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flat assembler > OS Construction > Check CPUID presence

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Fulgurance



Joined: 27 Nov 2017
Posts: 62
Hello, i have little question. To detect if CPUID if present, OsDev website recommend this code:

Code:
 pushfd                               ;Save EFLAGS
    pushfd                               ;Store EFLAGS
    xor dword [esp],0x00200000           ;Invert the ID bit in stored EFLAGS
    popfd                                ;Load stored EFLAGS (with ID bit inverted)
    pushfd                               ;Store EFLAGS again (ID bit may or may not be inverted)
    pop eax                              ;eax = modified EFLAGS (ID bit may or may not be inverted)
    xor eax,[esp]                        ;eax = whichever bits were changed
    popfd                                ;Restore original EFLAGS
    and eax,0x00200000                   ;eax = zero if ID bit can't be changed, else non-zero
    ret    


I understand all except AND instruction. Why use AND instruction set EAX to 0 when ID bit can't change ? If i understand good AND instruction, bit set to 1 only if all compared bits are value 1. But all eflag bits don't always have bits set to 1, no ?
Post 19 Dec 2018, 01:12
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HaHaAnonymous



Joined: 02 Dec 2012
Posts: 1181
Location: Unknown
It is easier to understand in binary:
Code:
mov eax,10110011b
and eax,00100000b
; eax now is 00100000b
ret
    

Code:
mov eax,1111b
and eax,0010b
; eax now is 0010b
ret
    
Post 19 Dec 2018, 01:22
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Fulgurance



Joined: 27 Nov 2017
Posts: 62
Oh yes ! It's very simple finally! Cool Why i don't have thinking that x)
Post 19 Dec 2018, 02:39
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