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Update blog to d57657ac29

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2016-01-02 15:12:00 +00:00
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_posts/2015-08-25-entering-longmode.md
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@@ -99,19 +99,37 @@ We compare the value in `eax` with the magic value and jump to the label `no_mul
```nasm ```nasm
check_cpuid: check_cpuid:
pushfd ; Store the FLAGS-register. ; Check if CPUID is supported by attempting to flip the ID bit (bit 21) in
pop eax ; Restore the A-register. ; the FLAGS register. If we can flip it, CPUID is available.
mov ecx, eax ; Set the C-register to the A-register.
xor eax, 1 << 21 ; Flip the ID-bit, which is bit 21. ; Copy FLAGS in to EAX via stack
push eax ; Store the A-register. pushfd
popfd ; Restore the FLAGS-register. pop eax
pushfd ; Store the FLAGS-register.
pop eax ; Restore the A-register. ; Copy to ECX as well for comparing later on
push ecx ; Store the C-register. mov ecx, eax
popfd ; Restore the FLAGS-register.
xor eax, ecx ; Do a XOR-operation on the A-register and the C-register. ; Flip the ID bit
jz .no_cpuid ; The zero flag is set, no CPUID. xor eax, 1 << 21
ret ; CPUID is available for use.
; Copy EAX to FLAGS via the stack
push eax
popfd
; Copy FLAGS back to EAX (with the flipped bit if CPUID is supported)
pushfd
pop eax
; Restore FLAGS from the old version stored in ECX (i.e. flipping the ID bit
; back if it was ever flipped).
push ecx
popfd
; 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
ret
.no_cpuid: .no_cpuid:
mov al, "1" mov al, "1"
jmp error jmp error