Master Boot Record (MBR) Overview from OSDev Wiki

22092022 0001 MBR x86 OSDev Wiki httpswikiosdevorgMBR28x8629 17 MBR x86 From OSDev Wiki A Master Boot Record MBR is the bootsector of a hard disk That is it is what the BIOS loads and runs when it boots a hard disk The MBR is the very first sector of the hard disk it contains an MBR Bootstrap program described below and a Partition Table Devices that emulate a hard disk during system initialization must also contain an MBR because they must also have Partition Tables even if they are not bootable The BIOS will only boot an MBR from a device if that device is in the boot sequence stored in CMOS and if the MBR on the device is formatted correctly On the other hand if a device is not in the boot sequence but has a drive number it is still possible for a Real Mode program such as another MBR or bootloader to load and boot that devices MBR directly Contents 1 MBR Bootstrap 2 MBR Format 3 Partition table entry format 4 Traditional MBR 5 Dual Booting 6 Generic MBRs 7 Building a Custom MBR Bootstrap program 71 Initial Environment 72 Immediate Priorities 8 Storing an MBR to the disk 9 x86 Examples 10 Comments 11 See Also 111 Articles 112 External Links MBR Bootstrap An MBR is loaded by the BIOS at physical address 0x7c00 with DL set to the drive number that the MBR was loaded from The BIOS then jumps to the very beginning of the loaded MBR 0x7c00 because that part of the MBR contains the bootstrap executable code Typical MBR bootstrap code will do the following relocate itself away from the 0x7c00 physical address using a memory copy and usually a far jump determine which partition or hard disk to boot from either by looking for the active partition or by presenting the user with a selection of installed operating systems to choose from if the user selected an inactive partition then set the selected partition entry to active and clear the active bits of other partition entries use BIOS INT 13h commands to rewrite the MBR if the partition table entries were modified use BIOS INT 13h commands to load the Volume Boot Record VBR the bootsector of the bootloader from the beginning of the selected partition to physical address 0x7c00 set DSSI pointing to the selected partition table entry jump to 0x7c00 with CS set to 0 and DL set to the drive number 22092022 0001 MBR x86 OSDev Wiki httpswikiosdevorgMBR28x8629 27 Note it is intended for the value of DL and the DSSI pointer to be passed all the way into the kernel for the kernels use This also means that the relocated MBR should not be overwritten during the boot process because the DSSI pointer is aimed at a partition table entry inside that MBR and needs to remain valid MBR Format Notes See the Partition Table article for the format of each partition table entry field It is important to remember that the partition table entries are not aligned on 32bit boundaries Naming the partition table entries as First through Fourth is for convenience only The partition table entries are not required to be in any kind of order Any one of the partitions may be active There is supposed to be one active partition table entry at most Windows seems to verify and require that the partition it boots from be marked active Most other operating systems dont seem to care about the active bit in the partition table entry Offset Size bytes Description 0x000 4401 MBR Bootstrap flat binary executable code 0x1B8 4 Optional Unique Disk ID Signature2 0x1BC 2 Optional reserved 0x00003 0x1BE 16 First partition table entry 0x1CE 16 Second partition table entry 0x1DE 16 Third partition table entry 0x1EE 16 Fourth partition table entry 0x1FE 2 0x55 0xAA Valid bootsector signature bytes 1 This can be extended to 446 bytes if you omit the next 2 optional fields Disk ID and reserved 2 The 4 byte Unique Disk ID is used by recent Linux and Windows systems to identify the drive Unique in this case means that the IDs of all the drives attached to a particular system are distinct 3 The 2 byte reserved is usually 0x0000 0x5A5A means readonly according to httpsneosmartnetwikimbr bootprocess Partition table entry format Offset1 Size bytes Description 0x00 1 Drive attributes bit 7 set active or bootable 0x01 3 CHS Address of partition start 0x04 1 Partition type 0x05 3 CHS address of last partition sector 0x08 4 LBA of partition start 0x0C 4 Number of sectors in partition 1 Relative to the start of the partition entry 22092022 0001 MBR x86 OSDev Wiki httpswikiosdevorgMBR28x8629 37 Traditional MBR The DOS FDISK program was the first to ever use an MBR so that MBR became the defacto standard It also automatically became the