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x64 Assembly: How to Check for CPUID Support

An example of how to check if a processor supports the CPUID instruction using x86-64 (64-bit) assembly.

This code was compiled and linked using the Microsoft Macro Assembler for Visual Studio 2022.

This example of checking for the CPUID instruction is done with rax, rbx and rflags — if the 22nd bit (bit 21) can be toggled then the cpu supports supports CPUID (all x64 processors support CPUID).

TITLE 'extern "C" long long isCPUID64();'
_TEXT	SEGMENT
isCPUID64 PROC
	
	push rbx         ; save rbx for the caller
	pushfq           ; push rflags on the stack
	pop rax          ; pop them into rax
	mov rbx, rax     ; save to rbx for restoring afterwards
	xor rax, 200000h ; toggle bit 21
	push rax         ; push the toggled rflags
	popfq            ; pop them back into rflags
	pushfq           ; push rflags
	pop rax          ; pop them back into rax
	cmp rax, rbx     ; see if bit 21 was reset
	jz not_supported
	
	mov rax, 1
	jmp exit
	
not_supported:
	xor rax, rax;

exit:
	pop rbx
	ret 0
isCPUID64 ENDP
_TEXT	ENDS
END

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