At the end of the ROM BIOS bootstrap routine, the BIOS reads and executes the first physical sector of the first floppy or hard disk on the system. This first sector of the hard disk is called the master boot record (or sometimes the partition table or master boot block). There is a small program at the beginning of this sector of the hard disk. The partition information, or partition table, is stored at the end of this sector. This program uses the partition information to determine which partition is bootable (usually the first primary DOS partition) and attempts to boot from it.
Master boot record (MBR) disks use the standard BIOS partition table. MBR disks do not support more than four partitions on each disk. The MBR partition method is not recommended for disks larger than 2 terabytes. It consists of a sequence of 512 bytes located at the first sector of a data storage device such as a hard disk. You can change a disk from GPT to MBR partition style as long as the disk is empty and contains no volumes. But, with the help of AOMEI Dynamic Disk Manager, you can directly convert a GPT disk with volumes to MBR disks.
The following is the structure of a basic MBR disk:
MBR viruses exploit the master boot code that runs automatically when the computer starts up. MBR viruses are activated when the BIOS activates the master boot code, before the operating system is loaded.
Many viruses replace the MBR sector with their own code and move the original MBR to another location on disk. Once the virus is activated, it stays in memory and passes the execution to the original MBR so that startup appears to function normally. Some viruses do not relocate the original MBR, causing all volumes on the disk to become inaccessible. If the active, primary partitions listing in the partition table are destroyed, the computer cannot start. Other viruses relocate the MBR to the last sector of the disk; if that sector is not protected by the virus, it might be overwritten during normal use of the computer, preventing the system from being restarted.