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Merge pull request #102 from phil-opp/cpuid-comments

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Improve comments for CPUID check
This commit is contained in:
Philipp Oppermann
2016-01-02 16:10:01 +01:00
parent 980c94202f 157b3b208d
commit d57657ac29
2 changed files with 62 additions and 26 deletions
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44
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
check_cpuid:
pushfd ; Store the FLAGS-register.
pop eax ; Restore the A-register.
mov ecx, eax ; Set the C-register to the A-register.
xor eax, 1 << 21 ; Flip the ID-bit, which is bit 21.
push eax ; Store the A-register.
popfd ; Restore the FLAGS-register.
pushfd ; Store the FLAGS-register.
pop eax ; Restore the A-register.
push ecx ; Store the C-register.
popfd ; Restore the FLAGS-register.
xor eax, ecx ; Do a XOR-operation on the A-register and the C-register.
jz .no_cpuid ; The zero flag is set, no CPUID.
ret ; CPUID is available for use.
; Check if CPUID is supported by attempting to flip the ID bit (bit 21) in
; the FLAGS register. If we can flip it, CPUID is available.
; Copy FLAGS in to EAX via stack
pushfd
pop eax
; Copy to ECX as well for comparing later on
mov ecx, eax
; Flip the ID bit
xor eax, 1 << 21
; 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:
mov al, "1"
jmp error

44
src/arch/x86_64/boot.asm
View File

@@ -111,19 +111,37 @@ check_multiboot:
; Throw error 1 if the CPU doesn't support the CPUID command.
check_cpuid:
pushfd ; Store the FLAGS-register.
pop eax ; Restore the A-register.
mov ecx, eax ; Set the C-register to the A-register.
xor eax, 1 << 21 ; Flip the ID-bit, which is bit 21.
push eax ; Store the A-register.
popfd ; Restore the FLAGS-register.
pushfd ; Store the FLAGS-register.
pop eax ; Restore the A-register.
push ecx ; Store the C-register.
popfd ; Restore the FLAGS-register.
xor eax, ecx ; Do a XOR-operation on the A-register and the C-register.
jz .no_cpuid ; The zero flag is set, no CPUID.
ret ; CPUID is available for use.
; Check if CPUID is supported by attempting to flip the ID bit (bit 21) in
; the FLAGS register. If we can flip it, CPUID is available.
; Copy FLAGS in to EAX via stack
pushfd
pop eax
; Copy to ECX as well for comparing later on
mov ecx, eax
; Flip the ID bit
xor eax, 1 << 21
; 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:
mov al, "1"
jmp error