Multiboot computer system

Since at least 20 years, I use on my workstation computers a multiboot configuration on their own storage device, regardless if they are desktops or laptops. This means I install multiple operating systems on the hardware, each of them can run as host OS (one at the time). My configuration is made with the intention of having each OS in a separate partition and the boot manager to be independent of the operating systems. In that way, a crash of an OS will not affect the others.

Figure 1
Boot manager
Figure 2
Figure 3

The simplest way to run an operating system different from the one in the computer is to build live distributions installed on a CD/DVD or USB stick. You can see some of these distributions in the link section at the end of this article: Windows 7 32 bit [1], Windows 7 64 bit [2], Windows 8 [3], Windows 8.1 [4], Windows 10 [5], System Tools [6], Parted Magic [7], Parted Magic 2019_01_03 [8]. The disadvantage of this method is that not all software programs can run on a live distribution.

The second easy way to run other operating systems is by using virtual machines, emulators of a real computer. A disadvantage of this method appears if the host operating system crashes. In this case you also cannot run the virtual machines. Often it is better to test applications on real hardware instead of virtual machines or live distributions to can evaluate their true performance and stability.

The multiboot configuration I present here is different and do not exclude the use of the other two methods mentioned above. Theoretically on a single computer it is possible to install an unlimited number of operating systems controlled by a single boot manager. As an example, let's see how to setup a configuration with three operating systems: Windows 10 (Windows 7), CentOS 8 and DOS on a computer with 9th generation Intel processor codename Coffee Lake [9] and a SSD storage of 240 GB. There are problems because installers for DOS and Windows 7 don't have USB 3.x drivers included. For using DOS you need a motherboard with input ports for PS2 mouse and keyboard (figure 4). If the input port is combo, you need also a PS2 splitter (figure 5).

Figure 4
PS2 ports
Figure 5
PS2 splitter
Figure 6

In figure 6 you can see the partition layout I use:
- first partition, primary 8 GB , FAT32 for DOS, boot manager and other system tools and programs;
- second partition, primary 40 GB, NTFS for Windows 10 or Windows 7;
- third partition, logical 20 GB, ext3 for CentOS 8;
- fourth partition, logical 4 GB, linux-swap for CentOS 8;
- fifth partition, logical 150 GB, NTFS for data, this section is seen by both Windows and Linux.

To setup this configuration you need the following software:
- GParted running from PartedMagic [7] [8] (always use the latest version) - it is used to make the partitions on internal storage;
- Clonezilla running from PartedMagic [7] [8] - it is used to backup / restore the Linux ext3 partition used by CentOS 8;
- BootIce running from a Windows live distribution [4] [5] - it is used to backup / restore the MBR (master boot record - first sector of disk);
- XOSL-OW 1.1.6 installation kit [10] - there is an instruction manual in the project website;
- DOS installation kit running from FDD, CD or USB;
- Windows 10 or Windows 7 installation kit running from DVD or USB;
- drivers for Windows, usually on CD/DVD;
- to install Windows 7 you need a Patcher provided by motherboard manufacturer to inject USB 3.x drivers in the installation kit;
- a backup / restore program running from a Windows live distribution [4] [5] used for Windows NTFS partition;
- CentOS 8 installation kit running from USB;
- a storage for backups, other than internal partition DATA, usually an USB stick.

The order of operations is as follows:
A. create first partition 8 GB FAT32 for DOS, the rest of internal storage space remains unallocated in this step. After every change in partition layout you must power off the computer;
B. install DOS (fdisk, format, sys);
C. install XOSL-OW and setup first menu in boot manager - DOS;
D. backup MBR with BootIce;
E. backup DOS partition;
F. create the rest of partitions with GParted;
G. set "hidden" flag on first partition DOS;
H. install Windows operating system, custom on NTFS 40 GB partition. The MBR will be overwritten during installation by setup program. Install all drivers in Windows. Test if Windows works well;
I. backup Windows partition;
J. restore MBR saved in step D;
K. setup second menu in boot manager - Windows, set "boot" and "hidden" partitions flags correctly (see XOSL-OW instruction manual);
L. backup MBR with BootIce;
M. install CentOS 8, choose custom partitioning, /dev/sda5 (20 GB) for root directory and /dev/sda6 (4 GB) for swap. Leave rest of partitions untouched. The MBR will be overwritten during installation. Change Linux boot loader to point on /dev/sda5 (grub2-install, grub2-mkconfig);
N. install ntfs-3g in CentOS 8 to get access on DATA partition;
O. restore MBR saved in step L, setup the third menu in boot manager - Linux. Set the partitions flags for hiding DOS partition when running Windows.
P. you can add on DOS partition several live distributions [6] that usually are running from an USB stick. This is the reason I made this partition of 8 GB.
Now you have three operating systems on your computer.

Note: This project is for people who have extensive knowledge of computer hardware and software. Changing the partition layout of an internal data storage will erase all data without possibility of recovering. The operations presented here must be performed on a new HDD/SSD or if it is an old one, you must backup all data before making the new configuration.

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[1] - Windows 7 32 bit Portable Edition
[2] - Windows 7 64 bit Portable Edition
[3] - Windows 8 Portable Edition
[4] - Windows 8.1 Portable Edition
[5] - Windows 10 Portable Edition
[6] - System Tools
[7] - Parted Magic
[8] - Parted Magic 2019_01_03
[9] - Intel Coffee Lake processors
[10] - XOSL-OW 1.1.6