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-automation, 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-automation drives

dosbox-automation 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-automation mounts them automatically on startup. The Tutorials 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 multiple images are placed in the same directory, they are all mounted and you can switch between them during gameplay - see Disk swapping below.

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:

Disk swapping

Many DOS games shipped on multiple floppy disks or CDs. During installation or gameplay, the game would prompt you to remove one disk and insert the next (e.g., "Please insert Disk 2"). On a real PC you would physically eject the disk and slide in the next one - in dosbox-automation, you press Ctrl+F4 / Cmd+F4 instead. The currently active image is printed to the log after each swap.

Swapping with automounting

Place multiple disk images in the same drive directory and dosbox-automation mounts them all automatically. For example, for a three-disk floppy game:

drives/
  a/
    disk1.img
    disk2.img
    disk3.img

When the game prompts you to insert the next disk, press Ctrl+F4 / Cmd+F4 to cycle to the next image.

The same approach works for multi-CD games - place all your CD images in drives/d/ and swap between them with the hotkey.

Swapping with the MOUNT command

You can also mount multiple images to the same drive letter manually:

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

Wildcard patterns are supported too:

mount D cd/*.cue

Press Ctrl+F4 / Cmd+F4 to cycle between them during gameplay.

Swapping booted disk images

The boot command also accepts multiple floppy images:

boot disk1.ima disk2.ima disk3.ima

The same Ctrl+F4 / Cmd+F4 hotkey swaps between them. See Booting from images for more on the boot command.

Using multiple CD-ROM drives

Some games supported multiple physical CD-ROM drives instead of swapping. Rather than mounting all CDs to one drive letter, you can mount each CD to its own consecutive drive letter:

mount D cd1.cue
mount E cd2.cue
mount F cd3.cue
mount G cd4.cue

This also works with automounting - place each CD image in its own drive directory:

drives/
  d/
    cd1.cue
  e/
    cd2.cue
  f/
    cd3.cue
  g/
    cd4.cue

This approach is needed for games that expect multiple CD drives to be available simultaneously, such as Under a Killing Moon (1994) which shipped on four CDs and could run with all four drives mounted at once.

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.

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.

mount C /path/to/game -freesize 1024

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

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

Use -t floppy if auto-detection picks the wrong type (see below).

Supported floppy image formats: .img, .ima, .dsk (raw sector images), .vfd (Virtual Floppy Disk), .flp (raw floppy image), and numeric extensions matching standard floppy sizes: .360, .720, .1200, .1440.

Files with .vfd, .flp, or numeric extensions are always auto-detected as floppy images, so -t floppy is not needed. For .img, .ima, and .dsk files, dosbox-automation checks the file size against known floppy disk geometries (see Floppy disk formats below) --- matching images are mounted as floppy drives; all others as hard disk 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-automation supports all standard PC floppy formats and several extended formats. The format is detected automatically from the image file size.

¹ Non-standard formats using 10 sectors per track instead of the usual 8 or 9. These cannot be created or read by standard MS-DOS and were used exclusively by booter games, notably Electronic Arts titles such as Seven Cities of Gold (1984) and Marble Madness (1984), as a form of copy protection.

² IBM's eXtended Density Format, used by IBM for distributing OS/2 and PC DOS 7. Uses variable sector sizes to squeeze extra capacity out of standard HD disks.

³ Microsoft's Distribution Media Format, used for distributing Windows and Office on fewer floppies. Uses 21 sectors per track. The 82-track variant has slightly higher capacity at the expense of compatibility.

⁴ Introduced in 1990 on the NeXTcube and IBM PS/2. Required special drives and disks that were significantly more expensive than standard HD hardware, so the format never saw widespread adoption.

Mounting CD-ROM images

mount D game.cue
mount D game.iso
mount D game.mds

The image type is auto-detected from the file extension (.cue, .iso, .bin, .mds), so -t iso is not needed. 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. See Disk swapping for the full details and examples.

Mounting hard disk images

mount C hdd.img

The image type is auto-detected from the file extension (.img, .ima, .dsk, .vhd), so -t hdd is not needed. For images that require explicit geometry:

mount C hdd.img -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:. The boot command also supports multiple floppy images for disk swapping.

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-automation 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-automation 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:

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