DOSBox-X adds TeleDisk floppy support in its September update

Jon Campbell and the DOSBox-X team have released version 2026.08.31 of their open-source emulator, bringing native support for TeleDisk floppy images and a host of low-level disk I/O improvements. The update is out today for Windows, macOS, and Linux.

For those who usually stick to standard DOSBox to fire up Doom and call it a day, DOSBox-X is a different beast entirely. Branched off from the main project in 2011, it targets strict hardware accuracy over pure gaming convenience. It handles Windows 3.x and 9x environments, NEC PC-98 architecture, 3Dfx Voodoo acceleration, and complex MIDI setups.

TeleDisk files finally mount natively

The headline addition for archivists is support for mounting TeleDisk (.td0) floppy image files. TeleDisk was a popular utility in the 1990s for creating exact sector-by-sector copies of floppy disks. It was frequently used to back up copy-protected software or disks with non-standard formatting that standard DOS copy commands would choke on.

The emulator also relaxes its sanity checks for floppy images to better accommodate these specially formatted disks.

INT 13h gets specific about disk failures

The developers made major changes to how the emulator handles low-level disk reads and writes. Previously, if a disk sector I/O operation failed, DOSBox-X would throw a catch-all 0x04 error code. Now, it returns typed INT 13h status codes. The INT 13h handler propagates the exact specific code (whether that is a seek failure, a controller failure, or a CRC data error) into register AH.

This matters because a lot of vintage software, particularly copy protection routines and diagnostic tools, expects the hardware to report exactly why a disk read failed. The update also fixes a bug where the AH register was left stale on write errors.

Floppy disk geometry also received a correction. The emulator's INT 13h CHS (cylinder-head-sector) decoding for floppies was accidentally using the hard disk layout, which relies on a 10-bit cylinder and 6-bit sector structure. Floppies use different geometry, and fixing this ensures obscure disk formats read correctly.

Memory layout and FreeDOS tweaks

Under the hood, the shell initialization process is now more accurate to real MS-DOS behavior. The emulator allocates the COMMAND.COM environment block after allocating the shell binary in both SHELL_Init and CONFIGSHELL_Init. This ensures the memory layout and ownership ordering match expected MCB-chain behavior, which keeps memory-resident programs (TSRs) happy.

The update cleans up several smaller bugs across the board. FreeDOS XCOPY will no longer fail to copy files when a destination directory is not specified. The developers also fixed a freeze that occurred when a savestate flagged a file as not found, along with correcting capture and savestate directory locations across different operating systems. It works, which is the nicest thing I can say about cross-platform savestate compatibility.

The project's source code also saw some housekeeping. The imageDisk subclasses (VFD, D88, NFD, El Torito floppy, and others) were refactored into their own source files, leaving just the base class in the bios_disk file. It is a maintenance release, but one that makes the emulator much more robust for handling weird edge cases in legacy software.