OSDev.org

How to do it?
On both.

Please read the big text near the top of the page that says "The OSDev.org Wiki - Got a question? Search this first!" and then click on it and follow the instructions on the wiki.

Edit: I can see you already have visited the wiki. In that case you should read it. There is a bare bones article, if you strip out the C part (but still use a cross-binutils), then you have a protected mode OS in assembler.

Does that applies to custom bootloader too?

My code:

.code16

mov ah, 02h
mov al, 1
mov ch, 0
mov cl, 0
mov dh, 0
mov dl, 0
mov bx, 0x1000
mov es, bx
mov bx, 0x00
int 0x13
jmp 0x1000:0x00

will this execute code start of first sector?

Please, read the wiki, it clearly states that the bare bones article gives you a multiboot ELF kernel loaded with GRUB, an existing bootloader. If you wanted help with a custom bootloader, you should have said that up front.

Please stop what you are doing and read the classic http://www.catb.org/esr/faqs/smart-questions.html article. It will help you help us help you.

But, seriously, STFW.

Sorry, I am curious and
I am sorry I messed up two topics.

For everyone: http://wiki.osdev.org/Real_mode_assembly_I end