Machine Instruction encoding

[sashti srinivasan]

Hello All,
 
   The following is a line in rtems' timer driver
 source.
     asm volatile(".byte 0x0F, 0x31" : "=A" (result));
   
   RTEMS mailing list clarified me saying that it is
encoding of pentium rdtsc instruction.  Please tell me
how the above statement differs from the one below.

         asm("rdtsc result");
 (Assuming the instruction mnemonic is correct)
 
   Only thing I found from gnu documentation for gcc
 that '=' specifies that result is a output of the
 instruction.  According to the document, there must
 be an instruction pnemonic string first inside the
 paranthesis.  But there is the directive .byte
 there.  Does this mean the opcode directly?
 
   The document also says that the arguments can be
 referred like %0, %1 etc.  But there is nothing like
 this here.  Is it because, since there is only one
 operand, it is assumed to follow the opcode?

   Any other reference than info pages of gcc provides
more details?
    
   Please clear my doubts.
 
 With Best Regards
 Srinivasan


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