The master boot sector always comprises at least four components:
- Start program (bootloader)
- Data media, disk signature (from Windows 2000)
- Master partition table
- MBR or boot signature (magic number)
The start program is contained in the first 446 bytes of the MBR. This rather tiny piece of software is activated when starting the PC and initiates the booting process. A more extensive procedure with standardized processing steps is launched, ending with an operating system ready for use (e.g. Windows). Working memory built into the computer is addressed in order to run this process. It can be addressed if the computer is able to use electrical energy for the necessary processing after being switched on.
Based on the data media or disk signature, Windows computers (from Windows 2000) identify a data medium with a partition table.
The partition table documents the allocation of data storage into separate sectors. It does so using four entries, each comprising 16 bytes, which provide information about the location and size of each partition. For example, where a C:\ partition or D:\ partition begins and ends. In addition, the table contains information about the time of data storage, such as “FAT32”, “LINUX Native” or “Dynamic Disk”. The order of the partitions in the partition table does not always reflect the physical arrangement on the hard drive.
The MBR or boot signature contains the strings “55” and “AA” in two bytes. A master boot record is recognized thanks to its characteristic coding, which can always be found at the end of the MBR sector. If this information is missing, the master boot sector won’t be identified, and the boot process will be aborted with an error message.