ZoomMultistompZDL

ZDL and Pedal Reverse-Engineering Status

Last updated: 2026-05-18

This is the current map of the Zoom .ZDL wrapper, the embedded TI C6000 ELF, and the runtime ABI details that matter for custom effects. Keep experiment transcripts out of this file; put durable state/edit-handler findings in STATE-ABI-PROGRESS.md.

Current Snapshot

What is working:

• The repo can build loadable custom .ZDL effects without Zoom’s SDK.
• dist/StChorus.ZDL is the current best Airwindows reference: it uses the host-managed ctx[3] arena for large state and was reported to sound like Airwindows StereoChorus.
• dist/ is kept release-focused. Hardware probe ZDLs are buildable, but are not shipped by default.
• The 830-file stock ZDL corpus in stock_zdls/ round-trips through the parser, aside from five stock files with stale embedded SIZE fields that canonicalize to the actual ELF length.

What is still experimental:

• dist/ToTape9.ZDL is not a finished port, but the no-divide full DSP now loads and runs on the test MS-70CDR. Current validation work is parameter initialization, preset behavior, and source-equivalence testing.
• dist/VerbTiny.ZDL is the first reverb candidate. It builds with ctx[3] state, no .fardata, and no object relocations; hardware result is pending.
• dist/Galactic.ZDL is a larger reverb candidate. It builds with about 528 KB of ctx[3] state, no .fardata, and no object relocations; hardware result is pending.
• dist/TEcho4.ZDL is a custom Airwindows-inspired Delay-category tape echo. It builds with a bounded ctx[3] stereo delay line, a compact Galaxy-derived FIR in a read-only named .const:* subsection, measured wow/flutter, a compact mono spring fallback, no .fardata, and a BPM+division tempo workflow. Desktop listening validated the echo path as promising and selected a closer Galaxy spring reference model: a 744 ms measured body, eight-line FDN, 0.92 feedback, 0.55 damping, and a 320 ms handoff. The pedal-safe implementation of that longer spring remains pending, so the ZDL spring defaults to zero. A downsampled distributed-FIR attempt was rejected by desktop A/B listening. Stock SPRING, SPRING63, and PARTICLE instead point toward a full-rate algorithmic diffusion and irregular-delay network. Hardware result is pending; true host tap-tempo plumbing for custom ZDLs is still unresolved.
• dist/OTT.ZDL is a custom Dynamics-category OTT-style 3-band compressor, not an Ableton port. It uses a small ctx[3] state block for crossover, envelope, and gain history, and builds with no .fardata, no .text, and no object relocations. Hardware result is pending.
• src/airwindows/tovinyl4/ is a source-only ToVinyl4 candidate. It builds manually or via build_all.py tovinyl4, but is not in release dist/ until hardware testing says it should be.
• Multi-page parameter editing is not a clean SDK contract yet. Stock LineSel handlers are useful for the first two knobs; object-defined ZOOM_EDIT_HANDLER freezes on interaction in T9NoAudio; synthesized LineSel clones still need an isolated page 2/3 tiny-DSP proof.
• Init-time parameter materialization is still unsolved. InitProbe proved the stock-style coefficient setup callback can be called from custom _init, but invoking a cloned LineSel edit handler from that custom init froze on boot.
• The custom stereo routing declaration remains unknown. Custom code can process stereo buffers once running, but the stock effect-level stereo mechanism has not been mapped.
• Category exposure has a host/browser dependency. On the MS-70CDR test pedal, Drive-category ToTape9 flashed successfully but only appeared in the on-device FX browser after a stock Drive effect was also installed.

ZDL File Layers

Layer	Current understanding
Wrapper	Starts with four zero bytes, SIZE, then INFO.
Header size	Standard files place ELF at offset 0x4c; extended files must honor the SIZE header-size field.
Extended payloads	Some stock files carry BCAB or CABI payloads between INFO and ELF. Their fields are preserved, not fully decoded.
Embedded executable	TI C6000 ELF32, little-endian, ET_DYN.
Relocations	.rela.dyn is used for descriptor pointers, image pointers, and code-address materialization. Object PCR_S21 relocations need different addend handling for section-local calls versus external symbols.
Exported symbols	Firmware finds names such as Dll_<Name>, Fx_<GID>_<Name>, init/onf/edit handlers, SonicStomp, effectTypeImageInfo, picture, and knob info.

