OS development 101 - How to make a bootloader part 1 - Hello World

I made a discord server for everyone interested in low level programming and malware.
Check it out:

Welcome to the first video in my new mini-series about bootloader development.
I’m planning to make 3 videos in total (maybe more, we’ll see how it goes).
Topics that will be covered in the series:
- Real Mode
- Protected Mode
- BIOS
- Global Descriptor Table
- CHS
- Loading Kernel

Enojy!

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My github:
OSDEV

Timestamps:

0:00 What happens at PC startup?
1:30 Real Mode
2:53 Setting up the project
5:20 Writting bootable code
13:00 Makefile and testing

How to write a bootloader from scratch
What happens during a PC boot sequence
Understanding the Power-On Self Test (POST)
How does the BIOS or UEFI work during boot
What is real mode in computing
How to write bootable code for a PC
Steps to create a simple bootloader
How does a BIOS initialize hardware components
What is the Master Boot Record (MBR)
Difference between MBR and GPT
How to load a 32-bit kernel with a bootloader
What are the first 512 bytes of a boot device
How does memory work in real mode
Writing a bootloader with a "Hello World" message
What are the last two bytes of the MBR
How does the boot order affect the boot process
What is the role of the PSU in the boot sequence
How does the BIOS/UEFI transfer control to the kernel
What is the purpose of the Power-On Self Test
Steps to load a kernel using a bootloader

[Hideko]

Hi 👋 just wanted to let you know that your content is very special. Tech Content Creators focus a lot on Hacking and Defense but Malware Analysis and Reverse Engineering need a little bit of attention so thanks !

[jordanarnould]

3:04 "This is the compiler for our assembly" that's called an assembler

[Lulxec]

your content is awesome, been wanting to learn assembly for a while and you've just gave me the motive to do so

[RebrandSoon]

This is very cool. I recognized some of the opcodes from my experimenting on and off a few years ago. Great work. Subbed.

[thermiusthermius]

Hi, I'm from Brazil and I was amazed by your content. I study operating systems, assembly, C/C++ and your video helped me understand how the boot process occurs.

[kevinmccallister]

I love this! can we have a full series where you build a simple OS?

[Bestfriend-qs]

The POST is performed by the BIOS/UEFI. Good job. Keep going.

[jannikwinkler]

Thank you soo much i am searching for this for years❤❤

[小张同学-vi]

you can also code in C and let the compiler create the assembly for you.

[insanelydigitalvids]

A very good video. Great content and easy to understand. Thank you!

[maxmuster]

I like to switch from the 16 bit Real Address Mode with 64 kb segment size for all segment register, into the 16 bit "Big" Real Mode with a segment size of 64 kb for CS and SS segment, but for DS, ES, FS and GS segment size of 4 gb with A20 address line on. This Mode is compatible to the 16 bit BIOS function of the mainboard and graphic BIOS and additional alows to write/draw into the linear framebuffer (located in the 4th gb) of VBE 2 or VBE 3 videomodes for high resolutions. Example 1920x1200x32 with 16: 10 aspect ratio for 28" LCD monitor using Radion 9750 PCIe with intel Core2quad CPU 2700mhz.

[LunaticEdit]

To be clear, this is more for how retro computers used to boot up (legacy BIOS boot). These days you use EUFI and boot from an .efi file (typically with digital signature requirements). I mean it's great to learn how things used to work, but if you're looking for a modern method this is not it.

[tech-german]

nice, waiting for next episode. Please post video for ARM64 also

[alexsurguy]

Would be nice to be able to figure out how to flash a bootloader to xbox series to boot on brand new SSD

[maxmuster]

I made some open source projects for DosBox using batch files and x86 assembly. Have fun.😊

[explore]

Great! Out of curiosity, what sources did you use to learn these concepts?

[maxmuster]

cli and sti are needed if we want to change an interrupt vector address offset/segment in memory like the timer interrupt 8 at the address 0:0020 = 4 bytes. If we don’t stop the timer interrupt and we try to override the old vector the interrupt can occur between changing the offset and changing the segment address and accidently jumping to a wrong address with no valid interrupt service routine(ISR). We have to prevent this.

[southernflatland]

Bruh, mov ax, 0x00?

xor ax, ax

Faster to manipulate CPU registers than load from RAM. And yes, that instruction clears the ax register.

[maxmuster]

We can write our string directly into the screen memory without using a bios function.

vga text screen default 80x25 with 16 colors and 8x16 character size and 8 text pages

mov ax, 0b800h
mov es, ax
mov di, Location
cld
mov si, OFFSET string ; ds:si
mov ah, color
mov cl, len
L1:
lodsb
stosw
dec cl
jnz L1
....

string DB "Hello World"
len = ($ - string)
Location = (Y * 160) + (X * 2)

[SXCgt]

hey there! its not really working out for me.
can you help with this error?
nasm -f bin .src/boot.asm -o .bin/boot.bin
.src/boot.asm: fatal: unable to open output file `.bin/boot.bin'
make: *** [Makefile:2: all] Error 1
its from terminal after running make all
edit: i fixed it. I just did spaces instead of tabs.