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Note: The column MBR (Master Boot Record) refers to whether or not the boot loader can be stored in the first sector of a mass storage device. The column VBR (Volume Boot Record) refers to the ability of the boot loader to be stored in the first sector of any partition on a mass storage device.
The VBR is loaded at memory location 0000h:7C00h [1] and with the following CPU registers set up when the prior bootstrap loader (that is, typically the BIOS or MBR, but possibly another boot loader) passes execution to it by jumping to 0000h:7C00h in the CPU's real mode. CS:IP = 0000h:7C00h (fixed) Some Compaq BIOSes erroneously use 07C0h ...
The MBR bootstrap code loads and runs (a boot loader- or operating system-dependent) volume boot record code that is located at the beginning of the "active" partition. The volume boot record will fit within a 512-byte sector, but it is safe for the MBR code to load additional sectors to accommodate boot loaders longer than one sector, provided ...
If the device is a hard disk, that will be an MBR. It is the code in the MBR which generally understands disk partitioning, and in turn, is responsible for loading and running the VBR of whichever primary partition is set to boot (the active partition). The VBR then loads a second-stage bootloader from another location on the disk.
The system files still have to be stored in the first two root directory entries on the disk, but the VBR will use only the first entry to load the first three sectors of IBMBIO.COM into memory and transfer control to it. [10] [nb 2] This part of IBMBIO.COM then contains a somewhat larger boot loader which: Loads the rest of itself into memory.
The VBR is often OS-specific; however, its main function is to load and execute the operating system boot loader file (such as bootmgr or ntldr), which is the second-stage boot loader, from an active partition. Then the boot loader loads the OS kernel from the storage device.
A bootloader, also spelled as boot loader [1] [2] or called bootstrap loader, is a computer program that is responsible for booting a computer. If it also provides an interactive menu with multiple boot choices then it's often called a boot manager .
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.