Jumping headlong into long mode

We are so close to Rust! Just a little bit of assembly code needed.

Our last task is to update several special registers called 'segment registers'. Again, we're not using segmentation, but things won't work unless we set them properly. Once we do, we'll be out of the compatibility mode and into long mode for real.

Updating the first three registers is easy:

; update selectors
mov ax, gdt64.data
mov ss, ax
mov ds, ax
mov es, ax

Here's a short rundown of these registers:

  • ax: This isn't a segment register. It's a sixteen-bit register. Remember 'eax' from our loop accumulator? The 'e' was for 'extended', and it's the thirty-two bit version of the ax register. The segment registers are sixteen bit values, so we start off by putting the data part of our GDT into it, to load into all of the segment registers.
  • ss: The 'stack segment' register. We don't even have a stack yet, that's how little we're using this. Still needs to be set.
  • ds: the 'data segment' register. This points to the data segment of our GDT, which is conveniently what we loaded into ax.
  • es: an 'extra segment' register. Not used, still needs to be set.

There's one more register which needs to be updated, however: the code segment register, cs. Should be an easy mov cs, ax, right? Wrong! It's not that easy. Unfortunately, we can't modify the code segment register ourselves, or bad things can happen. But we need to change it. So what do we do?

The way to change cs is to execute what's called a 'far jump'. Have you heard of goto? A jump is just like that; we used one to do our little loop when setting up paging. A 'far jump' is a jump instruction that goes really far. That's a little bit simplistic, but the full technical details involve stuff about memory segmentation, which again, we're not using, so going into them doesn't matter.

Here's the line to make our far jump:

; jump to long mode!
jmp gdt64.code:long_mode_start

Previously, when we used jne to set up paging, we passed it a label to jump to. We're doing the same here, but this time, the label is long_mode_start. We'll define that in a minute. Before we do, we should talk about the other part of this instruction: gdt64.code:. This is another label, the one to the code entry of our GDT. This foo:bar syntax is what makes this a long jump; we're also providing our GDT entry when we jump. When we execute this, it will then update the code selector register with our entry in the GDT!

I've always loved this part of the boot process. It's very visual for me; your OS makes a long leap of faith, and comes out the other side realizing that it has more abilities than it thought! A classic tale of bravery.

But where is this long_mode_start that we're jumping to? Why, defined at the bottom of our file, of course! Put this at the end of boot.asm:

section .text
bits 64


A new section! It's another text section, like the rest of our code. But there's a new bits 64 declaration: we're in honest-to-goodness 64-bit mode now!

Finally, we have our long_mode_start label, and then a humble hlt instruction to stop execution.

With this set up, we are now officially in long mode! Congrats! Let's do a small thing to prove it. Modify this code to look like this:


    mov rax, 0x2f592f412f4b2f4f
    mov qword [0xb8000], rax


We have a new fancy register, rax! Like eax is a 32-bit version of ax, rax is a 64-bit version of eax. The 'e' in eax stood for 'extended', the 'r' in rax stands for... register. Can't make this stuff up.

Anyway, we put a mystery sixty-four-bit value into rax, and then write it into 0xb8000. If you recall from earlier, that's the upper-left part of the screen. The qword bit stands for 'quad-word', aka, 64-bit. A word is 16 bits, a double word is 32 bits, so a quad word is 64-bits.

What does it say? Well, you'll have to run it and find out. 😊

Our next step is going to be a big one: moving to writing Rust code! I hope you've enjoyed this tour of assembly and legacy computer junk. You've made it through the toughest bits: getting started is the hardest part.