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boot sector wikipedia jump to content main menu main menu move to sidebar hide navigation main page contents current events random article about wikipedia contact us contribute help learn to edit community portal recent changes upload file special pages search search appearance donate create account log in personal tools donate create account log in contents move to sidebar hide top 1 the ibm pc and compatible computers 2 unified extensible firmware interface uefi 3 damage to the boot sector 4 partition tables 5 operation 6 boot sector viruses 7 see also 8 notes 9 references 10 external links toggle the table of contents boot sector 26 languages العربية беларуская català čeština deutsch español فارسی français hrvatski italiano 日本語 한국어 кыргызча မြန်မာဘာသာ nederlands norsk bokmål polski português русский slovenčina svenska ไทย українська tiếng việt 粵語 中文 edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia redirected from boot sector this article is about the generic concept of boot sectors for the mbr in pcs see master boot record for the vbr in pcs see volume boot record sector of persistent data storage device this example show various components of gnu grub distributed over sectors of a hard disk when grub is installed on a hard disk boot img is written into the boot sector of that hard disk boot img has a size of only 446 bytes a boot sector is the sector of a persistent data storage device e g hard disk floppy disk optical disc etc which contains machine code to be loaded into random access memory ram and then executed by a computer system s built in firmware e g the bios usually the first sector of the hard disk is the boot sector regardless of sector size 512 or 4096 bytes and partitioning flavor mbr or gpt the purpose of defining one particular sector as the boot sector is inter operability between firmware and various operating systems the purpose of chain loading first firmware e g the bios then code in the boot sector and then for example an operating system is maximal flexibility the ibm pc and compatible computers edit on an ibm pc compatible machine the bios selects a boot device then copies the first sector from the device which may be a mbr vbr or any executable code into physical memory at memory address 0x7c00 on other systems the process may be quite different unified extensible firmware interface uefi edit uefi does not rely on boot sectors but instead loads the boot loader directly from an efi application file in the efi system partition 1 additionally the uefi specification contains secure boot which requires the uefi code to be digitally signed damage to the boot sector edit in case a boot sector receives physical damage the hard disk will no longer be bootable unless used with a custom bios that defines a non damaged sector as the boot sector however since the very first sector additionally contains data regarding the partitioning of the hard disk the hard disk will become entirely unusable except when used in conjunction with custom software partition tables edit a disk can be partitioned into multiple partitions and on conventional systems it is expected to be there are two definitions on how to store the information regarding the partitioning a master boot record mbr is the first sector of a data storage device that has been partitioned the mbr sector may contain code to locate the active partition and invoke its volume boot record a volume boot record vbr is the first sector of a data storage device that has not been partitioned or the first sector of an individual partition on a data storage device that has been partitioned it may contain code to load an operating system or other standalone program installed on that device or within that partition the presence of an ibm pc compatible boot loader for x86 cpus in the boot sector is by convention indicated by a two byte hexadecimal sequence 0x55 0xaa called the boot sector signature at the end of the boot sector offsets 0x1fe and 0x1ff this signature indicates the presence of at least a dummy boot loader which is safe to be executed even if it may not be able actually to load an operating system it does not indicate a particular or even the presence of file system or operating system although some old versions of dos 3 relied on it in their process to detect fat formatted media newer versions do not boot code for other platforms or cpus should not use this signature since this may lead to a crash when the bios passes execution to the boot sector assuming that it contains valid executable code nevertheless some media for other platforms erroneously contain the signature anyway rendering this check not 100 reliable in practice the signature is checked for by most system bioses since at least the ibm pc at but not by the original ibm pc and some other machines even more so it is also checked by most mbr boot loaders before passing control to the boot sector some bioses like the ibm pc at perform the check only for fixed disk removable drives while for floppies and superfloppies it is enough to start with a byte greater or equal to 06h and the first nine words not to contain the same value before the boot sector is accepted as valid thereby avoiding the explicit test for 0x55 0xaa on floppies since old boot sectors e g very old cp m 86 and dos media sometimes do not feature this signature despite the fact that they can be booted successfully the check can be disabled in some environments if the bios or mbr code does not detect a valid boot sector and therefore cannot pass execution to the boot sector code it will try the next boot device in the row if they all fail it will typically display an error message and invoke int 18h this will either start up optional resident software in rom rom basic reboot the system via int 19h after user confirmation or cause the system to halt the bootstrapping process until the next power up systems not following the above described design are cd roms usually have their own structure of boot sectors for ibm pc compatible systems this is subject to el torito specifications commodore 128 software on commodore dos disks where data on track 1 sector 