MBRBootcode

MBR bootcode refers to the executable payload contained in the Master Boot Record of BIOS-based computers. Its primary purpose is to start the boot process by locating and loading an operating system or a subsequent bootstrap loader from the disk.

The MBR occupies the first sector of a storage device, which is 512 bytes in size. The first 446 bytes hold the bootloader code, the next 64 bytes contain the partition table (four 16-byte entries), and the final two bytes hold the boot signature 0x55AA. The partition table describes up to four primary partitions and their attributes, including the bootable flag on an active partition.

During boot, the BIOS loads the MBR into memory (commonly at address 0x7C00) and transfers control to the code at that location. The boot code typically searches the partition table to identify an active primary partition (boot flag set) and then loads the first sector of that partition, often called the boot sector or volume boot record, into memory and transfers control to it. The loaded boot sector may directly start an operating system or chainload a more complex bootloader, such as GRUB, LILO, or Syslinux, which then loads the OS kernel and initial ramdisk or other stages.

MBR bootcode is a legacy component of the BIOS boot path and is increasingly supplemented or replaced by UEFI with GPT in modern systems. Nevertheless, BIOS compatibility modes on many machines still rely on MBR boot sectors. Security concerns exist because the MBR can be modified by malware or bootkits, potentially loading unauthorized code before the operating system.

Common bootloaders that use MBR bootcode include GRUB, Syslinux, and LILO, which implement the actual OS loading beyond the initial 446-byte bootstrap.