From: johnl@taugh.com   
      
   It appears that John Dallman  said:   
   >] * "Trap and Emulate” is an illusion of compatibility"   
   >]   * Performance differential is too great for most applications   
   >   
   >This is inevitably true nowadays, but wasn't when the idea was invented.   
      
   For at least 20 years IBM's mainframes have used what they call millicode. The   
   relatively simple instructions are implemented in hardware, and everything else   
   in millicode, including the more complication instructions, I/O, and other   
   system features. Millicode runs on the same CPU using the same instruction set   
   as regular code with some extra registers and instructions to handle aspects of   
   the hardware not visible to regular programs. It is stored in dedicated memory   
   which is loaded at boot time so it's easy to update.   
      
   I gather there have been instructions that were implemented in millicode, then   
   moved into hardware in the next CPU generation since they were used enough for   
   the speed to matter.   
      
   Here's a 2012 slide deck:   
      
   https://public.dhe.ibm.com/eserver/zseries/zos/racf/pdf/ny_metro   
   naspa_2012_10_what_and_why_of_system_z_millicode.pdf   
      
   --   
   Regards,   
   John Levine, johnl@taugh.com, Primary Perpetrator of "The Internet for   
   Dummies",   
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