accumulator.  If it finds a match,
it loads bytes from tables at $1da6 and $1d97, pushes them on the stack,
and the JMPs away.  This code is pushing a return address onto the stack.
When the code at CHRGET returns, it'll return to that address.  Because of
a quirk of the 6502 architecture, the address pushed must be the target
address minus one.</p>
<p>The first byte in the first table at $1d97 (which has the auto-label
L1D97) is $b4.  The first byte in the second table is $1d.  So the first
address we want is $1db4 + 1 = $1db5.</p>
<p>Select the line at $1db5, and use Actions &gt; Hint As Code Entry Point.
More code appears, but again it's only a few lines.  Let's dress this one
up a bit.  Set a label on the code at $1db5 called "FUNC".  At $1d97, edit
the data item (double-click on "$b4"), click "Single bytes", then type "FUNC"
(note the text field gets focus immediately, and the radio button
automatically switches to "symbolic reference" when you start typing).
Click OK.  The operand at $1d97 should now say <code>&lt;FUNC-1</code>.
Repeat the process at $1da6, this time clicking the "High" part radio button
below the symbol entry text box,
to make the operand there say <code>&gt;FUNC</code>.  (If it says
<code>&lt;FUNC-152</code>, you forgot to select the High part.)</p>

<p>We've now changed the first entry in the table to symbolic references.
You could repeat these steps for the remaining items, but there's a faster
way.  Click on the line at address $1d97, then shift-click the line at
address $1da9 (which should be <code>.FILL 12,$1e</code>).  Select
Actions &gt; Format Split-Address Table.</p>
<p>The message at the top should indicate that there are 30 bytes
selected.  In Address Characteristics, click the "adjusted for RTS/RTL"
checkbox.  As soon as you do, the first line of the Generated Addresses
list should show the symbol "FUNC".  The rest of the addresses will look like
<code>(+) T1DD0</code>.  The "(+)" means that a label was not found at
that location, so a label will be generated automatically.</p>
<p>Down near the bottom, check the "add code entry hint if needed" checkbox.
Because we saw the table contents being pushed onto the stack for RTS,
we know that they're all code entry points.</p>
<p>Click OK.  The table of address bytes at $1d97 should now all be
references to symbols -- 15 low parts followed by 15 high parts.  If you
scroll down, you should see nothing but instructions until you get to the
last dozen bytes at the end of the file.  (If this isn't the case, use
Edit &gt; Undo, then work through the steps again.)</p>
<p>The formatter did the same steps you went through earlier -- set a
label, apply the label to the low and high bytes in the table, add a
code entry point hint -- but did several of them at once.</p>

<p>We don't want to save this project, so select File &gt; Close.  When
SourceGen asks for confirmation, click Discard & Continue.</p>


<h3>Going Deeper</h3>

<p>Start a new project.  Select "Generic 6502".  For the data file, navigate
to the Examples directory, then from the Tutorials directory
select "Tutorial2".</p>
<p>The first thing you'll notice is that we immediately ran into a BRK,
which is a pretty reliable sign that we're not in a code section.  The
generic profile puts a code entry point hint on the first byte, but that's
wrong here.  This particular file begins with <code>00 20</code>, which
could be a load address (some C64 binaries look like this).  So let's start
with that assumption.</p>
<p>Click on the first line of code at address $1000, and select
Actions &gt; Remove Hints.  The $20 got absorbed into a string.  The string
is making it hard to manipulate the next few bytes, so let's fix that by
selecting Edit &gt; Toggle Data Scan (Ctrl+D).  This turns off the feature
that looks for strings and .FILL regions, so now each uncategorized byte is
on its own line.</p>
<p>You could select the first two lines and use Actions &gt; Edit Operand
to format them as a 16-bit little-endian hex value, but there's a shortcut:
select the first line only, then Edit &gt; Format As Word (Ctrl+W).  It
automatically grabbed the following byte and combined them.  Since we believe
$2000 is the load address for everything that follows, click on the line
with address $1002, select Actions &gt; Set Address, and enter "2000".  With
that line still selected, use Actions &gt; Hint As Code Entry Point
(Ctrl+H then Ctrl+C) to identify it as code.</p>
<p>That looks better, but it's branching off the bottom of the screen
(unless you have a really tall screen or small fonts) because of all the
intervening data.  Use Edit &gt; Toggle Data Scan to turn the string
finder back on.</p>

