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# Storage

## How DOS uses drives

DOS identifies storage devices by drive letters, not by names or mount points. Each drive letter (A: through Z:) maps to exactly one storage device or partition. Unlike modern operating systems, there is no single unified file tree — each drive is its own independent root.

The letter assignments follow a fixed convention:

- A: and B: are reserved for floppy disk drives. If only one physical floppy drive is present, B: acts as a “phantom” mapped to the same drive — DOS prompts you to swap disks when you access it.

- C: is the first hard disk partition. A single physical hard disk can be divided into multiple partitions, each getting its own letter (C:, D:, E:, etc.).

- The first available letter after the hard disk partitions is typically assigned to the CD-ROM drive. On a system with a single hard disk partition, the CD-ROM is usually D:; with two partitions, it becomes E:, and so on.

In DOSBox Staging, you assign drive letters yourself when you mount directories or disk images, so you have full control over the layout. The conventions above are worth following because many games assume them — especially C: for the hard disk and D: for the CD-ROM.

## DOS storage media

If you grew up with modern computers where everything lives on a single internal SSD, the DOS storage landscape might seem alien. DOS PCs used three types of storage media, each with its own drive letter.

### Floppy disks

Floppy disks (drives A: and B:) were the primary way software was distributed. Games came on one or more floppies and usually had to be installed to the hard disk before you could play — unlike a console cartridge, you couldn’t just insert a floppy and go. Two form factors existed: the flexible 5.25-inch disk (1981–late 1980s) and the sturdier 3.5-inch disk (late 1980s–mid 1990s). See Floppy disk formats for the full list of supported sizes.

### Hard disks

Hard disks (drive C: and up) held the operating system and installed games. Capacities ranged from 10–20 MB in 1985 to over 1 GB by the mid-1990s. The DOS FAT16 filesystem limits individual partitions to 2 GB — larger disks needed multiple partitions.

### CD-ROMs

CD-ROMS (typically drive D:) appeared in the early 1990s and became mainstream by 1994–1995. A single CD held 650–700 MB — roughly 500 floppy disks’ worth. This enabled full-motion video, voice acting, and CD-quality audio soundtracks. Some games ran entirely from the CD; others installed partially to the hard disk.

## DOSBox Staging drives

DOSBox Staging provides two special drives in addition to whatever you mount:

- Z: contains the built-in programs and commands (MOUNT, CONFIG, MIXER, etc.). It is always present and always in the PATH. See DOS commands & programs for the full list.

- Y: contains a small set of bundled third-party utilities (DEBUG, DELTREE, XCOPY). It is automounted by default with its `Y:\DOS` directory added to the PATH.

## Automounting

The easiest way to set up drives is automounting. Place your game files in a `drives/` directory structure, and DOSBox mounts them automatically on startup. The Getting Started guide demonstrates this approach step by step.

### Directory structure

Create lowercase single-letter subdirectories under `drives/` to mount as DOS drives:

    drives/
      c/          -> mounted as C:
      d/          -> mounted as D:

The `drives/` folder is looked up relative to the current working directory or from the built-in resources directory.

### Floppy and CD-ROM images

Automounting also supports disk images for floppy and CD-ROM drives:

- Place IMG or IMA floppy images in `drives/a/` or `drives/b/` to mount them as floppy drives.
- Place ISO, CUE/BIN, or MDS/MDF CD-ROM images in any drive directory (e.g., `drives/d/`) to mount them as CD-ROM drives.

When image files are detected, DOSBox automatically uses the appropriate mount type.

### Drive configuration files

Each drive directory can have an accompanying configuration file named `[letter].conf` (e.g., `c.conf`) placed alongside the directory. Example:

    [drive]
    type     = dir
    label    = MYGAME
    readonly = off

Available options:

Option | Values | Description
--- | --- | ---
`type` | `dir`, `overlay`, `floppy`, `cdrom`, `iso` | Drive type. `iso` is an alias for `cdrom`.
`label` | any string | Custom volume label.
`path` | PATH specification | Extend the DOS PATH (e.g., `path = %path%;c:\tools`).
`override_drive` | single letter (a–y) | Mount to a different drive letter.
`readonly` | `on`, `off` | Mount as read-only.
`verbose` | `on`, `off` | Show mount command output during startup.

## Manual mounting

For more control, use the `MOUNT` command directly, either at the DOS prompt or in the `[autoexec]` section of your config file. Type `MOUNT /?` at the DOS prompt for the full reference.

Note

`IMGMOUNT` is deprecated. Use `MOUNT` for both directories and disk images.

### Mounting directories

    mount C /path/to/game/files

This mounts a host directory as a DOS hard disk drive. This is the most common way to make game files accessible to DOS.

By default, directory-mounted hard disks report approximately 250 MB of free space. Some games check for free disk space and refuse to install if there isn’t enough. Use the `-freesize` flag to adjust the reported free space (in MB for hard disks, KB for floppies):

    mount C /path/to/game -freesize 1024

#### Volume labels

Some games — particularly those with copy protection — check the volume label of the drive. Use the `-label` flag to set it explicitly:

    mount A /path/to/floppy -t floppy -label DISK1

This is most commonly needed for floppy-based games that verify the disk label during copy protection checks.

