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Add some explanation for the long mode check

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Philipp Oppermann
2016-02-01 23:57:09 +01:00
parent c2c863c7f3
commit d6efe62c80
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posts/2015-08-25-entering-longmode.md
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@@ -163,6 +163,7 @@ check_long_mode:
mov al, "2"
jmp error
```
It tries to invoke a CPUID function to test if the long mode is available. But this function is not part of CPUIDs core functions, therefore we need to test if the so-called extended functions are available before. So the first `cpuid` call tests whether the function is available that checks long mode support. And then the second `cpuid` call uses that function to test whether long mode support is available. Whew!
### Putting it together
We just call these check functions right after start: