LinuxWhat is the ST file format?
An .st file is a raw Atari ST floppy disk image - a sector-for-sector binary copy of an Atari ST floppy disk, stored without compression or metadata. The format originated with the PacifiST emulator (circa 1994) and is now supported by virtually every Atari ST emulator.
The file is a flat binary dump of all floppy sectors in sequential order, mirroring the Atari ST FAT12 filesystem as it existed on physical media. Because there is no file header, the format is identified entirely by file size: 368,640 bytes (single-sided double-density), 737,280 bytes (double-sided double-density), or 1,474,560 bytes (high-density). Byte ordering is big-endian, reflecting the Motorola 68000 CPU of the Atari ST.
The simplicity of .st is also its main limitation: it stores no timing or track-layout metadata, so it cannot represent copy-protected disks. The .STX format (PASTI) handles copy-protected originals, while .MSA (Magic Shadow Archiver) stores equivalent data with compression.
Emulators that support .st natively include Hatari, Steem SSE, and STonX.
Security & safety
RISK: LOWA .st file is a passive disk image with no executable code outside the emulator. Risk exists only if a malicious Atari ST program is inside the image - the emulator sandbox prevents host OS compromise. Download images from reputable archives (Atarimania.com, folk.ntnu.no).
Format details
in a nutshell- Smalltalk Source Code File - Some Smalltalk environments (Squeak, Pharo predecessors) use .st for source code text files.
- GraphicConverter / Stuff It Archive (rare) - Historical macOS associations; .st was sometimes used for StuffIt archives in very early Mac software, now obsolete.
Programs that open ST files
Windows2 apps
Hatari
Open-source Launch Hatari, go to Floppy Disks → Drive A, browse to the .st file and select it. The Atari ST will see it as an inserted floppy.
Steem SSE
Open-source Open Steem SSE, click the floppy icon for Drive A and select the .st file. Latest: v4.2.0 (July 2025), R8 (Jan 2026). 
macOS1 app
Hatari
Open-source Use Homebrew ('brew install hatari') or download the macOS build from the Hatari site; mount via Floppy Disks menu. Technical details
deep spec| Internal format | Raw sector dump - no container, no header, no metadata |
| Byte order | Big-endian (Motorola 68000 CPU architecture) |
| Magic bytes | None - raw sector data starts at byte 0; format identified by file size alone |
| Filesystem | Atari ST FAT12 |
| Size - single-sided DD | 368,640 bytes (80 tracks × 9 sectors × 512 B × 1 side) |
| Size - double-sided DD | 737,280 bytes (80 tracks × 9 sectors × 512 B × 2 sides) |
| Size - double-sided HD | 1,474,560 bytes |
| Sector layout | Sequential: track 0 side 0, track 0 side 1, track 1 side 0 - interleaved sides in track order |
| Max root directory entries | 112 (typical FAT12 DD floppy) |
| Copy-protection support | None - cannot represent protected disks; STX (PASTI) format required for those |
| Compressed equivalent | MSA (Magic Shadow Archiver) stores identical sector data with compression |
| Emulator support | Supported natively by virtually all Atari ST emulators: Hatari, Steem SSE, STonX |
| Released | ~1994 (with PacifiST emulator for MS-DOS) |
| Latest version | No versioning - raw dump format |
| Open standard | Yes · royalty-free |
| Specification | www.atari-forum.com |
ST conversions
Community Q&A
asked by usersNo questions yet - be the first to ask about ST files.
