four partition entries (up to 16 with 16 bytes each), a disk signature (6 bytes), a disk timestamp (6 bytes), an Advanced Active Partition (18 bytes) or special multi-boot loaders have to be supported as well in some environments. In floppy and superfloppy Volume Boot Records, up to 59 bytes are occupied for the Extended BIOS Parameter Block on FAT12 and FAT16 volumes since DOS 4.0, whereas the FAT32 EBPB introduced with DOS 7.1 requires even 87 bytes, leaving only 423 bytes for the boot loader when assuming a sector size of 512 bytes. Microsoft boot sectors therefore traditionally imposed certain restrictions on the boot process, for example, the boot file had to be located at a fixed position in the root directory of the file system and stored as consecutive sectors, conditions taken care of by the SYS command and slightly relaxed in later versions of DOS. The boot loader was then able to load the first three sectors of the file into memory, which happened to contain another embedded boot loader able to load the remainder of the file into memory. When Microsoft added LBA and FAT32 support, they even switched to a boot loader reaching over two physical sectors and using 386 instructions for size reasons. At the same time other vendors managed to squeeze much more functionality into a single boot sector without relaxing the original constraints on only minimal available memory (32 KB) and processor support (). For example, DR-DOS boot sectors are able to locate the boot file in the FAT12, FAT16 and FAT32 file system, and load it into memory as a whole via CHS or LBA, even if the file is not stored in a fixed location and in consecutive sectors. The VBR is often OS-specific; however, its main function is to load and execute the operating system boot loader file (such as bootmgr or ntldr), which is the second-stage boot loader, from an active partition. Then the boot loader loads the OS kernel from the storage device. If there is no active partition, or the active partition's boot sector is invalid, the MBR may load a secondary boot loader which will select a partition (often via user input) and load its boot sector, which usually loads the corresponding operating system kernel. In some cases, the MBR may also attempt to load secondary boot loaders before trying to boot the active partition.