Question about the first 100MB on SD (for boot-up)

Hi People :slight_smile: Just got my BananaPi M3 and downloaded a Crux-image. I figured out, that there is a gab of about 100MB before the first partition (the VFat one). As well I found out, that it contains very important stuff for booting. I like to know, what exactly is going on, when the BananaPi M3 is booting up, and as well I like to study the source of the boot-program, stored in that gap. What ever it contains, is it used by Linux AFTER kernel boot, or is it stuff, only FOR the boot-process, and forgot and/or replaced, after the kernel has started? Is it kind of comparable to a PC-BIOS? Where to find the docs, and sources? Best rgds, and thanx for any help!