intermezzOS workshop

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![rhizome](../public/images/rhizome.jpg)

## .subtitle[The Boot Process]

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# Roadmap

- multiboot headers
- hello world
- making an iso / running in qemu
- paging
- gdt
- jump to long mode
- hello rust

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# multiboot headers

- Hardware loads the BIOS
- The BIOS loads a bootloader (actually two)
- The bootloader wants to load your kernel
- Multiboot is a spec for kernels + bootloaders

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# multiboot headers

Request:

```http
GET / HTTP/1.1
Host: intermezzos.github.io
```

Response:

```http
HTTP/1.1 200 OK
Content-Type: text/html

<html><p>sup</p></html>
```

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# multiboot headers

Multiboot headers are kind of like HTTP response headers. They let the
bootloader and the kernel talk to each other, exactly like the HTTP spec
lets a client and server talk to each other.

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# hello world

```x86asm
start:
    mov word [0xb8000], 0x0248 ; H
    mov word [0xb8002], 0x0265 ; e
    mov word [0xb8004], 0x026c ; l
    mov word [0xb8006], 0x026c ; l
    mov word [0xb8008], 0x026f ; o
```

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# making an iso

- the bootloader has to load the OS from somewhere
- ISO is the file format for CD-ROMs
- We take our code, and make a CD-ROM out of it

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# running in qemu

- qmeu lets us pretend to have a virtual computer
- we boot it off that CD-ROM
- if we're not on an x86_64 system, it emulates one

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# the path to long mode

- GRUB starts us in 32-bit mode
- we want to be in 64-bit mode
- that takes work

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# the path to long mode

- 16 bit: real mode
- 32 bit: protected mode
- 64 bit: long mode (compatibility)
- 64 bit: long mode (forreals)

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# paging

- a huge topic in CS
- conversion between memory addresses and address spaces
- physical address, virtual address
- required for long mode
- we'll do the most basic possible thing
- after we turn this on, we're in compatibility mode

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# gdt

- "global descriptor table"
- entirely legacy but required by the hardware
- three entries in the table
- `lidt`: special assembly instruction

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# jump to long mode

- my favorite part of the boot process
- `jmp` in a way that should be impossible
- "oh wait, yeah, I'm totally a 64-bit enabled computer"

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# hello rust

Now that we're in long mode, we can use Rust.