<p>There are four strings starting at address $2004, each of which is
followed by $00.  These look like null-terminated strings, so let's make
it official.  But first, let's do it wrong.  Click on the line with
address $2004 to select it.  Hold the shift key down, then double-click
on the operand field of the line with address $2031 (i.e. double-click on
the words "last string").</p>
<p>The Edit Data Format dialog opens, but the null-terminated strings
option is not available.  This is because we didn't include the null byte
on the last string.  To be recognized as one of the "special" string types,
every selected string must match the expected pattern.</p>
<p>Cancel out of the dialog.  Hold the shift key down, and double-click
on the operand on line $203c (<code>$00</code>).
You should see "Null-terminated strings (4)" as an available
option now.  Click on that, then click OK.  The strings are now shown
as .ZSTR operands.</p>

<p>It's wise to save your work periodically.  Use File &gt; Save to create
a project file for Tutorial2.</p>

<h4>Pointers and Parts</h4>

<p>Let's move on to the code at $203d.  It starts by storing a couple of
values into direct page address $02/03.  This appears to be setting up a
pointer to $2063, which is a data area inside the file.  So let's make it
official.</p>
<p>Select the line at address $2063, and use Actions &gt; Edit Label to
give it the label "XDATA".  Now edit the operand on line $203d, and set it
to the symbol "XDATA", with the part "low".  Edit the operand on line $2043,
and set it to "XDATA" with the part "high".  (Note the symbol text box
gets focus immediately, so you can start typing the symbol name as soon
as the dialog opens; you don't need to click around first.)  If all
went well, the operands should now read <code>LDA #&lt;XDATA</code>
and <code>LDA #&gt;XDATA</code>.</p>
<p>Let's name the pointer.  Edit the operand on line $203f, enter the
symbol "PTR1", and in the Symbol Shorcuts section, click "Set operand
AND create project symbol".  Click OK.  Note that operand on line $2043
has changed to "PTR1+1".</p>
<p>Double-click on the JSR on line $2045 to jump to L209A.  This just
loads a value from $3000 into the accumulator and returns, so not much
to see here.  Hit the back-arrow in the toolbar to jump back to the JSR.</p>
<p>The next bit of code masks the accumulator so it holds a value between
0 and 3, then doubles it and uses it as an index into PTR1.  We know PTR1
points to XDATA, which looks like it has some 16-bit addresses.  The
values loaded are stored in two more zero-page locations, $04-05.</p>
<p>$04 is being displayed as "PTR1+2", which isn't quite right, so let's
fix that.  Double-click on the operand of address $204e, set the symbol
to "PTR2", and again click "Set operand AND create project symbol".
Click OK.  That looks better.</p>

<p>The next bit of code copies bytes from PTR2 to $0400, stopping when it
hits a zero byte.  Looks like this is copying null-terminated strings.
This confirms our idea that XDATA holds 16-bit addresses, so let's
format it.  Select lines $2063 to $2066, and Actions &gt; Edit Operand.
It should say "8 bytes selected" at the top.  Select "16-bit words,
little-endian", and then from the Display As box, select "Address".
Click OK.  XDATA should now be four <code>.dd2</code> 16-bit addresses.
If you scroll up, you'll see that the .ZSTR strings near the top now have
labels that match the labels in XDATA.</p>
<p>Now that we know what XDATA holds, let's rename it.  Change the label
to STRADDR.  The symbol parts in the operands at $203d and $2041 update
automatically.</p>

