ACBM File

What are ACBM files and how to open them

Are you having problems opening an ACBM 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 ACBM files.

What is an ACBM file?

An .ACBM file is an IFF Amiga Contiguous BitMap file.

The IFF Amiga Contiguous BitMap (extension .ACBM) is a graphics file format from the Amiga era used to store entire bitmaps in a way that's faster for BASIC programs to load. It is a variant of the IFF (Interchange File Format) image type, tailored for screen images on Amiga computers using contiguous bitplane layout rather than the interleaved layout in standard ILBM files.

An .ACBM file is produced by AmigaBasic or related utilities (for example LoadILBM-SaveACBM) when exporting or saving screen images in this contiguous-plane format. After creation, it is typically loaded by programs like LoadACBM, or other software that supports Amiga's IFF "ACBM" FORM. Because the layout stores one full bitplane after another (bitplane 0, then bitplane 1, ..., up to bitplane n), reading or writing in BASIC can be faster, especially when file I/O calls are costly (as was the case in older Amiga setups).

Internally, an .ACBM file begins with the ASCII signature "FORM" at offset 0 and "ACBM" at offset 8. It uses most of the same structure as an ILBM-FORM IFF image, except that the BODY chunk (which in ILBM stores interleaved bitplane data per row) is replaced by an ABIT chunk. The ABIT chunk contains bitplane data stored contiguously, in sequence for bitplane 0 through bitplane n.

How to open ACBM files

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

While we have not verified the app ourselves yet, our users have suggested a single ACBM opener which you will find listed below.

Last updated: August 16, 2026

Various apps that use files with this extension

These apps are known to open certain types of ACBM files. Remember, different programs may use ACBM 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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