Making Simple Bootloader using x86 Assembly

In this video I will demonstrate how to write a simple bootloader from scratch using the x86 Assembly Programming Language with the NASM assembler and afterwards running it with QEMU

[GiovanniCKC]

i love how to doesn't just go full on-- we aren't jumping to kernel or initializing a gdt or some such, we literally just are shown the most vital basic skeleton for a nasm program running as a bootloader with the ending magic word and whatever. super nice!

[GlorytoTheMany]

You could point SI to hello and then use a LODSB loop to read characters in line, then you don't need to increment SI. By the way, if you do increment SI, you could use INC. It's more ergonomic in assembly and probably even takes less bytes. Infinite loops are quite bad for modern CPUs (also old ones I suppose), use CLI HLT to shut it down.

[markusfassbinder]

wow, I wanted to do something like that, and I randomly found this video.
good video btw!

[GrindAlchemyTech]

I really appreciate you.... thank you♌🙌🏽💗

[narkroshi]

This is the perfect mix of concise and descriptive

[adderek]

That reminds me my fights to get into protected mode and then flat addressing mode. And to handle FAT32 and EXT2. Those were the times. Nowadays things are so complex - so many building blocks are there...

[eugrus]

Great content! Would be very interesting if you could illustrate the differences between a BIOS-executable, COM, MZ, PE and ELF with such minimalistic code!

[maxmuster]

We can write into the screen memory at address b800:0000 start in the upper left corner with ASCII byte and attribut byte with colors, no software interrupt needed.

[platoek]

It would be great, if you put the links to the docs you used in the video in the description, for all future videos, and maybe some existing ones. Like the link to the document you have opened at the right in this video.

[RegisMichelLeclerc]

First, you'll have to manage your stack if you want to go further:
mov ax, cs
mov ds, ax
dec ax
mov ss, ax
mov sp, 0x000e ; 16 bytes of stack is tiny but enough

Then write the message:
cld ; direction=forward for lodsb
call write ; here's the magic...
db "hello world", 0
jmp $ ; back from write, jump to whatever code to execute next...

The write procedure looks like that:
write:
pop si ; get address of the string
mov ah, 0x0e ; teletype: out char
print: ; loop label
lodsb ; same as mov al, [si] followed by inc si
jz end_str ; check if zero, no need for cmp
int 0x10 ; out char
jmp print ; short jump
end_str:
jmp si

Actually, you want to put your stack just under 0x8000 (0x7cff:00fe) so to load your kernel at 8000:0000, the boot loader only reads the sectors to the memory, nothing more

[sudeshryan]

just curious:
where does the cpu get the actual character from at this stage? does the cpu/bios has a basic font file embedded in it by the manufacturer?

[semmelsamu]

Well explained, just one question: Why can you define "Hello World" only after executing the print code? "Hello World" shouldn't be defined yet e.g in line 8, right?

[ntoslinux]

Thanks subscribed. What is the top toolbar on your desktop which shows the 1/2/3/4 etc.

[shawnm]

How does it know where to start without main ? does it just start at whatever line is after org ?

[kingivan]

Can you explain why you use cmp on line 11? *Edit*: Nevermind, I didn't see the jne. Nice video!

[its_code]

Please make full assembly language tutorial 😢🎉❤❤❤❤❤❤

[deepphasejuniorgamerz]

Can I use this code for making my own operating system

[tylerbradbury]

how is he using what looks like i3 on windows?

[sto]

I don't understand why I'm seeing an upraise of youtubers making assembly language videos (not that it's a bad thing). All the new gens should be programming the easy, lame and slow stuff like Python and Rust. Keep it up.