JPK File

What are JPK files and how to open them

Are you having problems opening a JPK 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 JPK files.

What is a JPK file?

JPK files have multiple uses, and JAM Packer Compressed Disk Image is one of them. Read more about the other uses further down the page.

JAM Packer Compressed Disk Image

A .JPK file labeled "JAM Packer Compressed Disk Image" is a compressed floppy disk image format used on the Atari ST computer. It was created by Andrew Knipe and Tim Knipe as part of the JAM Packer system designed to compact entire disk contents using custom algorithms. The format serves to reduce storage space while preserving everything on the disk so the image can later be decompressed and written back or emulated.

Files in this format are generated by the JAM Packer compressor. After compression, they can be decompressed by compatible tools to restore the original disk image, which is used with Atari ST emulators or written back to floppy disks via imaging hardware. ACDSee is known among software that can open this type of .JPK file. Use of this file typically involves preservation, archiving or sharing of full Atari ST disks in a compressed state.

Technically, a JAM Packer Compressed Disk Image begins with the ASCII signature "JPK!" at byte offset 0x0 (hex), corresponding to the first four bytes being 0x4A 0x50 0x4B 0x21. That header identifies the file as belonging to this format.

How to open JPK files

We have identified one JPK opener that is compatible with this specific type of JPK file.

Programs that open JAM Packer Compressed Disk Image files

Last updated: August 7, 2026

All known file formats using extension .JPK

While JAM Packer Compressed Disk Image is a popular type of JPK-file, we know of 2 different uses of the .JPK file extension. Different software may use files with the same extension for different types of data.

Jeepers Key/index

We know that one JPK format is Jeepers Key/index. We have not yet analyzed in detail what these files contain and what they are used for. We're working on it.

While we have not verified the apps ourselves yet, our users have suggested three different JPK openers which you will find listed below.

Various apps that use files with this extension

These apps are known to open certain types of JPK files. Remember, different programs may use JPK files for different purposes, so you may need to try out a few of them to be able to open your specific file.

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