Build OS from Scratch 1

I've been always curious as to how a computer works, all the way from the bottom level to the top level. We use computers everyday, so we are familiar with the user-level, i.e., the top-most level, but how many people in the world actually know what is going on in the most deep down level?

I took a course during my undergrad how a computer and OS works, but it has been too long since then, and I don't remember much. Furthermore, when I was taking that course, I lacked much of the necessary knowledge to really absorb the materials; I wasn't even familiar with the most basic shell commands, such as cp, mv, etc.

Now that I think about it, that course was really something I want to learn now; unfortunately, I can't access the course materials any more. Thankfully, there are abundant other resources that are accessible from simple Google search, so I am going to dive into these very low level materials one by one.

I will be starting a series of short blog post to summarize what I learn in my own words, starting with this one. For this post, I am referring to this excellent document.

Boot process looks for bootable sector in any available disk or media. The bootable sector is flagged by magic number 0xAA55 in the last two bytes. The boot sector refers to the first sector in the media.

Let's install qemu to emulate a computer, and nasm to compile assembly.
$ sudo apt-get install qeum nasm -y

Next, create a simple boot sector that prints 'Hello' by first creating assembly source file hello.asm
;
; A simple  boot  sector  that  prints a message  to the  screen  using a BIOS  routine.
;
mov ah, 0x0e    ; teletype mode (tty)
mov al, 'H'     ; char to write to
int 0x10        ; print char on sreen
mov al, 'e'
int 0x10
mov al, 'l'
int 0x10
mov al, 'l'
int 0x10
mov al, 'o'
int 0x10
jmp $           ; Jump to the  current  address (i.e.  forever).
;
; Padding  and  magic  BIOS  number.
;
times  510-($-$$) db 0  ; Pad  the  boot  sector  out  with  zeros
                        ; $ means address at the beginning of the line
                        ; $$ means address at the beginning of the session (file)
dw 0xaa55               ; Last  two  bytes  form  the  magic  number ,
; so BIOS  knows  we are a boot  sector.

and compile to binary format
$ nasm hello.asm -f bin -o hello.bin

For more details on int 0x10, refer to here.

To boot this sector, simply run
$ qemu-system-x86-64 hello.bin

To view the boot sector in HEX, run
$ od -t x1 -A n hello.bin

You should see it boots up successfully by printing out 'Hello'!!