0 began with a magic number corresponding to string cbm 2 ibm mainframe computers place a small amount of boot code in the first and second track of the first cylinder of the disk and the root directory called the volume table of contents is also located at the fixed location of the third track of the first cylinder of the disk other non ibm compatible pc systems may have different boot sector formats on their disk devices operation edit further information booting on ibm pc compatible machines the bios is ignorant of the distinction between vbrs and mbrs and of partitioning the firmware simply loads and runs the first sector of the storage device 3 if the device is a floppy or usb flash drive that will be a vbr if the device is a hard disk that will be an mbr it is the code in the mbr which generally understands disk partitioning and in turn is responsible for loading and running the vbr of whichever primary partition is set to boot the active partition the vbr then loads a second stage bootloader from another location on the disk furthermore whatever is stored in the first sector of a floppy diskette usb device hard disk or any other bootable storage device is not required to immediately load any bootstrap code for an os if ever the bios merely passes control to whatever exists there as long as the sector meets the very simple qualification of having the boot record signature of 0x55 0xaa in its last two bytes this is why it is easy to replace the usual bootstrap code found in an mbr with more complex loaders even large multi functional boot managers programs stored elsewhere on the device which can run without an operating system allowing users a number of choices in what occurs next with this kind of freedom abuse often occurs in the form of boot sector viruses boot sector viruses edit since code in the boot sector is executed automatically boot sectors have historically been a common attack vector for computer viruses to combat this behavior the system bios often includes an option to prevent software from writing to the first sector of any attached hard drives it could thereby protect the master boot record containing the partition table from being overwritten accidentally but not the volume boot records in the bootable partitions 4 depending on the bios attempts to write to the protected sector may be blocked with or without user interaction most bioses however will display a popup message giving the user a chance to override the setting the bios option is disabled by default because the message may not be displayed correctly in graphics mode and blocking access to the mbr may cause problems with operating system setup programs or disk access encryption or partitioning tools like fdisk which may not have been written to be aware of that possibility causing them to abort ungracefully and possibly leaving the disk partitioning in an inconsistent state nb 1 as an example the malware notpetya attempts to gain administrative privileges on an operating system and then would attempt to overwrite the boot sector of a computer 5 6 the cia has also developed malware that attempts to modify the boot sector in order to load additional drivers to be used by other malware 7 another malware that overwrites boot sector is the memz see also edit master boot record mbr volume boot record vbr notes edit one fdisk utility written to be aware of bios boot sector protection features is dr dos fdisk r2 31 and higher which will detect this scenario and display additional interactive messages to guide the user through it in contrast to other fdisk utilities dr dos fdisk is not only a partitioning tool but can also format freshly created partitions as fat12 fat16 or fat32 this reduces the risk to accidentally format wrong volumes citation needed references edit uefi osdev wiki wiki osdev org retrieved 2020 09 26 commodore 128 programmer s reference guide bantam books 1986 pp 446 667 isbn 0 553 34292 4 smith roderick w 2010 04 14 migrate to grub 2 ibm com retrieved 2013 03 05 intel desktop boards bios settings dictionary pdf intel retrieved 2013 09 01 new ransomware variant nyetya compromises systems worldwide blog talosintelligence com 27 june 2017 retrieved 2018 05 28 in an era of global malware attacks what happens if there s no kill switch cio dive retrieved 2018 05 28 cia developed windows malware that alters boot sector to load more malware information security newspaper 2017 09 01 retrieved 2018 05 28 external links edit mary landesman boot sector viruses archived from the original on 2011 07 07 retrieved 2006 08 18 microsoft how to protect boot sector from viruses in windows knowledgebase denny lin 15 june 1994 inexpensive boot sector virus detection and prevention techniques retrieved 13 august 2015 kaspersky lab boot sector viruses virus encyclopedia malware descriptions classic viruses archived from the original on 2007 08 22 retrieved 2006 06 05 arman catacutan glossary of virus terms boot viruses archived from the original on 2006 12 11 retrieved 2006 11 07 greg o keefe sample to build a boot program on x86 real mode susam pal writing boot sector code using gnu assembler pierre ancelot bootsector assembly code with detailed explanation cite web cs1 maint deprecated archival service link retrieved from https en wikipedia org w index php title boot_sector oldid 1376802993 categories computer file systems bios booting hidden categories articles with short description short description is different from wikidata all articles with unsourced statements articles with unsourced statements from september 2026 cs1 maint deprecated archival service this page was last edited on 26 september 2026 at 08 39 utc page was rendered with parsoid text is available under the creative commons attribution sharealike 4 0 license additional terms may apply by using this site you agree to the terms of use and privacy policy wikipedia is a registered trademark of the wikimedia foundation inc a non profit organization privacy policy about wikipedia disclaimers contact wikipedia legal safety contacts code of conduct developers statistics cookie statement mobile view search search toggle the table of contents boot sector 26 languages add topic