What is the DSK file format?
A .dsk file contains a disk image. .dsk is a generic file extension - it is not native to any single program and is not standardized across the industry. The extension most commonly stores images of floppy disks from the vintage home-computer era, including Apple II, Amstrad CPC, and ZX Spectrum disks, though it is also applied to images of hard disks and other storage media.
Because .dsk is an umbrella extension, the internal format varies by origin. The most widely recognized structured variant is the Amstrad CPC/ZX Spectrum format, which begins with the ASCII header MV - CPCEMU or EXTENDED CPC DSK File at offset 0. A plain raw image - such as an Apple II DOS 3.3 floppy - carries no header at all; it is simply the disk's sectors laid end-to-end. A .dsk image can be one of the following types:
- Raw - a 1:1, byte-by-byte sector copy of the disk
- Logical - a copy of disk contents as seen by the file system (logical volume)
- Partition - a copy of only part of a given disk, such as user data excluding system areas
Disk images are typically created to obtain a data backup copy, which can then be used to restore data. Raw images are also used in digital forensics to preserve evidence. Because .dsk is a generic image format, a file created in one program may not be compatible with another, even when both support the .dsk extension. Using appropriate tools, .dsk images can be mounted as virtual disks and browsed as if they were physical storage. Related formats include .IMG, .ISO, .VHD, and .NIB (Apple II nibble image).
Security & safety
RISK: MEDIUMThe .dsk container itself is passive sector data, but the disk it images can hold executable software - a retro game image is usually harmless, but a PC floppy image can contain old boot-sector viruses or DOS malware that some antivirus still flags. Risks are mostly about source and obsolescence: download retro .dsk images from reputable preservation archives, scan extracted files before running, and remember that a damaged or non-standard .dsk may simply not open in the wrong tool (it's not corrupt - it's the wrong variant for that program).
Format details
in a nutshellPrograms that open DSK files
Windows5 apps
WinImage
Paid Open a raw PC floppy .dsk to browse, extract, or inject files; can also write the image back to a physical disk. 
7-Zip
Open-source Try opening a FAT-formatted .dsk as an archive to list/extract files without mounting. 
IsoBuster
Freemium Open the .dsk to inspect its file system and extract files; good for recovering data from old images. 
AppleWin
Open-source For Apple II .dsk images: load the .dsk into Drive 1 and boot the emulated Apple II.
CiderPress II
Open-source Modern Apple II disk-image tool: open Apple .dsk/.po/.do/.2mg to browse and extract files. 
macOS2 apps
Apple Disk Utility
Built-in Can mount/convert some raw .dsk images (Try File > Open Disk Image); for emulator-specific .dsk use the matching emulator instead.
OpenEmulator / Virtual ][
Open-source For Apple II/retro .dsk images on macOS: load the disk image into the emulated drive. 
Linux1 app
dskutil / cpctools / Linux loop mount
Open-source Raw FAT .dsk: sudo mount -o loop file.dsk /mnt. CPC/Spectrum images: use cpctools/dsktools. Apple II: AppleCommander. Technical details
deep spec| Format category | Binary sector-by-sector disk image - an umbrella extension with no single internal standard |
| Magic bytes | None for raw images; Amstrad CPC/Spectrum variant begins with ASCII `MV - CPCEMU` or `EXTENDED CPC DSK File` at offset 0 |
| Header structure | Absent in raw mode (sectors begin at byte 0); Amstrad CPC variant opens with a 256-byte disk information block followed by 256-byte track information blocks |
| Typical raw file size | 143 KB (Apple II 140 KB disk), 720 KB (3.5-inch DD floppy), 1.44 MB (3.5-inch HD floppy) - determined entirely by source disk geometry |
| Compression | Not built into the format; raw `.dsk` images are uncompressed binary copies of disk sectors with no inherent encoding |
| MIME type | application/octet-stream |
| Apple II raw variant geometry | 35 tracks × 16 sectors × 256 bytes each (DOS 3.3, 143,360 bytes); sector interleave order matches the original Disk II controller |
| Amstrad CPC track layout | Up to 80 tracks per side, single- or double-sided; standard geometry uses 9 sectors × 512 bytes per track; extended format supports variable sector counts per track |
| MS-DOS floppy geometry | 720 KB images: 80 tracks, 2 sides, 9 sectors × 512 bytes; 1.44 MB images use 18 sectors per track at the same track and side count |
| Identification method | No universal magic number; variant is determined by file size, known geometry table, and the emulator or imaging tool that produced it |
| Side count | Single-sided (SS) or double-sided (DS) depending on the source disk; most emulator-distributed images are single-sided 35- or 40-track floppies |
| Mounting on Linux | Raw `.dsk` images can be mounted as loopback block devices using `mount -o loop disk.dsk /mnt` when the filesystem inside is recognized |
| Primary modern use | Software preservation and vintage-computer emulation - the extension is the standard interchange format for Apple II, Amstrad CPC, ZX Spectrum, and CP/M emulators |
| Extension name conflict | The `.dsk` extension is also claimed by Turbo Pascal compressed disk sets and several proprietary backup utilities, causing format ambiguity when no header is present |
| Encoding of sector data | Sectors are stored in physical or logical order as raw bytes; no checksum or error-correction data from the original disk controller is preserved in plain raw images |
| Released | Used since the 1980s home-computer era; no single standard |
| Specification | en.wikipedia.org |
DSK conversions
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