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<title>Instruction and Data Analysis - 6502bench SourceGen</title>
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<h1>6502bench SourceGen: Instruction and Data Analysis</h1>
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<p><a href="index.html">Back to index</a></p>
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<h2><a name="analysis-process">Analysis Process</a></h2>
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<p>Analysis of the file data is a complex multi-step process. Some
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changes to the project, such as adding a code entry point hint or
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changing the CPU selection, require a full re-analysis of instructions
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and data. Other changes, such as adding or removing a label, don't
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affect the code tracing and only require a re-analysis of the data areas.
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And some changes, such as editing a comment, only require a refresh
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of the displayed lines.</p>
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<p>It should be noted that none of the analysis results are stored in
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the project file. Only user-supplied data, such as the locations of
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code entry points and label definitions, is written to the file. This
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does create the possibility that two different users might get different
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results when opening the same project file with different versions of
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SourceGen, but these effects are expected to be minor.</p>
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<p>The analyzer has the following steps (see the <code>Analyze</code>
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method in <code>DisasmProject.cs</code>):</p>
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<ul>
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<li>Reset the symbol table.</li>
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<li>Merge platform symbols into the symbol table, loading the files
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in order.</li>
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<li>Merge project symbols into the symbol table, stomping on any
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platform symbols that conflict.</li>
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<li>Merge user label symbols into the table, stomping any previous
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entries.</li>
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<li>Run the code analyzer. The outcome of this is an array of analysis
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attributes, or "anattribs", with one entry per byte in the file.
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The Anattrib array tracks most of the state from here on. If we're
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doing a partial re-analysis, this step will just clone a copy of the
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Anattrib array that was made at this point in a previous run. (This
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step is described in more detail below.)</li>
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<li>Apply user-specified labels to Anattribs.</li>
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<li>Apply user-specified format descriptors. These are the instruction
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and data operand formats.</li>
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<li>Run the data analyzer. This looks for patterns in uncategorized
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data, and connects instruction and data operands to target offsets.
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The "nearby label" stuff is handled here. All of the results are
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stored in the Anattribs array. (This step is described in more
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detail below.)</li>
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<li>Remove hidden labels from the symbol table. These are user-specified
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labels that have been placed on offsets that are in the middle of an
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instruction or multi-byte data item. They can't be referenced, so we
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want to pull them out of the symbol table. (Remember, symbolic
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operands use "weak references", so a missing symbol just means the
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operand is shown as a hex value.)</li>
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<li>Resolve references to platform and project external symbols.
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This sets the operand symbol in Anattrib, and adds the symbol to
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the list that is displayed in .EQ directives.</li>
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<li>Generate cross-reference lists. This is done for file data and
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for any platform/project symbols that are referenced.</li>
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<li>If annotated auto-labels are enabled, the simple labels are
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replaced with the annotated versions here. (This can't be done earlier
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because the annotations are generated from the cross-reference data.)</li>
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<li>In a debug build, some validity checks are performed.</li>
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</ul>
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<p>Once analysis is complete, a line-by-line display list is generated
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by walking through the annotated file data. Most of the actual strings
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aren't rendered until they're needed.</p>
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<h3><a name="auto-format">Automatic Formatting</a></h3>
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<p>Every offset in the file is marked as an instruction byte, data byte, or
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inline data byte. Some offsets are also marked as the start of an instruction
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or data area. The start offsets may have a format descriptor associated
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with them.</p>
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<p>Format descriptors have a format (like "numeric" or "string") a
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sub-format (like "hexadecimal" or "null-terminated"), and a length. For
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an instruction operand the length is redundant, but for a data operand it
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determines the width of the numeric value or length of the string. For
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this reason, instructions do not need a format descriptor, but all
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data items do.</p>
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<p>Symbolic references are format descriptors with a symbol attached.
