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Explain the CPUID check

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Philipp Oppermann
2016-02-01 23:40:45 +01:00
parent cbf4534eca
commit b9c94baaaa
1 changed files with 7 additions and 2 deletions
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9
posts/2015-08-25-entering-longmode.md
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@@ -130,13 +130,18 @@ check_cpuid:
; Compare EAX and ECX. If they are equal then that means the bit wasn't
; flipped, and CPUID isn't supported.
xor eax, ecx
jz .no_cpuid
cmp eax, ecx
je .no_cpuid
ret
.no_cpuid:
mov al, "1"
jmp error
```
Basically, the `CPUID` instruction is supported if we can flip some bit in the [FLAGS register]. We can't operate on the flags register directly, so we need to load it into some general purpose register such as `eax` first. The only way to do this is to push the `FLAGS` register on the stack through the `pushfd` instruction and then pop it into `eax`. Equally, we write it back through `push ecx` and `popfd`. To flip the bit we use the `xor` instruction to perform an [exclusive OR]. Finally we compare the two values and jump to `.no_cpuid` if both are equal (`je` – “jump if equal”). The `.no_cpuid` code just jumps to the `error` function with error code `1`.
Don't worry, you don't need to understand the details.
[exclusive OR]: https://en.wikipedia.org/wiki/Exclusive_or
### Long Mode check
Now we can use CPUID to detect whether long mode can be used. I use code from [OSDev][long mode detection] again: