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A disk operating system for MSX computers, forked from MSX-DOS 2.31
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Konamiman v2.1 beta 2
- Nextor will now try to load MSXDOS2.SYS if NEXTOR.SYS is not found in the boot drive.

- The method for selecting partitions for automatic mapping has changed from requiring a NEXTOR.DAT file in the root directory to having the "active" flag set in the partition table.

- Now the first 9 partitions of a device will be scanned during the automatic mapping procedure, this includes extended partitions.

- FDISK allows to change the "active" flag of new and existing partitions.

- FDISK now always creates extended partitions, even if 4 or less partitions are defined.

- FDISK now creates FAT16 partitions with a partition type code of 14 (FAT16 LBA) instead of 6 (FAT16 CHS).

- The numeric keyboard can now be used both when booting and when changing disks in disk emulation mode.

- Russian keyboard is now properly recognized (numeric keys only).

- Introduced the boot key inverters.

- Introduced the one-time boot keys.

- Introduced the NEXBOOT.COM tool to set the RAM based one-time boot keys.

- Introduced the RAM based one-time disk emulation mode.

- The method to enter the old disk emulation mode (now named "persistent") has changed from requiring a NEXT_DSK.DAT file in the root directory to storing a pointer to the emulation data file in the partition table of the device.

- Pressing the 0 key at boot time will delete the pointer to the emulation data file in the partition table, thus permanently disabling the emulation mode - no need to manually do anything else.

- When Nextor is waiting for a disk key press after having pressed GRAPH in disk emulation mode, now you can press GRAPH again to cancel the disk change.

- The first Nextor kernel to boot now clears the screen before invoking the driver initialization.

- ARG is no longer used as temporary work area by the Nextor kernel, this should improve the compatibility of games in disk emulation mode.

- Fix: drive was remapped in case of error (even if it was retried successfully).

- Fix: boot sector checksum calculation had a bug that caused "Wrong disk" errors.

- Fix: #1 pressing CTRL+STOP while Nextor was trying to load NEXTOR.SYS hanged the computer.

- Fix: #23 computer hanged when booting with one single drive letter (e.g. when using single-device controller in a computer without internal disk drive).

- Fix: #29 wrong stack management hangedd the computer when a file handle was read or written to a number of times.

- Fix: computer crashing when more than one Nextor kernel was present as soon as the extended BIOS hook was called (for example, when loading COMMAND2.COM).

- BREAKING CHANGE: The address of CODE_ADD, used by the CALLB0 routine, has changed to F1D0h (it was F84Ch).

- Fix: there was Nextor kernel code in the 1K free area in pages 0 and 3, so putting anything here caused problems, e.g. DOS 1 mode didn't work.

- _GPART now returns the status byte of the partition, and allows to retrieve the device sector number that holds the partition table entry instead of information about the partition.
Latest commit 6ecd074 Feb 11, 2019
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docs v2.1 beta 2 Apr 26, 2019
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.gitignore v2.1 beta 2 Apr 26, 2019 v2.1 beta 1 Apr 25, 2019 v2.1 beta 1 Apr 25, 2019


Nextor is a disk operating system for MSX computers. It is built on top of the source code of MSX-DOS 2.31, released in 1991.

The source code of Nextor is published with permission from the MSX Licensing Corporation under certain terms. Please take a moment to read the license terms for details.

Please visit the Nextor section in Konamiman's MSX page for binaries.

Repository structure

Note that there is no master branch, but branches for each major version of Nextor (v2.0 and v2.1 currently).

  • source: The source code of Nextor itself.

    • kernel: The kernel ROM, includes the FDISK tool.

    • command: NEXTOR.SYS, COMMAND2.COM and the command line tools that were originally supplied with MSX-DOS.

    • tools: The new command line tools created for Nextor.

  • wintools: Windows tools needed for building Nextor. Includes the source for two custom made tools: mknexrom (C) and SymToEqus (C#).

  • docs: Documentation for both users and developers.

How to build Nextor

You need:

  1. A Windows machine (if you don't have one see "No Windows?" below)
  2. SDCC (, targetting the Z80 processor, to build FDISK.
  3. .NET Framework 2.0 or higher (for the SymToEqus tool in the wintools folder)
  4. The wintools folder must be added to the PATH environment variable

To build the Nextor kernel

Run the compile.bat script located in the source\kernel folder. If the FDISK tool has not been compiled already (the fdisk.dat and fdisk2.dat files do not exist in the bank5 folder), they will be compiled on the fly.

The generated kernel base file and the complete ROM files will be generated in the bin\kernels folder. One ROM file will be generated for each folder existing in the source\kernel\drivers folder.

To build the FDISK tool only

If you make a change in the FDISK tool, you can compile it without having to compile the full kernel again. Just run the compile.bat script in the source\kernel\bank5 folder (do NOT run compfdsk.bat). The ROM files in bin\kernels will be appropriately updated.

To build the command line tools

Run the compile.bat script in the source\tools folder. The tools will be generated in the bin\tools folder.


Run the compile.bat script in the source\command\msxdos folder. The file will be generated in the bin\tools folder.


Run the compile.bat script in the source\command\command folder. The file will be generated in the bin\tools folder.

At this time there's no specific script (other than the original makefile) for building the original MSX-DOS command line tools.

No Windows?

If your machine doesn't run Windows you can still build Nextor by using Xesco's Nextor builder.

You can’t perform that action at this time.