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I came across this code and need to understand what it is doing. It just seems to be declaring two bytes and then doing nothing...
uint64_t x;
__asm__ __volatile__ (".byte 0x0f, 0x31" : "=A" (x));
Thanks!
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In computer programming, an inline assembler is a feature of some compilers that allows low-level code written in assembly language to be embedded within a program, among code that otherwise has been compiled from a higher-level language such as C or Ada.
asm volatile ("" ::: "memory") AFAIK is the same as the previous. The volatile keyword tells the compiler that it's not allowed to move this assembly block. For example, it may be hoisted out of a loop if the compiler decides that the input values are the same in every invocation.
Is inline assembly part of the C standard (ANSII or ISO) or not? There is no such thing as ANSII. ANSI is the American National Standards Institute, which issued the original C standard in 1989. All later versions of the standard (1990, 1999, 2011) have been issued by ISO.
This is generating two bytes (0F 31) directly into the code stream. This is an RDTSC instruction, which reads the time-stamp counter into EDX:EAX, which will then be copied to the variable 'x' by the output constraint "=A"(x)
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0F 31 is the x86 opcode for the RDTSC (read time stamp counter) instruction; it places the value read into the EDX and EAX registers.
The _ _ asm__ directive isn't just declaring two bytes, it's placing inline assembly into the C code. Presumably, the program has a way of using the value in those registers immediately afterwards.
http://en.wikipedia.org/wiki/Time_Stamp_Counter
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