Beware: This text is partially from 1996 and on, so the parts mentioning OS/2 may be of limited interest.

Multiple operating systems in a PC

If anybody asks me what operating system I want in a PC, I playfully answer "all of them". But you must do it right. First you must understand the way PC hard disks can be divided in primary and secondary partitions. Using DOS and Win 95/98 Fdisk, each hard disk can have a maximum of one primary and one secondary (extended) partition. The secondary partition can in turn be divided in a number of logical drives. But, using the OS/2 Fdisk or other boot selector software, each hard disk can contain a maximum of 4 primary partitions or 3 primary and one secondary.


An elegant way to make multi OSs coexist is to divide hard disks in partitions. Of course it takes some trouble to repartition and reformat the drive, so one aid to partition the hard disk, is the program Partition Magic.

DOS and Windows 95 / 98 must boot from a primary C: partition. OS/2, Windows NT/2000, and Linux can boot from a logical drive in an extended partition or from a secondary drive. 

Sharing one partition

Some operating systems may also coexist on the same primary partition: 

  • DOS together with Win 95/98 and NT/2000 on the same FAT partition. When NT starts, you get an option to boot DOS. If you choose DOS, your previously installed Win 95 will boot. At the text "starting Windows 95", press F8 or F4 and you get to the previously installed DOS.
  • DOS together with OS/2 on the same FAT partition. This is called "Dual Boot" by IBM.
  • Any of the above together with Linux or BeOS on the same FAT partition, using a boot diskette.

Separate partitions

For this I used one PC at work and my own at home, in February 1996. First, a P75 with 16 MB RAM, then a 486DX4 with 12 MB RAM. Each has 2 EIDE ports connected to 2 IDE disks and one IDE CD-ROM drive.

Start programs
In order to choose the starting partition, it's necessary to have a special program with a start menu. I used OS/2 Boot Manager mostly to select between OS/2 and Win 95. If you decide to use Boot Manager, make sure you get the whole OS/2 pack with the excellent manual. Now (1998) you can get it more or less for free on the used software market. You can use just the Boot Manager, without installing the OS/2 if you prefer. However, there are also other start programs that work in a similar way. One is called System Commander, but I haven't tried it.

First I thought of just having one partition and using OS/2 Dual Boot. It should be possible to configure a system that dualboots between Win95 and Warp on the same primary FAT partition. In such a setup, of course, there has to be different directories for Win95 and the Windows 3 or Win-OS2 code for running Win 16 apps under Warp. 

I decided, however, to use separate partitions and Boot Manager, letting Warp and Win 95 boot from separate system partitions, invisible to each other. 

Then I created other partitions for Win 16 programs and data that can be used by both operating systems. 

The greatest disadvantage with these setups is that the hard disks aren't used efficiently because of all the small partitions. So there is little space left for programs on these disks. Another disadvantage is that most 16 bit Win apps must be installed twice, under both Warp Win-OS2 and Win 95, so that all DLLs and INIs get right. So I have choose only to install the most important apps twice. 

This was my spec of a 16 MB RAM P75, at my job:

  • first IDE disk, 520 MB
    C: 180 Mb primary partn Win 95 system 
    C: 140 Mb primary partn HPFS OS/2 Warp Connect fullpack system 
    D: 200 Mb secondary partn (data & common non-Win95 programs) 
  • second IDE disk, 200 MB 
    E: 200 MB FAT secondary partn, misc data
  • Drive F: is CD-ROM

With the above solution I can choose from boot manager: - OS/2 - Win95 

On the second computer at my job, I created one more partition booting DOS, thus choosing between 3 operating systems. A monster setup? I let Warp boot from a secondary partition. From the DOS session I can even start a small Windows 3.1 installation if I want to. 

This was my spec of my home 12 MB RAM 486DX4:

  • first IDE disk, 420 MB
    C: 160 Mb primary partn, Win 95 system 
    C:   7 Mb primary partn, DOS 
    E: 140 Mb secondary partn, (data & common non-Win95 programs plus Windows 3.1) 
    F: 110 Mb secondary partn, HPFS OS/2 Warp system (Warp Connect would need another 40 MB). The F: drive is only visible from Warp sessions. When I install Warp in a secondary partn, I make sure the Win95 partn is NOT ACTIVE when I install WinOS2 in Warp, because if Warp install can see the Win95 WINDOWS directory, things may screw up. 
  • second IDE disk, 120 MB 
    D: 120 MB FAT primary partn, misc. data 
  • G:  is CD-ROM Driveletter 

With the above solution I can choose from boot manager: - Warp - DOS 6 - Win95 

It works fine. When I choose to boot DOS, I can start Windows 3.1 on the E: drive. If I install ATM under both Win95 and Win3 I can store the font files for both Windows systems in E:\PSFONTS. Maybe, that directory also can be used for ATM fonts under Warp. 

160 MB seems to be a minimum for the Win 95 system partition, and I recommend at least 250 MB. Indeed a basic Win 95 installation takes only 60 MB or so, but it swells with new fonts and DLLs, and Win 95 easily uses 30 MB for swap files. 

In that setup, I never decided if I should install Win-OS2 in the Warp partition an delete the Windows 3.1 installation in drive E: or let Warp run the Windows 3.1 code installed drive E:. Anyways, who wants Windows 3 on a PC fast enough to run alternatives? 

