CROMFS File

What are CROMFS files and how to open them

Are you having problems opening a CROMFS file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your CROMFS files.

What is a CROMFS file?

A .CROMFS file is a Compressed ROM FileSystem Image file.

Compressed ROM FileSystem Image files store a read-only Linux filesystem archive optimized for high compression and random access. They are produced by the "Cromfs" tool, which builds a filesystem image that contains data, directory hierarchy, and metadata fully compressed. This format is intended for long-term offline storage or ROM‐based systems where files will not be modified but must still be accessed quickly and by arbitrary order.

The image is created once using the mkcromfs program and then mounted or accessed in user space via the cromfs-driver. Once produced, the archive cannot be changed; new files cannot be added, and write operations are not supported. Internally, most data (file contents, directories, inodes, block index tables) is compressed using the LZMA algorithm. Files are broken into fragments, and many fragments from different files may share storage blocks (so duplicate content is stored only once), helping reduce size.

Technical structure begins with a superblock identifying the format version and locations of key areas (root directory inode, inode table, block lists, etc.). Data is stored in "fblocks" (solid compressed blocks), with a table of "blocks" that reference fblocks and offsets into them for each file fragment. The inode records contain metadata (mode, timestamps, uid/gid, size, etc.) and data locators pointing to the block list. One signature string appears at the start of the file: ASCII "CROMFS" (6 bytes) at offset 0.

How to open CROMFS files

Important: Different programs may use files with the CROMFS file extension for different purposes, so unless you are sure which format your CROMFS file is, you may need to try a few different programs.

We have not yet verified any programs for Windows that work with this specific file format. If you know one, please use the 'Suggest a program' link below. Thanks!

Last updated: September 18, 2026

Help us help others

File.org helps thousands of users every day, and we would love to hear from you if you have additional information about CROMFS file formats, example files, or compatible programs. Please use the links below or email us at submit@file.org to discuss further.