standard for the minimum level of functionality of an MBR It was never changed after it was first introduced The whole point of the FDISK program is to manipulate the MBRs of the hard disks attached to a system When FDISK partitions a blank disk it writes an MBR to sector 0 of the drive When FDISK adds a new partition to a disk it adds an entry into the Partition Table in the MBR When FDISK makes a partition active it sets the flag byte in the Partition Table entry to 0x80 The MBR that FDISK uses is coded to relocate itself to 0x00000x0600 examine the byte at offset 0x1be 0x1ce 0x1de and 0x1ee to determine the active partition load only the first sector of the active partition which is expected to contain a DOS bootsector to 0x00000x7c00 hence the previous relocation set SI jump to 0x7c00 transferring control to the DOS bootsector Dual Booting A typical system may have several hard disks on it and each hard disk can have 4 standard partitions without going into the extra complication of Extended Partitions Each of those partitions could theoretically hold its own distinct bootable OS and filesystem However the standard x86 boot sequence will only ever boot the MBR from the C disk the first disk found during the disk detection phase And the standard MBR will only allow a single active partition on that disk and will only ever boot that one partition This is really completely inadequate It is not possible to modify the BIOSCMOS boot sequence but you can change the MBR So one solution is to replace the standard MBR with a custom Dual Booting MBR For a good example see John Fines SMBMBR in the External Links below A simple dual booting MBR will allow the user to select any partition on the current drive to boot A more complicated dual booting MBR will also allow the user to select other hard disks or even to specifically choose other partitions on other hard disks to boot For one MBR to load and run a different MBR off a different drive is called chain loading If all the MBRs are dual booting then the user can cycle through all the disks and choose to boot the correct partition from the correct drive A dual booting MBR is a huge improvement over the standard MBR The only problem is that an MBR bootstrap is limited to a little over 400 bytes of code Such a tiny program is insufficient to create a nice user friendly dual booting system that has commercial appeal It is enough space to hack up an interface that is good enough for the person who wrote it One way around this size limitation is to note that a bootloader can contain a lot more code than an MBR So a custom MBR can try to load a preferred bootloader if it can find one on some partition somewhere and that preferred bootloader can have a very pretty userfriendly interface that allows the user to select any partition off any drive to boot 22092022 0001 MBR x86 OSDev Wiki httpswikiosdevorgMBR28x8629 47 It is also nice if there is only one partition on only one drive ie there is no choice for a user to make if the MBR will boot that one partition automatically without bothering the user with prompts Generic MBRs As said above the MBR that is written to the disk by the DOS FDISK program is the most generic one But every disk partitioning app must write some sort of MBR to the disk and every generic bootloader may well need a custom MBR Fortunately there was a standard for them to conform to the FDISK MBR and they all did conform to it Some of them may be Dual Booting or have other features but they will all end up loading your bootloader at the standard address 0x7c00 with the DL register set to the boot drive number and DSSI pointing at the correct partition table entry of the correct MBR Building a Custom MBR Bootstrap program When the BIOS transfers control to the MBR bootstrap code the system is in Real mode The MBR will probably also run entirely in Real mode so it is important to understand Real mode addressing Selfrelocation is one of the things C cant do and most modern C compilers cant create code that is compatible with Real mode anyway So an MBR must be written in Assembly It is necessary to build an MBR that is exactly 512 bytes long How this is done depends on your assembler and linker The last 2 bytes must be the special boot pattern 0x55 followed by 0xAA and as said above the bootstrap portion of the MBR must be less than 447 bytes long You must also copy at least one partition table entry into the Partition Table portion of the MBR Initial Environment When the BIOS loads and runs an MBR it is loaded into memory at physical address 0x7c00 This is usually 0x00000x7c00 CS 0 offset address 0x7c00 However some