From: not_valid@nospicedham.comcast.net   
      
   "Bartc"  wrote in message news:fxdZB.23554$4O3.2385@fx15.am4...   
      
   > On 16/12/2017 17:40, James Van Buskirk wrote:   
      
   > > The above numbering doesn't reflect the   
   > > overlap between RBP and R13 or RSP and R12.   
      
   > What overlap is that: that the bottom 3 bits of the register code? And why   
   > is it important, as all bottom 8 registers will 'overlap' with the top 8?   
      
   I refer you to 325462.pdf, Vol. 2A p. 2-11, table 2-5.  An SIB byte is   
   required for R12-based addressing and R13 can't be used without   
   displacement, although the displacement can be zero.   
      
   Whatever numbering you choose for the registers, different ABIs   
   will use them for different purposes, so ordering them to match   
   one ABI doesn't make all that much sense to me.  Look at Agner   
   Fog's PDF on calling conventions.   
      
   I emphasize that I have no objection to replacing all the register   
   names with numbers (6 for 2 for free for all and all that) but do   
   believe that using a numbering scheme other than the one already   
   hard-coded into the processor family is counterproductive.   
      
   References:   
   https://www.intel.com/content/www/us/en/architecture-and-technol   
   gy/64-ia-32-architectures-software-developer-manual-325462.html   
   http://www.agner.org/optimize/calling_conventions.pdf   
   https://www.youtube.com/watch?v=nhEhMinbd8g   
      
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