When a computer boots, the BIOS passes the control to the first-boot device-it may be the hard disk, CD-ROM, floppy disk, or Flash drive. MBR is the first sector of the hard disk and is only 512 bytes in size (Figure 2). This sector consists of code required to boot a PC. MBR consists of 446 bytes of primary bootloader code and 64 bytes of the partition table. The partition table records the information regarding the primary and extended partitions. Boot loading is implemented in GRUB as Stage 1, Stage 2, Stage 1.5 (optional), etc. The primary bootloader area (446 bytes)contains Stage 1, which in turn directs you to Stage 2 (i.e., the menu.lst configuration file, which has the list of operating systems on the machine).