I understand that the BIOS is in ROM, and the manufacturer "typically" designs it to point to the first sector of the "active" storage device's first sector or CHS (0,0,1)*physical and CHS (0,0,0)*logical which is either an MBR or VBR based on your preference of partitioning.


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 to actually load an operating system.

The boot signature identifies the boot loader using a two-byte hexadecimal sequence, so I'm guessing the signature has to be an offset in the same sector? Therefore then assuming the boot loader must be in this same sector?

  • MBR (1983) predates LBA (1986ish) by a few years, making it rather difficult to have MBR dependent on LBA. – 8bittree Sep 21 '15 at 16:07
  • Yes I understand that, but HOW is the BIOS identifying the boot sector using only a two-bytes?! If the BIOS can do it in two bytes, why aren't we using two bytes for CHS or LBA to identify sectors as does the BIOS. – Jordan Davis Sep 21 '15 at 16:14
  • I just updated the question to make it more clear. – Jordan Davis Sep 21 '15 at 16:18
  • The boot sector signature isn't used to find where on the disk the boot sector is located (it's assumed to be the first sector, or else nonexistant), it's used after reading in that first sector to check if that first sector is, indeed, a boot sector, or if it's some mystery thing that the BIOS should ignore. – 8bittree Sep 21 '15 at 16:23
  • Got it! yea was just reading that it identifies the boot loader and that by default goes to the sector. – Jordan Davis Sep 21 '15 at 16:27

The basic order of operations that happens while a BIOS is trying to find something to boot is:

  1. Load first sector (512 bytes) of the device you're trying to boot from (HDD, SSD, Floppy Disk, Optical Disc, etc) into memory
  2. Check if the 511th and 512th bytes are 0x55 and 0xAA, respectively.
    • If not, go back to step 1 and try the next device, or give up if there aren't any more.
    • If yes, start executing code at the beginning of this sector, thus passing control to (hopefully) a boot loader/manager.

You might find the OSDev wiki page on the Boot Sequence useful. The MBR page also has a useful table explaining the layout of that first sector. I've recreated it here with some simplification:

Offset | Size (bytes) | Description
    0  | 436          | MBR Bootstrap (flat binary executable code) 
0x1b4  |  10          | Optional "unique" disk ID
0x1be  |  64          | MBR Partition Table, with 4 entries
0x1fe  |   2          | (0x55, 0xAA) "Valid bootsector" signature bytes

Note that the BIOS doesn't necessarily pay any attention to or even know about the disk ID or the partition table.

enter image description here

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  • Well I know it doesn't load the entire disk into memory, but I know what you mean... but what is it checking for in bytes 511 and 512? I mean I know the boot signature is located there but what is the boot signiture declaring to the BIOS? – Jordan Davis Sep 21 '15 at 16:47
  • That link is really good by the way. – Jordan Davis Sep 21 '15 at 16:51
  • The BIOS simply assumes that if the boot signature is there, that there is bootloader code at the beginning of that sector. Note that it is an assumption. There is a 1/65536 chance that there is random garbage that just happens to put 0x55AA in the right spot. – 8bittree Sep 21 '15 at 16:55
  • Haha :D kk got it, and thats at the very end of the sector being that it's bytes (511,512) correct? – Jordan Davis Sep 21 '15 at 16:59
  • 1
    This answer on Stack Overflow suggests some reasoning behind those numbers (alternating pattern in binary). But it still seems to boil down to just being magic numbers. – 8bittree Sep 21 '15 at 17:36

BIOS code is in ROM (or EEPROM these days). It loads first sector from the disk (#0 in LBA notation or c=0,h=0,s=1 in CHS notation), verifies that last two bytes are 0x55 and 0xAA and transfers control to this sector.

So, MBR is actually identified by its address, #0. And 55 AA signature is just for verification. If first sector is zero-filled (as for new HDDs), BIOS can detect this by missing 55 AA signature and try to boot from another disk, or PXE, or ROM BASIC, or whatever.

0x55 0xAA is not an offset of MBR, actual offset is zero.

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  • so your saying if the device doesn't have an MBR than it's zero-filled therefore missing the 55AA signature? – Jordan Davis Sep 21 '15 at 16:44

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