Stock corpus observations:

Observation	Result
Stock files in corpus	830
Header payload 56 bytes	754
Header payload 232 bytes	24
Header payload 312 bytes	52
PT_LOAD segments with memsz > filesz	0
Stock effects with 9 user parameters	114
Largest observed executable PT_LOAD	18,016 bytes
Largest observed .fardata	220 bytes

Loader rule for custom builds: keep memsz == filesz, keep .fardata tiny, and avoid assuming a normal C runtime zero-fill/startup path.

Runtime Model

The audio function receives a ctx pointer in the first C6000 argument register. The useful custom map is:

Field	Meaning
ctx[1]	parameter float table
ctx[2]	stock state/scratch header; small derived blocks are writable
ctx[3]	large descriptor: base, end, span/length
ctx[4]	dry/input buffer
ctx[5]	effect/wet buffer
ctx[6]	output accumulator
ctx[11] / ctx[12]	preserve the stock magic shuttle
ctx[13] / ctx[14]	stock-used, still unresolved

Audio data is float32. The known processing shape is 8 samples per channel per callback: LLLLLLLL RRRRRRRR.

For details on ctx[2], ctx[3], descriptor-size probes, and ToTape9 split results, see STATE-ABI-PROGRESS.md.

Parameter Model

Slot	Meaning
params[0]	on/off as float
params[4]	normalizer, commonly 1 / max
params[5]	user knob 1 raw value
params[6]	user knob 2 raw value
params[7]..params[13]	user knobs 3..9

Parameter-scaling caution: different handler/descriptor combinations can expose different raw ranges. Do not claim source-equivalent control laws until the raw knob scale for that effect’s handler path has been confirmed on hardware.

Airwindows Port Boundary

A build is not a 1:1 Airwindows port until all of these are true:

1. The manifest matches source parameters: names, order, defaults, labels, and control laws.
2. The DSP kernel is the source algorithm, with only documented mechanical changes for the C674x toolchain.
3. Persistent state survives across audio calls like the source plugin instance state.
4. Hardware has tested load, bypass, preset switching, parameter edits, and audio output.

StereoChorus is the best current reference. ToTape9 shows the next boundary: state and helper-light DSP can run, but source-equivalent claims still need parameter lifecycle tests and a desktop comparison harness. VerbTiny and Galactic start the same process for reverb-sized state.

Highest-Priority Open Questions

• Does the current ToTape9 zero-as-unmaterialized fallback and corrected 0..0.14 raw handler scaling stop first-touch Bias/Output and zero-output startup issues? The first edit-calling object-defined init shim crashed on boot; setup-only InitProbe is safe, so the next step is mapping the missing edit-handler init context.
• Do the new VerbTiny and Galactic reverb candidates load and pass audio on the test MS-70CDR, and do synthesized page 2 controls survive interaction?
• Do synthesized LineSel-cloned page 2/3 edit handlers update params[7..13] correctly in an isolated tiny-DSP probe?
• What declares stock-style stereo routing for custom effects?
• Do stock tempo/tap-tempo descriptor flags work for custom effects beyond UI presentation?
• What are the lifecycle rules for ctx state during bypass, preset switch, reload, and duplicate instances?
• What exactly are ctx[11] and ctx[12], and how dangerous is skipping the shuttle in complex effects?
• What are the semantics of extended BCAB and CABI header payloads?

Useful References

• build/ABI.md - low-level linker/runtime ABI reference.
• SAFE-DSP-RULES.md - hardware-safe DSP constraints.
• AIRWINDOWS-EXACT-PORTS.md - exact-port rules.
• AIRWINDOWS-1TO1-PORT-ROADMAP.md - current porting plan.
• TI-PDF-NOTES.md - distilled TI C6000 manual notes.

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