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The symbol reference also specifies low/high/bank, for partial symbol
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references like <code>LDA #>symbol</code>.</p>
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<p>Every offset marked as a start point gets its own line in the on-screen
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display list. Embedded instructions are identified internally by
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looking for instruction-start offsets inside instructions.</p>
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<p>The Anattrib array holds the post-analysis state for every offset,
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including comments and formatting, but any changes you make in the
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editors are applied to the data structures that are saved in the project
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file. After a change is made, a full or partial re-analysis is done to
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fill out the Anattribs.</p>
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<p>Consider a simple example:</p>
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<pre>
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.ORG $1000
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JMP L1003
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L1003 NOP
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</pre>
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<p>We haven't formatted anything yet. The data analyzer sees that the
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JMP operand is inside the file, and has no label, so it creates an
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auto-label at offset +000003 and a format descriptor with a symbolic
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operand reference to "L1003" at +000000.</p>
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<p>Now we edit the label, changing L1003 to "FOO". This goes into the
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project's "user label" list. The analyzer is
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run, and applies the new "user label" to the Anattrib array. The
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data analyzer finds the numeric reference in the JMP operand, and finds
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a label at the target address, so it creates a symbolic operand reference
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to "FOO". When the display list is generated, the symbol "FOO" appears
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in both places.</p>
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<p>Even though the JMP operand changed from "L1003" to "FOO", the only
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change actually written to the project file is the label edit. The
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contents of the Anattrib array are disposable, so it can be used to
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add labels and "fix up" numeric references. Generated labels and
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format descriptors are never added to the project file.</p>
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<p>If the JMP operand were edited, a format descriptor would be added
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to the user-specified descriptor list. During the analysis pass it would
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be added to the Anattrib array at offset +000000.</p>
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<h3><a name="undo-redo">Interaction With Undo/Redo</a></h3>
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<p>The analysis pass always considers the current state of the user
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data structures. Whether you're adding a label or removing one, the
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code runs through the same set of steps. The advantage of this approach
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is that the act of doing a thing, undoing a thing, and redoing a thing
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are all handled the same way.</p>
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<p>None of the editors modify the project data structures directly. All
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changes are added to a change set, which is processed by a single function.
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The change sets are kept in the undo/redo buffer indefinitely. After
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the changes are made, the Anattrib array and other data structures are
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regenerated.</p>
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<p>Data format editing can create some tricky situations. For example,
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suppose you have 8 bytes that have been formatted as two 32-bit words:</p>
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<pre>
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1000: 68690074 .dd4 $74006968
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1004: 65737400 .dd4 $00747365
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</pre>
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<p>You realize these are null-terminated strings, select both words, and
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reformat them:</p>
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<pre>
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1000: 686900 .zstr "hi"
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1003: 74657374+ .zstr "test"
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</pre>
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<p>Seems simple enough. Under the hood, SourceGen created three changes:</p>
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<ol>
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<li>At offset +000000, replace the current format descriptor (4-byte
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numeric) with a 3-byte null-terminated string descriptor.</li>
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<li>At offset +000003, add a new 5-byte null-terminated string
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descriptor.</li>
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<li>At offset +000004, remove the 4-byte numeric descriptor.</li>
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</ol>
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<p>Each entry in the change set has "before" and "after" states for the
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format descriptor at a specific offset. Only the state for the affected
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offsets is included -- the program doesn't take a complete state snapshot
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(even with the RAM on a modern system that would add up quickly). When
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undoing a change, before and after are simply reversed.</p>
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<h2><a name="code-analysis">Code Analysis</a></h2>
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<p>The code tracer walks through the instructions, examining them to
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determine where execution will proceed next. There are five possibilities
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for every instruction:</p>
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<ol>
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<li>Continue. Execution always continues at the next instruction.
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Examples: <code>LDA</code>, <code>STA</code>, <code>AND</code>,
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<code>NOP</code>.</li>
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<li>Don't continue. The next instruction to be executed can't be
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determined from the file data (unless you're disassembling the
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system ROM around the BRK vector).
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Examples: <code>RTS</code>, <code>BRK</code>.</li>
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<li>Branch always. The operand specifies the next instruction address.
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Examples: <code>JMP</code>, <code>BRA</code>, <code>BRL</code>.</li>
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<li>Branch sometimes. Execution may continue at the operand address,
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or may execute the following instruction. If we know the value of
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the flags in the processor status register, we can eliminate one
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possibility. Examples: <code>BCC</code>, <code>BEQ</code>,
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<code>BVS</code>.</li>
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<li>Call subroutine. Execution will continue at the operand address,
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and is expected to also continue at the following instruction.
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Examples: <code>JSR</code>, <code>JSL</code>.</li>
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</ol>
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<p>Branch targets are added to a list. When the current run of instructions
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is exhausted (i.e. a "don't continue" or "branch always" instruction is
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reached), the next target is pulled off of the list.</p>
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<p>The state of the processor status flags is recorded for every
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instruction. When execution proceeds to the next instruction or branches
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to a new address, the flags are merged with the flags at the new
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location. If one execution path through a given address has the flags
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in one state (say, the carry is clear), while another execution path
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sees a different state (carry is set), the merged flag is
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"indeterminate". Indeterminate values cannot become determinate through
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a merge, but can be set by an instruction.</p>
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<p>There can be multiple paths to a single address. If the analyzer
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sees that an instruction has been visited before, with an identical set
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of status flags, the analyzer stops pursuing that path.</p>
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<p>The analyzer must always know the width of immediate load instructions
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when examining 65816 code, but it's possible for the status flag values
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to be indeterminate. In such a situation, short registers are assumed.
