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   (always use the latest version) - it is used to make the partitions on internal storage;
- Clonezilla running from PartedMagic   - it is used to backup / restore the Linux ext3 partition used by CentOS 8;
- BootIce running from a Windows live distribution   - it is used to backup / restore the MBR (master boot record - first sector of disk);
- XOSL-OW 1.1.6 installation kit  - 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   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  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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