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Superfloppy Utility for OS/2

Utility to convert FAT super floppies with more than 512 bytes per sector (usually magneto opticals) for usage with N512DASD.FLT

I made this tool to share data on 640MB magneto-opticals with different operating systems. It should work for media with 1024 bytes per sector too. It runs with OS/2 and Windows NT.

Version: 1.0
- mo640 now expects a drive letter instead of a device number
- reinserting of media after conversion no longer required
Version: 0.9a
- MO640 now works with LVM
- WinNT support
→ Installation → Usage → WinNT → Constraints → Download → Todo → Background → Details Contact


Place the file MO640.EXE somewhere in your PATH and GCC30S.DLL somewhere in your LIBPATH.

You need the following drivers installed in your CONFIG.SYS:


Optionally you may create program object for the conversion on your desktop.


The behavior of 512 bytes per sector media is unchanged. You can share super floppies and partitioned media with other operating systems. You do not need this utility in this case.

My recommendation:
Use FAT superfloppy for data exchange and HPFS partitioned for OS/2 only.


Normally you do not need any utility to access opticals with WinNT. However, there is one good reason for a WinNT port of this tool: if you forgot to convert a disk back before using it with Win.

The WinNT version is called mo640nt.exe and is included within this package.



This program is distributed under the terms of the GNU GENERAL PUBLIC LICENSE.

ZIP file with binaries, source and this guide: Version 1.0
(Untested with OS/2 Warp4 and older.)

Old version: Version 0.9a


Of course, it ist not very comfortable to convert the media always. And maybe, you forgot to convert it back before using it with Win...


The OS/2 driver model for (optical) devices with >512 bytes per sector:

  1. The 'official' way
    IBM tells you to use OPTICAL.DMD. Well, this driver is very old. It only supports FAT super floppies and uses at least no write caching.
  2. With Fujitsu devices
    If you have a Fujitsu MO-drive there is a OS/2 driver available from their homepage. It supports all FAT based formats I know, but it also uses no caching. And unfortunately it has a bug (like the DOS driver too). This bug ist not very common but it will destroy the integrity of the FAT file system from time to time - so your data is unsafe.
  3. The N512DASD driver - the only way for HPFS formatted media
    The removable media support of IBM's NEWDASD package (and newer fix packs) only includes 512 bytes per sector media. If you have optical devices greater than 230 MB you will usually have 1024 or 2048 bytes per sector. The filter driver N512DASD.FLT will logically convert them into devices with two or four times as many sectors of 512 bytes.
    This works fine but the resulting formats are incompatible with other operating systems. In this case MO640 can help.

What does it exactly?

The N512DASD driver changes the number of sectors on the media, so all references within the file system become invalid. Fortunately FAT uses mainly cluster based references. So the only job is to modify the boot block this way that the clusters have more sectors of smaller size (512) resulting in the same cluster size. Only a few changes are necessary.

Example for 640 MB super floppy:

offset before after description
0B 0008 0002 bytes per sector (2048->512)
0D 08 20 sectors per cluster (8->32)
0E 0100 0400 reserved sector at the beginning (boot block) (1->4)
16 2600 9800 sectors per FAT (38->152)
20 4EBC0400 38F11200 total number of sectors (310350->1241400)

The implementation is a little bit more sophisticated. In addition to the example short number of sectors and hidden sectors are processed. Usually they are zero.


Suggestions, help, complaints (but not too much:-): Mail address: mueller (at) maazl (dot) de

Original homepage: http://www.maazl.de/project/mo640/index.html