Question

again I have a question that is not so easily googlable. Let’s assume BIOS has loaded my bootloader from designated device, how do I now find out which device it is so I can load the rest of my os into ram?

Answer 1

If it’s MBR bootstrap (And possibly also otherwise? I’m not 100% sure) DL will contain
the drive number i.e. you can do something like this (substitute your own memory layout and so on):

EntryPoint: 
    xor ax, ax
    mov ss, ax
    mov ds, ax
    mov sp, 0x7c00
.DiskReset:
    mov ah, 0
    int 0x13
    jc .DiskReset
    mov ax, 0x50 ; load to 0x500 linear address. It has unused space up to 0x7bff
    mov es, ax
    xor bx, bx
    mov ax, 0x023B ; count = 0x3b = 59, the maximum..
    mov cx, 0x0002
    xor dh, dh ; leave (dl = drive number) intact
    int 0x13
    jnc .ReadDone
    mov si, ReadError
    call PrintString16
    jmp .DiskReset
.ReadDone:
    ; ......