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Move 64-bit assembly to own file

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Philipp Oppermann
2015-08-22 15:25:12 +02:00
parent 5b09bda567
commit 0503c59198
1 changed files with 11 additions and 2 deletions
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13
_posts/2015-08-19-entering-longmode.md
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@@ -388,6 +388,8 @@ Now there is just one last step left to enter the true 64-bit mode: We need to l
[equ]: http://www.nasm.us/doc/nasmdoc3.html#section-3.2.4 [equ]: http://www.nasm.us/doc/nasmdoc3.html#section-3.2.4
```nasm ```nasm
global start
extern long_mode_start
... ...
lgdt [gdt64.pointer] lgdt [gdt64.pointer]
@@ -398,7 +400,15 @@ Now there is just one last step left to enter the true 64-bit mode: We need to l
mov es, ax mov es, ax
jmp gdt64.code:long_mode_start jmp gdt64.code:long_mode_start
```
The actual `long_mode_start` label is defined as `extern`, so it's part of another file. The `jmp gdt64.code:long_mode_start` is the mentioned far jump.
I put the 64-bit code into a new file to separate it from the 32-bit code, thereby we can't call the (now invalid) 32-bit code accidentally. The new file (I named it `long_mode_init.asm`) looks like this:
```nasm
global long_mode_start
section .text
bits 64 bits 64
long_mode_start: long_mode_start:
; print `OKAY` to screen ; print `OKAY` to screen
@@ -411,8 +421,7 @@ bits 32
``` ```
_Congratulations_! You have successfully wrestled through this CPU configuration and compatibility mode mess :). You should see a green `OKAY` on the screen. If you still have energy, here are some notes on this last step: _Congratulations_! You have successfully wrestled through this CPU configuration and compatibility mode mess :). You should see a green `OKAY` on the screen. If you still have energy, here are some notes on this last step:
- the `jmp gdt64.code:long_mode_start` is the mentioned far jump - As the CPU expects 64-bit instructions now, we use `bits 64`
- As the CPU expects 64-bit instructions now, we to switch to `bits 64` before `long_mode_start`. Don't forget to switch back to `bits 32` after it because our other functions are 32-bit functions.
- We can now use the extended registers. Instead of the 32-bit `eax`, `ebx`, etc. we now have the 64-bit `rax`, `rbx`, … - We can now use the extended registers. Instead of the 32-bit `eax`, `ebx`, etc. we now have the 64-bit `rax`, `rbx`, …
- and we can write these 64-bit registers directly to memory using `mov qword` (quad word) - and we can write these 64-bit registers directly to memory using `mov qword` (quad word)