BIOSes load to 0x7c00x0000 CS 0x07c0 offset address 0 which resolves to the same physical address but can cause problems A good practice is to enforce CSIP with a far jump near the beginning of your bootstrap code The MBR will probably need to immediately relocate itself anyway and that is a good time to enforce CSIP The BIOS passes very little useful information directly to the MBR In fact the only important number is the value in DL the drive number It needs to be passed to all later calls to BIOS function INT 13h so that byte in DL probably needs to be saved carefully The values in all the other registers and in most of memory are undefined Immediate Priorities It is important to immediately set up a stack and also to set the rest of the CPUs segment registers DS ES FS GS properly Setting up a stack involves pointing the SSSP pair of registers at some memory that is not being used for anything else and the address must be even The other segment registers should usually be set to 0 See the Memory Map x86 article to understand what memory is available during boot low memory from 0x500 to 0x7ffff generally In general you do not want to fragment your available memory or the memory of each 64K page if possible The MBR needs a stack and a place to relocate itself to The MBR will be loading a bootloader at 0x7c00 so it is reasonably convenient for the MBR to relocate itself either to somewhere around the 0x500 to 0x600 address 22092022 0001 MBR x86 OSDev Wiki httpswikiosdevorgMBR28x8629 57 range or to 0x7a00 ie just below 0x7c00 The stack can then point to 0x7c00 if the MBR is not at 0x7a00 or the stack can be just below the relocated MBR Storing an MBR to the disk To write an MBR to the first sector of a disk you must use special disk IO tools because the MBR by definition is not inside any disk partition The MBR only exists on the raw device There are quite a few disk editing tools available some are listed in Disk Image Utilities x86 Examples Assuming one knows how to read sectors from disk and has setup a function allowing them to do so and assuming the inputs to said function are EBX 32bit LBA Address CX Sector Count ESDI Buffer BYTE bootDrive Drive Number An extremely simple MBR complying to the standard could look like bits 16 org 0x0600 start cli We do not want to be interrupted xor ax ax 0 AX mov ds ax Set Data Segment to 0 mov es ax Set Extra Segment to 0 mov ss ax Set Stack Segment to 0 mov sp ax Set Stack Pointer to 0 CopyLower mov cx 0x0100 256 WORDs in MBR mov si 0x7C00 Current MBR Address mov di 0x0600 New MBR Address rep movsw Copy MBR jmp 0LowStart Jump to new Address LowStart sti Start interrupts mov BYTE bootDrive dl Save BootDrive CheckPartitions Check Partition Table For Bootable Partition mov bx PT1 Base Partition Table Entry 1 mov cx 4 There are 4 Partition Table Entries CKPTloop mov al BYTE bx Get Boot indicator bit flag test al 0x80 Check For Active Bit jnz CKPTFound We Found an Active Partition add bx 0x10 Partition Table Entry is 16 Bytes dec cx Decrement Counter jnz CKPTloop Loop jmp ERROR ERROR CKPTFound 22092022 0001 MBR x86 OSDev Wiki httpswikiosdevorgMBR28x8629 67 mov WORD PToff bx Save Offset add bx 8 Increment Base to LBA Address ReadVBR mov EBX DWORD bx Start LBA of Active Partition mov di 0x7C00 We Are Loading VBR to 0x07C00x0000 mov cx 1 Only one sector call ReadSectors Read Sector jumpToVBR cmp WORD 0x7DFE 0xAA55 Check Boot Signature jne ERROR Error if not Boot Signature mov si WORD PToff Set DSSI to Partition Table Entry mov dl BYTE bootDrive Set DL to Drive Number jmp 0x7C00 Jump To VBR times 218 nop Pad for disk time stamp DiskTimeStamp times 8 db 0 Disk Time Stamp bootDrive db 0 Our Drive Number Variable PToff dw 0 Our Partition Table Entry Offset times 0x1b4 nop Pad For MBR Partition Table UID times 10 db 0 Unique Disk ID PT1 times 16 db 0 First Partition Entry PT2 times 16 db 0 Second Partition Entry PT3 times 16 db 0 Third Partition Entry PT4 times 16 db 0 Fourth Partition Entry dw 0xAA55 Boot Signature Of course this is an extremely simplified MBR and is only able to load the first active partition Comments See Also Articles System Initialization x86 External Links Fines MBR code on osdever httpwwwosdevernetdownloadsbootsectorssmbmbr03zip OSRC httpwwwnondotorgsabreosarticlesTheBootProcess for more boot sector info Ranish Partition Manager httpwwwranishcompart Disk partitioning software Retrieved from httpswikiosdevorgindexphptitleMBRx86oldid23491 Categories X86 Bootloaders This page was last modified on 29 March 2019 at 1453 22092022 0001 MBR x86 OSDev Wiki httpswikiosdevorgMBR28x8629 77 This page has been accessed 147403 times