# Bootloaders

The Linux kernel is not a standalone executable. It must be loaded into memory
by a bootloader, which performs early system initialization and transfers
control to the kernel. On BIOS-based systems, this typically involves a boot
sector, while on UEFI systems the kernel may be loaded as an EFI executable via
the EFI stub.

Common Bootloaders are GRUB and Systemd-boot.

Here is a latex diagram of the boot process:

$$
\text{BIOS/UEFI} \rightarrow \text{Bootloader} \rightarrow \text{Kernel} \rightarrow \text{Initramfs (loaded by bootloader)} \rightarrow \text{Userspace}
$$



## Comparison of bootloaders

Below is a comparison of some of the most common bootloader's used for Linux, 
along with their support:

| BIOS                 | UEFI | MBR     | GPT     | Multiboot | Filesystem support |
|----------------------|------|---------|---------|-----------|--------------------|
| GRUB                 | Yes  | Yes     | Yes     | Yes       | Built in           |
| systemd-boot         | No   | Yes     | Yes     | Yes       | Built in           |
| EFI boot stub        | No   | Yes     | Yes     | Yes       | Firmware           |
| Limine               | Yes  | Yes     | Yes     | Yes       | Limited            |
| rEFInd               | No   | Yes     | Yes     | Yes       | Extensible         |
| Syslinux             | Yes  | Partial | Yes     | Partial   | Limited            |
| Clover               | Yes  | Yes     | No      | Yes       | Extensible         |
| GRUB Legacy          | Yes  | No      | Yes     | No        | Limited            |
| LILO                 | Yes  | No      | Partial | Yes       | Limited            |
| Unified kernel image | No   | Yes     | Yes     | Yes       | Firmware           |