PartUtil

Overview

Download link: partutil_2025-07-07.zip

If you want to have more than one operating system installed on your hard drive you can partition it and install an operating system in each partition. FDISK supports this by letting you create partitions and set one of them to be active.

When running DOS this can cause drive letter mappings to change. Later versions of DOS usually will be able to see and map drive letters to the earlier versions of DOS, while earlier versions of DOS will ignore the newer filesystems that they don't understand. If you have additional drive letters in an extended partition those are also affected. Imagine having a drive letter for common utilities that keeps moving around each time you switch DOS versions ... that is annoying.

The general solution to this is to install a boot manager that can hide or unhide partitions as needed. Hiding a partition means changing the partition type in the MBR to something that the active operating system does not recognize, and thus no drive letter is assigned to it. Boot managers are a good solution for later systems but I really don't want to install one on a PCjr. I want something that I know runs under any version of DOS, runs on any class of PC, and only touches the MBR. (No special installation procedure or hidden data on the drive should be needed.)

My solution to this was to write a simple partition switcher - you told it what primary partition you wanted to activate, and it would unhide that partition, set it active, and hide the other primary partitions. That worked for years and I managed to lose the source code, so when I wanted to add features to it I had to start from scratch. And here we are ...

Usage

PartUtil is a stand-alone program that you just run - no installation is needed. It runs under all versions of DOS and is less than 20KB in size.

No changes will ever be made to your hard drive without giving you a chance to confirm the changes.

At startup it will read the drive geometry from the BIOS and read the Master Boot Record (MBR) from sector zero of the first hard drive. It will then show you the partition table of the hard drive.

The current and pending partition table display:

• The drive geometry will be in Cylinder/Head/Sector (CHS) format. That may not be the actual drive geometry, as the BIOS may be doing some form of translation. For this tool that does not matter.
• If a partition is active it will have an asterisk in the "Active" column. Only one partition should ever be marked active; more than one active partition is an error. It is possible to not have any active partitions but that means nothing on this hard drive should be bootable.
• The "Size" column is in megabytes (MB) and it should always be accurate to within one tenth of a megabyte. The "Start CHS" and "End CHS" columns may not be accurate due to the limits of CHS addressing, especially on larger drives. This tool doesn't alter or use the CHS fields and they are only there for informational purposes. (DOS FDISK always works in cylinders.)
• PartUtil always shows the existing MBR as it appears on disk first. If you have made changes then it will show the pending changes next.

Basic commands:

• setActive and clearActive: Mark or unmark a partition as active.
• hide or unhide: Hide or unhide a partition.
• show: Show the current MBR and any pending changes.
• revert: Undo any pending changes to the MBR.
• write: Write pending changes to the MBR. (You will be asked to confirm the changes.)
• reboot: Reboot the machine.
• quit: Quit the program. (Ctrl-C also works.) Pending changes will not be saved.

Hide and unhide only work on a limited set of partition types. If you need to hide or unhide a partition type that PartUtil refuses to touch use the setType command described later.

Save and restore commands:

• saveCurrentMBR: Save the current MBR on the disk to a file.
• loadPendingMBR: Load pending changes from a file on the disk.
• savePendingMBR: Save pending changes to a file on the disk.

Saving the current MBR to a file allows you to recover from accidents when using this tool, FDISK, or other partition editors. You should specify a file on a floppy disk for safety; after an accident your hard drive might not be readable until the partition table is correctly restored.

SavePendingMBR and loadPendingMBR let you save or load proposed MBRs to/from files. This is useful when you are crafting your dual boot setup; instead of hiding, unhiding and setting a partition active each time you can save the proposed changes in a file and load them when needed.

Expert mode commands:

• setType: Force a partition to be a specific partition type. Useful for hiding or unhiding a partition when PartUtil doesn't understand the partition type.
• delete: Deletes a partiton from the partition table. FDISK will only delete DOS partitions, limiting its usefulness when setting up a new hard drive.
• wipeMBR: Write zeros to the entire MBR. This is "the nuclear option" and should only be needed when installing a new hard drive.

To get into expert mode start PartUtil using the "-expert" option on the command line.

How to dual (or triple!) boot

To setup your drive, use FDISK or another partition editor to create the partitions for your operating systems. You might need to use wipeMBR or delete to get rid of partitions that FDISK won't touch. DOS FDISK also won't create a new DOS partition if it sees an existing one so you can use PartUtil to hide the other DOS partitions as you partition and install the various versions of DOS.

Note that each version of DOS has rules for where and how it can be installed. For example, DOS 2.1 must be within the first 32MB of the drive. You might need to try a few experiments until you find a setup that works.

To switch partitions:

• Unhide the partition you want to boot from.
• Mark that partition as active.
• Hide the other partitions that you don't want drive letters for.
• Review the changes then write them to the hard drive.
• Reboot!

After testing and getting comfortable consider automating the process. With the savePendingMBR command you can write the pending changes to a file. Then later you can use loadPendingMBR, write the changes to disk and reboot without having to use the hide, unhide and setactive commands. PartUtil will read from the keyboard or a redirected file so you can put the commands in a file and replay them.

Here you can see my test system which has DOS 3.3 in partition 1, DOS 6.3 in partition 2, and DOS 2.1 in partition 4. DOS33.MBR is a previous pending MBR that I saved to a file that has partition 1 active and unhidden and partitions 2 and 4 hidden. If I run the DOS33.BAT batch file the machine will switch over to DOS 3.3.

Disclaimer

Editing the MBR of a hard drive can lead to data loss. Doing it by hand is not advised, which is why we have tools. But even with a well written tool things can go wrong.

This tool minimizes risk by only manipulating the active flag on a partition and only marking specific partition types and hidden or visible. The other parts of the partition table and MBR are not touched.

As always, ensure you have a back up of your data and the MBR before using this or any similar tool that can cause data loss. I have tested it fairly well and I use it on my personal systems, but I take no liability for the code or user errors.

Created July 7th, 2025
(C)opyright Michael B. Brutman, mbbrutman at gmail.com