What is OBR

OBR (OS boot record) is the operating system boot sector, typically located on the hard disk's 0 cylinder 1 head 1 sectors (defined for DOS only, systems that are booting from multiple booting are located in the first sector of the corresponding primary partition/extension partition), the first sector that the operating system can directly access. It also includes a bootstrapper and a BPB parameter record table called the BIOS Parameter block.

Each logical partition has a OBR in a standard system state, and its parameters vary depending on the size of the partition and the category of the operating system. The primary task of the bootstrapper is to determine whether the first two files in the root directory of this partition are operating system boot files (such as Msdos or Io.sys and Msdos.sys originating from Msdos win9x/me). That is, the first file is read into memory and the control is handed over to the file. The BPB parameter block records important parameters such as the starting sector, end sector, file storage format, hard disk media descriptor, root directory size, fat number, allocation unit (allocation, early also called cluster).

OBR are generated by advanced formatters (such as DOS format.com, etc.).