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| 1 | +##OS Notes |
| 2 | + |
| 3 | +###Boot sequence |
| 4 | + |
| 5 | +- a computer is turned on, it runs through a series of diagnostics (POST - Power On Self Test). |
| 6 | +- BIOS looks for a bootable devices which contains byte sequence 0x55 0xAA at offset 510 511 and loads (usually 1st sector) into memory at 0x0000:0x7c00 (segment 0). CPU is currently in Real Mode. |
| 7 | +- assuming booting from a hard drive, we have only 446 bytes available |
| 8 | +- if we executes the kernel directly -> it is called Single Stage Bootloader (it is nearly impossible to do since a lot of things need to be done like setuping GDT, IDT, loading/jumping 32bit kernel ...) |
| 9 | +- consists a single 512 byte bootloader above, however, it only loads and executes another loader Second Stage Bootloader which is used to setup/load 32bit kernel |
| 10 | + |
| 11 | +**Reference** |
| 12 | + |
| 13 | +-[Wiki boot sequence](https://en.wikipedia.org/wiki/Booting#Boot_sequence) |
| 14 | +-[Wiki MBR layout](https://en.wikipedia.org/wiki/Master_boot_record#Sector_layout) |
| 15 | +-[OS Development - Bootloader](http://www.brokenthorn.com/Resources/OSDev5.html) |
