Clarify coverage wording and add ROM analysis guide

docs/LARGE_ROM_ANALYSIS_WORKFLOW.md

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+# Large ADSP-219x ROM Analysis Workflow
+
+This guide describes a practical workflow for reverse engineering a
+large raw ADSP-219x ROM image in radare2 using the `adsp219x`
+architecture plugin from this repository.
+
+It is written for the common case where you have a large blob, for
+example a 1 MB ROM dump, and you do not yet know:
+
+- where code starts and where data starts
+- how many independent code regions exist
+- where the main loops and dispatchers are
+- which PM words are instructions and which are tables
+
+The workflow below is intentionally conservative. For raw DSP ROMs,
+manual validation beats aggressive auto-analysis.
+
+## Assumptions
+
+- The ROM is for an ADSP-219x family DSP.
+- Instructions are 24-bit words.
+- The dump is raw program memory, not a richly annotated executable.
+- You already have a working radare2 installation and the
+  `adsp219x` plugin is installed or loadable with `-L`.
+
+## Load The ROM
+
+For an installed plugin:
+
+```bash
+r2 -a adsp219x -b 24 firmware.bin
+```
+
+For a local plugin build that is not installed:
+
+```bash
+r2 -a adsp219x -L ./r2plugin/asm_adsp219x.so -b 24 firmware.bin
+```
+
+Why the flags matter:
+
+- `-a adsp219x`: forces the custom architecture plugin
+- `-b 24`: tells radare2 to treat the code as 24-bit ADSP-219x words
+
+## ROM Format Sanity Check
+
+Before analyzing flow, check whether the byte layout looks correct.
+
+ADSP-219x code is usually stored either as:
+
+- packed 24-bit words: `aa bb cc dd ee ff ...`
+- padded 32-bit words with a leading zero byte
+
+If your disassembly looks completely implausible, confirm the ROM
+format first.
+
+Helpful first commands:
+
+```text
+px 64
+s 0
+pd 16
+```
+
+If the first few decoded instructions look impossible but the hex dump
+shows a repeating `00 xx xx xx` structure, you may be looking at a
+32-bit padded dump and need to strip padding before analysis.
+
+## High-Level Strategy
+
+For a large ROM, do not start with `aaa`.
+
+A better sequence is:
+
+1. validate the entry region manually
+2. mark only plausible functions
+3. follow explicit control flow
+4. distinguish code from tables
+5. expand analysis gradually
+6. use graph views only after local validation
+
+`aaa` on a large raw DSP ROM often creates a false sense of structure
+ by treating valid-looking data words as code.
+
+## Workflow Diagram
+
+```mermaid
+flowchart TD
+    A[Load ROM in r2] --> B[Check raw bytes and first instructions]
+    B --> C{Plausible code at reset area?}
+    C -- no --> D[Verify ROM packing, offset, endianness, dump source]
+    D --> B
+    C -- yes --> E[Create first function manually]
+    E --> F[Inspect linear disassembly and graph]
+    F --> G{Looks like real control flow?}
+    G -- no --> H[Do not mark as function, treat as possible data]
+    G -- yes --> I[Follow jumps, calls, DO UNTIL loops]
+    I --> J[Name regions and comment findings]
+    J --> K[Search for more entry points and dispatchers]
+    K --> L[Only then expand with broader analysis]
+    L --> M[Separate code islands from PM data tables]
+```
+
+## Step 1: Inspect The Reset Region Manually
+
+Start at address zero unless you have evidence of a different boot
+mapping.
+
+```text
+s 0
+pd 20
+pd 64
+```
+
+Look for signs of real code:
+
+- immediate register loads
+- `JUMP` or `CALL`
+- `DO ... UNTIL`
+- initialization of `I`, `M`, `L`, `CNTR`, status registers
+- branches to other regions
+
+Red flags that suggest you are not in code:
+
+- long stretches of decodeable but nonsensical instructions
+- no branches, no calls, no returns
+- bizarre register moves with no purpose
+- disassembly that looks random but hex bytes look regular
+
+## Step 2: Mark The First Function Manually
+
+Once the entry region looks plausible:
+
+```text
+s 0
+af
+pdf
+agf
+```
+
+Meaning:
+
+- `af`: define a function at the current address
+- `pdf`: print the current function linearly
+- `agf`: show the control-flow graph of the current function
+
+This is a better first move than `aaa`, because it forces you to
+validate one region before scaling out.
+
+## Step 3: Use Graphs Locally, Not Globally
+
+For a large ROM, graphs are most useful once you already believe the
+current region is code.
+
+Useful commands:
+
+```text
+agf
+VV
+afb
+```
+
+- `agf`: static graph of current function
+- `VV`: visual graph mode
+- `afb`: list/show basic blocks
+
+If `agf` produces a clean graph with obvious branches and loop edges,
+the region is probably code.
+
+If `agf` is trivial, chaotic, or makes no semantic sense, you may be
+looking at data or a bad function boundary.
+
+## Step 4: Expand By Following Explicit Flow
+
+After validating the first function, expand manually through explicit
+targets:
+
+```text
+pdf
+axt
+afl
+```
+
+Then jump to interesting destinations:
+
+```text
+s <target>
+af
+pdf
+agf
+```
+
+Prioritize:
+
+- call targets
+- jump targets
+- loop bodies
+- indirect branch setup code
+
+For ADSP-219x specifically, also watch for:
+
+- `DO ... UNTIL`
+- tight MAC loops
+- register setup for DAGs
+- memory access kernels
+
+## Step 5: Search For ADSP-219x-Specific Patterns
+
+Search directly for common control-flow patterns:
+
+```text
+/a JUMP
+/a CALL
+/a DO
+/a RTS
+/a RTI
+```
+
+Also inspect likely DSP kernel patterns:
+
+```text
+/a MR
+/a MX0
+/a MY0
+/a IO(
+```
+
+These searches are not perfect, but they help locate:
+
+- processing loops
+- dispatch logic
+- hardware initialization
+- coefficient loads
+
+## Step 6: Distinguish Code From Data
+
+In a large ADSP-219x ROM, many PM words are data, not instructions.
