# Boot Process

The boot process chain is the following

```
Power on -> Self-Test -> Find MBR/GUID -> Read MBR/GUID -> Find bootable
partition -> Start found bootloader -> Start OS
```

## Master Boot Recoder (MBR)

The MBR is 512 Bytes in size and starts at the first sector of the storage device.
(It contains the following sections)(https://www.writeblocked.org/resources/MBR_GPT_cheatsheet.pdf)

```
----------------------------
|bootstrap code (445 bytes)|
----------------------------
|partition table (64 bytes)|
|    - partition 1         |
|    - partition 2         |
|    - partition 3         |
|    - partition 4         |
|     each 16 bytes        |
|    4 partitions max      |
----------------------------
|  mbr signature (2 bytes) |
|  this is: 55 aa (le)     |
----------------------------
```

A partition represented in the MBR contains following hex values in succession
one after another, starting at the first one at byte position 0x00

1. Boot indicator (1 Byte), 0x80 means it is bootable and 0x00 means it is not
2. Starting Cylinder Head Sector (CHS, 3 Bytes), physical cylinder, head and sector number
3. Partition Type (1 Byte), indicating the file system
4. End of CHS address (3 Bytes), contains `FE FF FF`
5. Logical Block Addressing (LBA, 4 Bytes), logical starting address of the partition
6. Number of Sectors (4 Bytes) of the partition

### Find a partition using the LBA

The address can be calculated through the following formula

```
LBA value x sector size = starting byte
```

To calculate the size of the parition use the following formula

```
Number of sectors x sector size = parition size
```

> **NOTE**: Watch out for LE and LB.

## GPT

```
--------------------------------
| Protective MBR               |
--------------------------------
| Primary GTP Header           |
--------------------------------
| Partition Entry Array        |
--------------------------------
| Backup GPT Header            |
--------------------------------
| Backup Partition Entry Array |
--------------------------------

```