In "How to Boot Pure DOS under WinNT/2K/XP," the section below feels rather hard to operate. I got stuck when I reached here.
Booting DOS requires the DOS system boot sector and boot files. The key to installing DOS after WinNT/2K/XP has already been installed is to create them. Some of you may ask: after WinNT/2K/XP is installed, hasn't the content in the master boot sector already been replaced with non-DOS content? Then why can DOS still be booted from the startup menu mentioned above? This is because if DOS was previously installed in the system (including the MS-DOS 7.x that comes with Win9x), WinNT/2K/XP will automatically create a BOOTSECT.DOS file in the active boot partition (usually drive C) during installation. This file preserves the DOS boot information. After selecting DOS from the startup menu, WinNT/2K/XP uses this file to let DOS boot. Therefore, if BOOTSECT.DOS already exists in C:\( (that is, the active boot partition), there is no need to create it manually. You can directly copy the DOS boot files (such as MS-DOS 7.10) IO.SYS, MSDOS.SYS, and COMMAND.COM to C:\ (it is best not to transfer the system with the SYS command; see the next paragraph for details), and then modify BOOT.INI as described below.
As for DOS system boot files, everyone knows that you can transfer them with the SYS command that comes with DOS. However, if you do it directly like that, then only DOS will be able to boot, and the boot information for WinNT/2K/XP will be overwritten, causing it to fail to boot. In order to make both DOS and WinNT/2K/XP bootable, there are several methods. You can use a very practical tool called BOOTPART, which can be found under "System Tools" on this site. It is very powerful. For example, it can write information for a specified system into the boot sector, including DOS6, DOS7 (Win9x), and WinNT (/2K/XP). For example, if you want to write the boot information of MS-DOS 7.10 (Win98) into drive C, you can use the command BOOTPART WIN98 BOOT:C:. Therefore, the specific method is: first use the SYS command (such as SYS C

to transfer the DOS system files into drive C, then save the information currently in the master boot sector (that is, the DOS boot information) into the file C:\BOOTSECT.DOS. This can easily be done with a small tool called SRC Tools in the site's "Disk Tools" section. The SRCBOOT, SRCMBR, and SRCFAT in it are commands for saving, backing up, and comparing BOOT, MBR, and FAT respectively. The specific usage of the SRCTools collection can be found in a special article under the "DOS Articles" section. For example, SRCBOOT C: C:\BOOTSECT.DOS /S can save the boot sector of drive C into the file C:\BOOTSECT.DOS. At this point the BOOTSECT.DOS file has been created successfully. Next is restoring the WinNT/2K/XP boot information, which can be done with the BOOTPART tool mentioned above, such as BOOTPART WINNT BOOT:C:. After that, just carry out the following operation of modifying the BOOT.INI file.