tags : Computer Architecture, Operating Systems
I am writing this in 2020, so you’ll probably never need to understand this sidenote. IDK why I am writing this.
The motherboard has some type of ROM chip that holds the firmware. This is the software that the CPU first starts executing at power on. This firmware(BIOS / UEFI) is supposed to do initialization of the hardware, typically provides some diagnostic output, and provides a way for the user to configure the hardware, then locates and loads the =boot loader=(grub etc.) into the RAM, which in turn locates and loads the OS.
- Partitioning is typically the first step of preparing a newly installed disk, before any file system is created. The disk stores the information about the partitions’ locations and sizes in an area known as the partition table.
- Partitions are simply a way to split a block device into multiple ones.
- There are two worlds here;
(BIOS+MBR)and(UEFI+GPT), let’s try to understand them.
BIOS
BIOS simply loads and executes the first sector from the secondary storage, called the MBR. BIOS let’s you choose which disk/secondary storage if you wish to change it. (this is what you do when you want to format you computer etc.) MBR describes the partitions on the disk in a particular format, and contains a ‘boot loader’ or the start of the boot loader.
MBRalso only supports up to four primary partitions.- Limited size of 32 bits for block addresses.
- Only upto 2TiB HDD etc.
UEFI
GPT on the other hand is part of UEFI specification but can support BIOS as mentioned below. How UEFI works is completely different from how BIOS works. UEFI does not look for something in the first sector of the bootable storage device. Hecc, UEFI can boot remotely. What it does is make use of EFI system partition(ESP).
UEFI Firmwares by specification should be capable of reading the FAT12, FAT16 and FAT32 variants of the FAT format maintained as part of the UEFI specification. An ESP is really just any partition formatted with one of the UEFI spec-defined variants of FAT and given a specific GPT partition type to help the firmware find it. When you write a bootloader for native UEFI, you write in this executable format defined by UEFI spec.
With a BIOS firmware, your firmware level ‘boot menu’ is necessarily, the disks connected to the system at boot time — no more, no less. This is not true with a UEFI firmware. For handling boooting multiple targets, UEFI has the /UEFI boot manager/(efibootmgr command can be used to manipulate the UEFI bootmanager), NVRAM is non-volatile RAM that is used in EFI to store variables that need to persist between boots.
- 64 bits for block addresses in
GTP.
Support
Supporting MBR or GPT is task of the Operating System. UEFI Firmwares can also implement BIOS compatibility mode, where UEFI would go look for MBR on a disk.
- BIOS+MBR ✔️
- EFI+GPT ✔️ (the one you’re probably using rn)
- BIOS+GPT ✔️ (Linux&FreeBSD only)
- EFI+MBR ✔️ (BIOS compatibility mode)
Source: This blogpost, Wikipedia and few more blogposts.