#### Overlay mounts

An overlay mount adds a write layer on top of an existing drive. Modified files are stored in the overlay directory on the host, leaving the original drive data unchanged. This is useful for keeping a clean copy of game files while still allowing the game to save data:

    mount C /path/to/game
    mount C /path/to/saves -t overlay

### Mounting floppy images

    mount A floppy.img -t floppy

Supported floppy image formats: `.img`, `.ima` (raw sector images).

#### Floppy disk formats

The 5.25-inch disk was a flexible magnetic disk visible through a slot in its cardboard sleeve. The 3.5-inch disk housed the magnetic medium in a rigid plastic shell with a spring-loaded metal shutter. Capacities grew as recording density improved: DD (Double Density), HD (High Density), and ED (Extra Density) describe the magnetic coating and recording method. Some software distributors squeezed extra capacity using non-standard formats like Microsoft’s DMF (Distribution Media Format) at 1.68 MB or IBM’s XDF at 1.84 MB.

DOSBox Staging supports all standard PC floppy formats and several extended formats. The format is detected automatically from the image file size.

Size | Type | Capacity | Notes
--- | --- | --- | ---
5.25” | SS/DD | 160 KB | Original IBM PC (1981)
5.25” | SS/DD | 180 KB | 9 sectors/track variant
5.25” | DS/DD | 320 KB | Double-sided (1982)
5.25” | DS/DD | 360 KB | Standard 5.25” DD
5.25” | DS/HD | 1.2 MB | Standard 5.25” HD
3.5” | DS/DD | 720 KB | Standard 3.5” DD
3.5” | DS/HD | 1.44 MB | Standard 3.5” HD
3.5” | DS/HD | 1.68 MB | Microsoft DMF
3.5” | DS/HD | 1.72 MB | Microsoft DMF (82-track variant)
3.5” | DS/ED | 2.88 MB | Extra Density (rare)

### Mounting CD-ROM images

    mount D game.cue -t iso
    mount D game.iso -t iso
    mount D game.mds -t iso

Supported CD-ROM image formats:

- CUE/BIN — CUE sheet with data and optional audio tracks.
- ISO — Standard ISO 9660 images (data only).
- MDS/MDF — Alcohol 120% disc images.

For games with CD-DA music, use CUE/BIN or MDS/MDF format images with audio tracks. See CD-DA audio for details on supported audio track formats.

### Mounting multiple images

You can mount multiple images at the same drive letter:

    mount A disk1.img disk2.img disk3.img -t floppy

Wildcard patterns are also supported:

    mount D cd/*.cue -t iso

Press the `Ctrl`+`F4` / `Cmd`+`F4` hotkeys to cycle between the mounted images during gameplay. This is essential for multi-disk games that prompt you to insert the next disk.

### Mounting hard disk images

    mount C hdd.img -t hdd

For images that require explicit geometry:

    mount C hdd.img -t hdd -chs 304,64,63

FAT12, FAT16, and FAT32 filesystems are supported.

### Read-only mounts

Use the `-ro` flag to mount a drive as read-only:

    mount D /path/to/shared -ro

### Booting from images

For bootable disk images (real MS-DOS, Windows 9x, etc.), mount with `-fs none` using a drive number instead of a letter:

    mount 0 boot.img -t floppy -fs none
    boot -l a

    mount 2 win95.img -t hdd -fs none -chs 304,64,63
    boot -l c

Drive numbers 0 and 1 correspond to floppy drives A: and B:; 2 and 3 correspond to hard disks C: and D:.

Booter games

Some early games (roughly 1981–1986) bypassed DOS entirely and booted directly from floppy disk. These “booter” games took over the whole machine, using non-standard disk formats as a form of copy protection and to squeeze out every last byte of available memory. They need the `boot` command shown above rather than `mount`.

Notable booter games

- Alley Cat (1984)
- Archon (1984)
- BC’s Quest for Tires (1983)
- Bouncing Babies (1984)
- Dig Dug (1983)
- Flight Simulator 2.0 (1984)
- Hitchhiker’s Guide to the Galaxy (1984)
- Karateka (1986)
- King’s Quest (1984)
- Lode Runner (1983)
- M.U.L.E. (1983)
- Pac-Man (1983)
- Paratrooper (1982)
- Zork I (1982)

### DOS filesystem limitations

DOS filenames follow the 8.3 convention: a maximum of 8 characters for the name and 3 for the extension (e.g., `DUKE3D.EXE`). Names are case-insensitive, cannot contain spaces, and only a limited set of special characters is allowed. When mounting host directories, DOSBox Staging automatically truncates long filenames to 8.3 format for DOS programs.

The FAT16 filesystem used by DOS limits individual partitions to a maximum of 2 GB. Larger hard disk images must be split into multiple partitions. DOSBox Staging also supports FAT12 (used on floppy disks) and FAT32 (used by Windows 95/98, with a theoretical 2 TB limit).

### Unmounting drives

To unmount a previously mounted drive:

    mount -u D

## Configuration settings

Storage-related settings are in the `[dosbox]` section:

- `automount`
- `hard_disk_speed`
- `floppy_disk_speed`

For emulated drive sounds (floppy chatter, hard disk clicking), see Disk noise.