What is the ACC file format?
.acc files are GEM Desktop Accessories - small resident utility programs loaded automatically at startup by the GEM graphical environment. GEM (Graphics Environment Manager) was developed by Digital Research Inc. in 1985 and ran on IBM PC compatibles under MS-DOS/DR-DOS as well as on Atari ST computers running TOS/GEM. On the Atari ST, any .acc file placed in the root directory of the boot drive is scanned at startup and listed under the Accessories menu, giving users instant access to small utilities such as calculators, clocks, calendars, and mini text editors without leaving the desktop.
Technically, a .acc file is a standard GEMDOS executable in the same binary format as .PRG programs and .tos auto-run files, distinguished only by its extension. The file begins with the GEMDOS magic word 0x601A (a Motorola 68000 branch instruction) followed by segment size fields, a relocation table, and an optional symbol table. The Atari TOS loader uses the .acc extension to classify the binary as an accessory rather than a standalone application.
The format is a legacy artifact: GEM on the PC was discontinued in the early 1990s, and Atari ST hardware production ended in 1994. Some .acc files were packed with compression utilities such as Paccer or LHASFX. The format remains accessible today through Atari ST emulators such as Hatari or via OpenGEM running under DOSBox.
Note: the .acc extension is also used by some accounting software (such as FKJ Software Graphic Accounts) for data files - an entirely unrelated usage.
Security & safety
RISK: MEDIUMGEM .acc files are executables (machine code). In their native GEM/Atari ST environment they execute directly. Running in emulators (Hatari, DOSBox+OpenGEM) is sandboxed. Do not attempt to run .acc files directly on modern Windows or macOS; they will not execute and are harmless, but file disguising is always a risk. FKJ Graphic Accounts .acc data files are non-executable and safe.
Format details
in a nutshell- FKJ Software Graphic Accounts data file - Personal accounting software (UK, Windows) uses .acc for its financial data files; entirely unrelated to GEM executables.
- Altamira Composer image file - Altamira Composer (1990s Windows graphics app, now defunct) used .acc for image project files.
Programs that open ACC files
Windows3 apps
FKJ Software Graphic Accounts
Paid Opens .acc accounting data files created by Graphic Accounts. Irrelevant to GEM accessory .acc files.
Hatari (Atari ST emulator)
Open-source Emulate an Atari ST system; copy .acc to the emulated C:/ root; GEM will auto-load it in the Accessories menu.
DOSBox + OpenGEM
Open-source Run OpenGEM (GEM for DOS) inside DOSBox; place .acc in the GEM root directory to load as an accessory. 
macOS1 app
Linux1 app
Hatari (Atari ST emulator)
Open-source Available via apt/dnf; emulate Atari ST and load .acc in GEM Accessories. Technical details
deep spec| File signature | 0x60 0x1A at offset 0 - GEMDOS executable header (Motorola 68000 branch instruction to entry point) |
| Processor architecture | Motorola 68000 (big-endian) |
| Byte order | Big-endian |
| Encoding | Binary |
| Binary format | GEMDOS executable - same binary layout as .PRG (standalone app) and .TOS (auto-run); distinguished by .acc extension only |
| Compression | None in standard form; some files compressed post-link with Paccer or LHASFX |
| Typical file size | 2 KB - 200 KB |
| Load mechanism | Auto-loaded from root of boot drive at GEM startup; each .acc appears as an entry in the Accessories menu |
| Executable segments | Text (code), data, BSS; relocation table; optional symbol table |
| Platform | Atari ST/STE/TT/Falcon (TOS/GEM); DR-DOS + GEM Desktop on PC; OpenGEM (DOSBox) |
| Accessories limit | TOS 1.x supports up to 6 accessories loaded simultaneously (memory-dependent) |
| Released | 1985 (GEM 1.0, Digital Research) |
| Latest version | N/A (legacy format; GEM discontinued) |
ACC conversions
Community Q&A
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