SPARK / Acorn ARC file format

.arc · .spk · archive+compression · introduced 1989 by David Pilling

What is a SPARK / Acorn ARC file?

SPARK is the native archive format of Acorn's RISC OS, related to but distinct from the SEA ARC format. Files are stored with the trailing-byte type metadata that RISC OS uses instead of file extensions, which is why SPARK archives are still the only practical way to ship RISC OS software with proper file types intact.

Most readers reach this page after double-clicking a .arc file and finding their operating system unsure what to do with it. The good news: SPARK / Acorn ARC is a well-understood format with mature tooling on every major platform. It is supported in one click by mainstream archivers on Windows, macOS, and Linux.

How to extract a SPARK / Acorn ARC file

Opening a .arc file is straightforward on every modern operating system. The exact steps depend on which platform you are on:

On Windows

Locate the .arc file in File Explorer.
Right-click the file. If you have 7-Zip installed, choose 7-Zip → Extract Here. On Windows 11 the built-in extractor handles ZIP, 7z, RAR and several other formats directly through Extract All…
If prompted, pick a destination folder and click Extract.
The extracted files appear in a folder next to the original archive.

On macOS

Find the .arc file in Finder.
Install The Unarchiver from the Mac App Store, or Keka for both creating and extracting.
Right-click the file → Open With → The Unarchiver (or just double-click after Unarchiver is set as the default).
The extracted folder appears next to the archive.

On Linux

Use your file manager: right-click the archive → Extract Here (file-roller on GNOME, Ark on KDE).

On the command line, run one of:

7z x archive.arc    # works for almost any format

The files appear in the current directory.

How to create a SPARK / Acorn ARC file

To pack files into a .arc archive, you typically need an archiver that supports writing this format. Not every tool can create every format — for example, only WinRAR can create real RAR files. The sidebar lists the software known to read and write SPARK / Acorn ARC.

On the command line, the canonical create command is:

# Use 7-Zip, PeaZip, or the format's reference tool to create .arc files.

If you regularly create archives at scale, a dedicated archive automation suite can wrap this command in scheduled jobs, integrity checks, and offsite replication.

Strengths

Preserves RISC OS file types
Native to the platform

Weaknesses and limitations

RISC OS-specific use case
Tiny user base

Typical use cases

RISC OS software distribution
Acorn community downloads

Technical details

SPARK / Acorn ARC uses the Various (LZW, RLE, etc.) algorithm and is identified on disk by the magic byte sequence 1A. The standard MIME type is application/x-arc-spark. The format is an open specification and is subject to license restrictions for some uses (notably creation).

Frequently asked questions

Is SPARK / Acorn ARC safe to open?

Archive files are containers — they are only as safe as what they contain. A .arc from a trusted source is fine; one received from a stranger can hold malicious executables, scripts, or files engineered to exploit bugs in your archiver. Always keep your archiver updated, and never run unknown executables that come out of an archive. A safe workflow is to extract into a sandbox and scan the contents before opening anything.

What is the maximum file size?

Modern implementations support archives in the multi-terabyte range. The practical limit is your filesystem (FAT32 caps at 4 GB; ext4, NTFS, APFS, and exFAT all comfortably handle multi-TB files).

Can I password-protect a SPARK / Acorn ARC?

Some archivers can wrap SPARK / Acorn ARC files in an encrypted container, but the format itself does not always include built-in encryption. Check your archiver's documentation.

How does SPARK / Acorn ARC compare to other formats?

See our format comparison pages such as ZIP vs 7z, ZIP vs RAR, and tar.gz vs tar.xz for side-by-side feature tables.