MAME ZIP vs 7z vs folders on Android: choose by evidence
Keep an already verified lawful set in its known-good form unless you have a measured storage or loading problem. MAME searches matching folders, PKZIP archives, then 7-Zip archives; ZIP is often the simpler compatibility and decode-cost baseline, while 7z can save space but LZMA and solid archives can require more decompression work. An unpacked folder is easiest to inspect. Keep CHDs as CHD files in the required short-name folder and never place a supported archive inside another archive.
Do this in order
- 01
Record the exact MAME or core revision, Android frontend, rompath, free storage, and a targeted audit result for one authorized short name.
- 02
Keep a read-only backup of the verified source files and their provenance before changing archive format.
- 03
Confirm the frontend supports the candidate form: matching folder, standard PKZIP, or 7-Zip; reject encrypted, multi-segment, nested, and legacy implode archives.
- 04
Test one representative set in only one form at a time, then re-run the same version's audit rather than trusting archive extraction success.
- 05
Measure launch time, sustained emulation speed, temperature, and storage size on the actual Android device; do not infer speed from compression ratio.
- 06
Keep CHDs outside ZIP/7z in the expected short-name directory, choose the simplest passing form, and retain the last verified copy.
Decision and diagnostic table
| Storage form | Best reason to choose it | Main caution |
|---|---|---|
| Matching folder | Inspection and one-file diagnostics | More filesystem entries and easy accidental edits |
| PKZIP `.zip` | Common MAME baseline with moderate decode cost | Unsupported methods, encryption, and nesting still fail |
| 7-Zip `.7z` | Storage pressure after device measurement | LZMA and solid layout can increase repeated decode work |
| CHD | MAME disk-image format and random access | Keep as CHD in the documented folder; do not recompress into ZIP/7z |
Supported does not mean equally efficient
MAME treats a matching folder, ZIP, and 7z as alternative locations for ROM dumps, but its manual warns that LZMA is more CPU-intensive and solid compression can cause repeated decompression. Android thermal and storage behavior makes measurement more useful than a universal claim.
A smaller archive can be the wrong choice when it delays every launch or adds heat on a constrained device.
Archive structure has hard failure rules
MAME does not load ROM files from an archive inside another archive. It also rejects encrypted and multi-segment archives and the legacy PKZIP implode method.
If a download arrives as a bundle containing other archives, extraction may be a packaging step, but compatibility and rights must still be verified against the exact files and target build.
Format conversion does not repair a set
Repacking changes the container, not the ROM bytes, names, parent relationships, BIOS dependencies, or MAME version contract. Run the target build's audit after conversion and compare the result with the preserved baseline.
CHD is already a compressed random-access format designed for disk images. Putting it inside ZIP or 7z adds an unsupported or wasteful layer and breaks the expected folder layout.
Compression changes neither ownership nor permission. Convert only files you are entitled to possess, keep their rights and provenance record, and do not upload or share a repacked commercial set, BIOS collection, or CHD.