I did it partly for fun partly because of the work at helpdesk and beta testing and need to test different systems. 

Next time I want to create a similar setup on larger disks, including a 100 or 200 MB Linux partition, thus choosing between 4 OSs from, Boot Manager. According to my information, Linux, like Warp, can boot through Boot Manager from a secondary partition or a secondary drive. Thus breaking the FAT partition limit. I have never installed Linux, so I appreciate all further info on such a setup. I have also read that Linux can read files in an HPFS partition. Anybody know more about this? 

February 1996

Windows 95/98 file systems

Before you make custom solutions with Windows 95, you need to understand the FAT and FAT32 file systems. The first Win 95 could only use FAT and max partition size 2 GB. Windows 95 B and Windows 98 offers FAT32 as an alternative. When I tried FAT32 for a 3 GB partition, the hard disk access was at least twice as slow compared to FAT. Thus I do not recommend FAT32 with Windows 95. My Win 95 system now has a 3 GB drive, split on 3 partitions, each partn is 1 GB, using the FAT file system. This works fast and uses the hard disk efficiently. This is explained in Tom's Hardware Guide

May 1998

I have now tried FAT32 using one 3 GB partition under Windows 98 on a 64 MB RAM Pentium 200. It seemed to run acceptably smoothly, but with a slight delay. Judge for your self!

January 1999

I have now reformatted the same hard disk as 2 FAT 16 partitions. Suddenly the entire computer is considerably faster. I was wrong about FAT 32 running "smoothly". No more FAT 32 for me during this millennium!

November 1999


In 1996, I tried to set a record in number of booting operating systems. I set up 5 boot partitions on the 2 disks of my PC. IBM Boot Manager let me choose between: DOS, OS/2, Win95, Win NT 3.51, and Linux. Linux had one partition for the system and one for the virtual memory swap file. I set IBM Boot Manager to invoke the NT "boot manager", so NT started in a 2 step process. I had NT in a 100 MB partition for a while, and then erased it, since it was totally useless for me at home. 

In 1998 I mostly used Windows 95 for personal needs at home. I have a Pentium 200 and 64 MB RAM. Maybe when I get a 300 MHz CPU I'll think of NT as a serious alternative. For anybody who hasn't tried Linux, I recommend having a look. Linux is almost for free. Already in 1994 I used an old 386 as a local network mail server running Linux in text mode. 

Here's the setup I used to install 3 starting systems: Windows 95 - Windows NT 4 - Linux 

  • first IDE disk, 3 GB Quantum
    C: 1 GB FAT16 primary partn, Windows 95 system + programs installed under \Program files 
    D: 1 GB FAT16 secondary ptn, Various large applications, like MS Office and graphics programs 
    E: 1 GB FAT16 secondary ptn, Only data, all my documents

  • second IDE disk, 1 GB
    F: 512 MB FAT16 secondary ptn, Windows NT 4 system + a few programs. Most programs, I reinstalled from NT but in the same directories where they were already installed from Win 95 in drive C: and D: Thus no extra space was wasted. But they had to be reinstalled under NT for the DLL files end for the register. 
    Then , a Linux system partition, that has no drive letter. 
    Finally, a Linux swap system.

  • R: is the CD-ROM drive letter, "R" as in ROM.

After this installation was done, I decided to use the Linux Lilo start program. Thus, at each power on, I was prompted to choose between "DOS" and "Linux". If I chose "DOS", I immediately got the Windows NT menu, offering NT or Windows 95. Thus I got 3 starting systems, unfortunately it was a 2-step process. In the second hard disk I could, if I wanted, install 2 different Linuxes (Red Hat + whatever) both handled by the same Lilo system. 

May/July 1998


Since I wrote the above report, I have made similar setups a few more times with up to 5 choices on the Boot Manager menu.

If you install Win-OS2, on a system that has Windows 95, I recommend temporarily hiding the Win 95 WINDOWS directory during installation. With the OS/2 FDISK you can alternally hide primary C: partitions and you can also try renaming C:\WINDOWS to C:\INDOWS

It is very important that you understand that DOS, Win NT, OS/2 and Linux all have their own FDISK programs that can write to the partition table. Tampering with the partition table using alternating FDISKs may cause damage. If you use the OS/2 Boot Manager I recommend sticking to the OS/2 FDISK as much as possible. After installing DOS, Win 95 and Win NT, you must use the Microsoft FDISK to reactivate partition 1 (Boot Manager). If you install Linux you must use Linux FDISK to create the Linux partitions. If you use Win NT 3.51 or 4, there is a Disk administrating tool. The first time you start it you are asked if it may write a signature to the hard disk. If you use OS/2 Boot Manager, answer NO to that question! Else the partition table may be damaged.

OS/2 Warp 3 needs to boot inside the first 1024 cylinders of the hard disk. But Warp 4 does not have this limit.


February 1997

Per Åkesson


Computers can be socially hazardous. Sitting at a computer, it's easy to forget about the real world (people, friends, love, etc). I have an idea: Give computers to everybody and the world's overpopulation will stop, because people even forget about sex and how to make children! Then of course, we must use insemination, to prevent mankind from dying out totally. 

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