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Similarly, if the carry flag is unknown when an <code>XCE</code> is
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performed, we assume a transition to emulation mode (E=1).</p>
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<p>There are three ways in which code can set a flag to a definite value:</p>
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<ol>
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<li>By explicit instructions, like <code>SEC</code> or
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<code>CLD</code>.</li>
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<li>By immediate-operand instructions. <code>LDA #$00</code> sets Z=1
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and N=0. <code>ORA #$80</code> sets Z=0 and N=1.</li>
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<li>By inference. For example, if we see a <code>BCC</code> instruction,
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we know that the carry will be clear at the branch target address, and
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set at the following instruction. The instruction doesn't affect the
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value of the flag, but we know what the value will be at both
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addresses.</li>
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</ol>
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<p>Self-modifying code can render spoil any of these, possibly requiring a
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status flag override to get correct disassembly.</p>
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<p>The instruction that is most likely to cause problems is <code>PLP</code>,
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which pulls the processor status flags off of the stack. SourceGen
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doesn't try to track stack contents, so it can't know what values may
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be pulled. In many cases the PLP appears not long after a PHP, so
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SourceGen will scan backward through the file to find the nearest PHP,
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and use the status flags from that. If no PHP can be found, then all
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flags are set to "indeterminate". (The boot loader in the Apple II 5.25"
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floppy disk controller is an example where SourceGen gets it wrong. The
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code does <code>CLC</code>/<code>PHP</code>, followed a bit later by the
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<code>PLP</code>, but it's actually using the stack to pass the carry
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flag around. Flagging the carry bit as indeterminate with a status flag
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override on the instruction following the PLP fixes things.)</p>
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<p>Some other things that the code analyzer can't recognize automatically:</p>
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<ul>
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<li>Jumping indirectly through an address outside the file, e.g.
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storing an address in zero-page memory and jumping through it.</li>
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<li>Jumping to an address by pushing the location onto the stack,
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then executing an <code>RTS</code>.</li>
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<li>Self-modifying code, e.g. overwriting a <code>JMP</code> instruction.</li>
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</ul>
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<p>Sometimes the indirect jump targets are coming from a table of
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addresses in the file. If so, these can be formatted as addresses,
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and then the target locations hinted as code entry points.</p>
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<p>The 65816 adds an additional twist: 16-bit data access instructions
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use the data bank register ("B") to determine which bank to load from.
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SourceGen can't determine what the value is, so it currently assumes
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that it's equal to the program bank register ("K"). Handling this
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correctly will require improvements to the user interface.</p>
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<h3><a name="extension-scripts">Extension Scripts</a></h3>
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<p>Extension scripts can mark data that follows a JSR or JSL as inline
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data, or change the format of nearby data or instructions. The first
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time a JSR/JSL instruction is encountered, all loaded extension scripts
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are offered a chance to act.</p>
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<p>The first script that applies a format wins. Attempts to re-format
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instructions or data will fail. This rule ensure that anything explicitly
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formatted by the user will not be overridden by a script.</p>
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<p>If code jumps into a region that is marked as inline data, the
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branch will be ignored. If an extension script tries to flag bytes
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as inline data that have already been executed, the script will be
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ignored. This can lead to a race condition in the analyzer if
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an extension script is doing the wrong thing. (The race doesn't exist
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with inline data hints specified by the user, because those are applied
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before code analysis starts.)</p>
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<h2><a name="data-analysis">Data Analysis</a></h2>
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<p>The data analyzer performs two tasks. It matches operands with
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offsets, and it analyzes uncategorized data. Either or both of
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these can be disabled from the
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<a href="settings.html#project-props">project properties</a></p>
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<p>The data target analyzer examines every instruction and data operand
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to see if it's referring to an offset within the data file. If the
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target is within the file, and has a label, a format descriptor with a
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weak symbolic reference to that label is added to the Anattrib array. If
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the target doesn't have a label, the analyzer will either use a nearby
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label, or generate a unique label and use that.</p>
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<p>While most of the "nearby label" logic can be disabled, targets that
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land in the middle of an instruction are always adjusted backward to
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the instruction start. This is necessary because labels are only visible
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if they're associated with the first (opcode) byte of an instruction.</p>
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<p>The uncategorized data analyzer tries to find ASCII strings and
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opportunities to use the ".FILL" operation. It breaks the file into
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pieces, where contiguous regions hold nothing but data, are not split
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across a ".ORG" directive, are not interrupted by data, and do not
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contain anything that the user has chosen to format. Each region is
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scanned for matching patterns. If a match is found, a format entry
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is added to the Anattrib array. Otherwise, data is added as independent
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byte values.</p>
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</div>
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