From: redelm@ev1.net.invalid   
      
   Robert Prins  wrote in part:   
   > On 2018-09-15 18:09, Robert Redelmeier wrote:   
   >> I have some confidence that even MS w7-64 preserves XMM   
   >> registers across syscalls & task swaps.  However, your hot   
   >> silicon also has YMM and may need a VEX prefix to correctly   
   >> do XMM (overflow wrap).   
   >   
   > W7(-64) supports AVX since SP1, so that's not the problem,   
   > and all my code is either using legacy MMX registers or   
   > VEX encoded instructions using the XMM/YMM registers.   
   >   
   > What is "overflow wrap", and how would I detect it,   
   > and would this not also affect MMX instructions?   
      
   Every transition [upwards] in wordsize presents the machine   
   designer decisions of what to do with the new extra bits.   
   Essentially to use them for greater precision or parallel-wise.   
      
   Your YMM registers may not always behave exactly as XMM,   
   especially since they are wider.  Do they wrap or saturate on   
   overflow, or go into higher bits?  There may be a control word   
   to set the desired behaviour.  Or use a prefix to distinguish   
   XMM from YMM .  MMX is deep legacy and will be coded as such.   
      
   -- Robert R   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)