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The boot code in the VBR can assume that the BIOS has set up its data structures and interrupts and initialized the hardware. The code should not assume more than 32 KB of memory to be present for fail-safe operation; [1] if it needs more memory it should query INT 12h for it, since other pre-boot code (such as f.e. BIOS extension overlays, encryption systems, or remote bootstrap loaders) may ...
Some MBR code loads additional code for a boot manager from the first track of the disk, which it assumes to be "free" space that is not allocated to any disk partition, and executes it. A MBR program may interact with the user to determine which partition on which drive should boot, and may transfer control to the MBR of a different drive.
The boot partition (or boot volume) [5] is the disk partition that contains the operating system folder, known as the system root or %systemroot% in Windows NT. [6]: 174 Before Windows 7, the system and boot partitions were, by default, the same and were given the "C:" drive letter.
Drive description / mirroring flags (bits 3-0: zero-based number of active FAT, if bit 7 set. [4] If bit 7 is clear, all FATs are mirrored as usual. Other bits reserved and should be 0.) DR-DOS 7.07 FAT32 boot sectors with dual LBA and CHS support utilize bits 15-8 to store an access flag and part of a message.
A modern PC is configured to attempt to boot from various devices in a certain order. If a computer is not booting from the device desired, such as the floppy drive, the user may have to enter the BIOS Setup function by pressing a special key when the computer is first turned on (such as Delete, F1, F2, F10 or F12), and then changing the boot order. [6]
On system with BIOS firmware, the BIOS invokes MBR boot code from a hard disk drive at startup. The MBR boot code and the VBR boot code are OS-specific. In Microsoft Windows, the MBR boot code tries to find an active partition (the MBR is only 512 bytes), then executes the VBR boot code of an active partition.
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These strategies can be preselected by a user or OEM or be changed by a boot loader on the fly when launching DR-DOS. Under these issues, the boot drive can be different from A: or C: as well. The drive letter order can depend on whether a given disk is managed by a boot-time driver or by a dynamically loaded driver.