<p>Let's pause briefly to look at the cycle-count feature.  Use
Edit &gt; Settings to open the app settings panel, then select the
Asm Config tab.  Click the "Show cycle counts" checkbox, then click OK.</p>
<p>Every line with an instruction now has a cycle count on it.  The cycle
counts are adjusted for everything SourceGen can figure out.  For example,
the BEQ on line $205a shows "2+" cycles, meaning that it takes at least two
cycles but might take more.  That's because conditional branches take an
extra cycles if the branch is taken.  The BNE on line $2061" shows 3 cycles,
because we know that the branch is always taken.  (If you want to see why,
look at the value of the 'Z' flag in the "flags" column.  Lower-case 'z'
means the zero-flag is clear.)</p>
<p>The cycle-count comments are included in assembled output as well.  If
you add an end-of-line comment, it appears after the cycle count.</p>
<p>Hit Ctrl+S to save your project.  Make that a habit.</p>

<h4>Odds &amp; Ends</h4>

<p>The rest of the code isn't really intended to do anything useful.  It
just exists to illustrate some odd situations.</p>
<p>Look at the code starting at $206b.  It ends with a BRK at $2074, which
as noted earlier is a bad sign.  If you look two lines above the BRK,
you'll see that it's loading the accumulator with zero, then doing a BNE,
which should never be taken (note the cycle count for the BNE is 2).  The
trick is in the two lines before that, which use self-modifying code to
change the LDA immediate operand from $00 to $ff.  The BNE is actually
a branch-always.</p>
<p>We can fix this by correcting the status flags.  Select line $2072,
and then Actions &gt; Override Status Flags.  This lets us specify what
the flags should be before the instruction is executed.  For each flag,
we can override the default behavior and specify that the flag is
clear (0), set (1), or indeterminate (could be 0 or 1).  In this case,
we know that the self-modified code will be loading a non-zero value, so
in the "Z" column click on the button in the "Zero" row.  Click OK.  The
BNE is now an always-taken branch, and the code list rearranges itself
appropriately (and the cycle count is now 3).</p>

<p>Continuing on, the code at $2079 touches a few consecutive locations.  Edit
the label on line $2074, setting it to "VAR".  Notice how the references
to $2074 through $2077 have changed from auto-generated labels to
references to VAR.  For some projects this may be undesirable.  Use
Edit &gt; Project Properties, then in the Analysis Parameters box
un-check "Seek nearby targets", and click OK.  You'll notice that the
references to $2075 on have switched back to auto labels.  Furthermore, if
you scroll up, you'll see that the stores to PTR1+1 and PTR2+1 are now
just stores to $03 and $05.</p>
<p>The nearby-target behavior is generally desirable, because it lets you
avoid explicitly labeling every part of a multi-byte data item.  For now,
use Edit &gt; Undo to switch it back on.</p>

<p>The code at $2085 looks a bit strange.  LDX, then a BIT with a weird
symbol, then another LDX.  If you look at the "bytes" column, you'll notice
that the three-byte BIT instruction has only one byte on its line.  The
trick here is that the <code>LDX #$01</code> is embedded inside the BIT
instruction.  When the code runs through here, X is set to $00, then
the BIT instruction sets some flags, then the STA runs.  Several lines
down there's a BNE to $2088, which is in the middle of the BIT instruction.
It loads X with $01, then also continues to the STA.</p>
<p>Embedded instructions are unusual but not unheard-of.  When you see the
extra symbol in the opcode field, you need to look closely at what's going
on.</p>


<h3>Go Forth</h3>

<p>That's it for the tutorials.  There's significantly more detail on
all aspects of SourceGen in the manual.</p>
<p>While you can do some fancy things, nothing you do will alter the
data file.  The assembled output will always match the original.  So
don't be afraid to play around.</p>

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