+You must actively separate them.
+
+### Heuristic: likely code
+
+A region is likely code if it has:
+
+- incoming xrefs from jumps or calls
+- meaningful block structure
+- function-like boundaries
+- setup followed by control flow
+- loops, branches, and exits
+
+Check with:
+
+```text
+axt @ <addr>
+pdf
+agf
+```
+
+### Heuristic: likely data
+
+A region is likely data if it has:
+
+- regular numeric patterns in hex
+- no meaningful control flow
+- no incoming code xrefs
+- no returns or loop structure
+- many decodeable instructions that make no programmatic sense
+
+Check with both disassembly and raw bytes:
+
+```text
+pd 32
+px 64
+```
+
+If the hex dump looks more plausible than the disassembly, it is often
+a table.
+
+### ADSP-219x-specific data patterns
+
+Common PM data in DSP firmware:
+
+- FIR coefficients
+- IIR coefficients
+- FFT sine/cosine tables
+- lookup tables
+- packed constants
+- boot configuration words
+
+These often decode into valid-looking instructions by accident.
+
+## Step 7: Delay Global Auto-Analysis
+
+Only after you have mapped a few real code islands should you broaden
+analysis:
+
+```text
+aa
+afl
+```
+
+Prefer `aa` first.
+
+Use `aaa` only when:
+
+- the ROM format is confirmed
+- the entry region is valid
+- you already understand where major code regions are
+- the plugin behaves consistently on this image
+
+Why this matters:
+
+- `aa` is less aggressive
+- `aaa` can create junk functions in large raw ROMs
+- false positives are expensive to clean up mentally
+
+## Step 8: Name What You Understand
+
+As soon as a region is understood, name and annotate it.
+
+Useful commands:
+
+```text
+afn entry_init
+afn main_loop
+afn io_dispatch
+CCu probable coefficient table
+CCu hardware init and watchdog setup
+```
+
+Naming reduces rework and makes graph navigation much easier.
+
+## Step 9: Build A Region Map
+
+For a 1 MB ROM, keep a rough map as you go:
+
+- boot/reset code
+- hardware init
+- interrupt vector area
+- main loop
+- DSP kernels
+- dispatch tables
+- PM data tables
+- obvious unused or padding regions
+
+This can live in:
+
+- r2 comments
+- a notebook
+- a separate markdown file
+
+The important thing is to stop treating the ROM as one continuous
+thing. Large firmware becomes manageable once you divide it into
+regions.
+
+## Step 10: Revisit Ambiguous Areas Later
+
+Do not force a conclusion too early.
+
+When a region is ambiguous:
+
+- leave it unnamed or mark it as tentative
+- inspect surrounding xrefs first
+- compare with neighboring validated code
+- revisit it after understanding more of the firmware
+
+Good reverse engineering on large DSP ROMs is iterative.
+
+## Recommended First 15 Minutes
+
+If you want a concrete first-pass routine for a 1 MB ROM:
+
+### 1. Open the ROM
+
+```bash
+r2 -a adsp219x -b 24 firmware.bin
+```
+
+### 2. Check the first bytes and first instructions
+
+```text
+s 0
+px 64
+pd 32
+```
+
+### 3. If plausible, define the first function
+
+```text
+af
+pdf
+agf
+```
+
+### 4. Enter visual graph mode
+
+```text
+VV
+```
+
+### 5. Follow obvious branch targets manually
+
+```text
+s <target>
+af
+pdf
+agf
+```
+
+### 6. Search for loops and calls
+
+```text
+/a DO
+/a CALL
+/a JUMP
+```
+
+### 7. Compare suspicious regions with hex
+
+```text
+pd 32
+px 64
+```
+
+### 8. Only then widen analysis
+
+```text
+aa
+afl
+```
+
+## When To Suspect A Bad Decode
+
+Pause and reassess if you see:
+
+- no meaningful flow anywhere near the entry region
+- every region looks equally nonsensical
+- branch targets never lead to reasonable code
+- graph mode shows nonsense everywhere
+- the ROM appears to decode but nothing behaves like firmware
+
+Then check:
+
+- ROM packing
+- dump alignment
+- whether the image is compressed or encrypted
+- whether the base offset is wrong
+- whether the file contains headers before the actual code
+
+## Practical Command Cheat Sheet
+
+Open:
+
+```bash
+r2 -a adsp219x -b 24 firmware.bin
+```
+
+Start region:
+
+```text
+s 0
+pd 20
+pd 64
+px 64
+```
+
+Define and inspect function:
+
+```text
+af
+pdf
+agf
+```
+
+Visual graph:
+
+```text
+VV
+```
+
+Search:
+
+```text
+/a JUMP
+/a CALL
+/a DO
+/a RTS
+```
+
+Cross-references:
+
+```text
+axt
+axt @ <addr>
+```
+
+Broader analysis:
+
+```text
+aa
+afl
+```
+
+Naming:
+
+```text
+afn main_loop
+CCu probable PM coefficient table
+```
+
+## Summary
+
+For a large ADSP-219x ROM:
+
+- do not trust auto-analysis first
+- validate the entry region manually
+- graph only locally at first
+- follow explicit flow edges
+- use both disassembly and hex dumps
+- treat PM as mixed code and data
+- expand analysis gradually
+
+The core rule is simple:
+
+**Local confidence first, global analysis later.**