I was studying for my Linux+ exam and while taking notes I was attempting to explain the different stages of the grub bootloader to myself in my notes and I got hung up on a specific fact. To my understanding on hard drive with a mbr partition table the first 512 bytes (inner disk) store the partition table and the first stage of the bootloader (in my case grub). I wanted to say:
MBR style disks utilize the first 512 bytes of the drive for storing the partition table and the stage 1 bootloader. The stage 1 bootloader is the first of 2 parts of the bootloaders portion of the boot sequence. First the BIOS/UEFI looks within the first readable sector (512 bytes) of a mbr partitioned drive for the location of stage 1.5 of the bootloader. Stage 1 is essentially a pointer to tell BIOS/UEFI where to look for the second part of the bootloader. The second portion of the bootloader is stored on the first readable sector of the hard drive (usually several kib in size).
But I'm not able to confirm through research that it is correct to say the first stage or second stage is the first readable sector and I don't know if it would vary depending on which components perspective, the operating system or the firmware in the case that the operating system can't read the mbr.
To sum it up my questions are:
When people say "the first readable sector" are they talking from the operating system or firmware (BIOS/UEFI) perspective?
Does a difference exist between what the operating system can see of a mbr disk in comparison to what the computers fimware can see (UEFI/bios)?
For GPT Disks does the BIOS Boot partition only exist for backwards compatibility with BIOS?
Some of the graphics I was looking at to understand this topic:
Credits: anchor.com.au