c - Why should I use 'rdtsc' differently on x86 and x86_x64? -

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i know rdtsc loads current value of processor's time-stamp counter 2 registers: edx , eax. in order on x86 need (assuming using linux):

    unsigned long lo, hi;     asm( "rdtsc" : "=a" (lo), "=d" (hi));     return lo;

and x86_x64:

        unsigned long lo, hi;         asm( "rdtsc" : "=a" (lo), "=d" (hi) );          return( lo | (hi << 32) );

why that? can explain me?

in x86-64 mode, rdtsc clears higher 32 bits of rax. compensate bits have shift hi left 32 bits.


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