On 4/16/2013 12:43 PM, Gibney, Dave wrote:
I don't get to work at this level often, but I am always interested. How can Millicode be faster than the equivalent using the hardware instructions? As I understand Millicode, that is really all it is (using the hardware instructions) plus any overhead in context switching to the Millicode "environment". For the MVC/MVCL option, I can imagine a macro which generates an MVC loop, or unroll the loop into a sequence of MVC, or generate the MVCL depending on several criteria. I currently don't have the knowledge to determine the criteria and I would expect the criteria to change over time
Some millicode instructions will outperform their PoOp-code counterparts because millicode has access to hardware features not available to ordinary code. For example, MVCL(E) has the ability to move data under certain conditions without loading it into cache. (You can't do that with looping MVC.) Millicode routines also have access to the MVCX instruction which performs a variable-length MVC -- something ordinary programs cannot do without using the EXecute instruction. Furthermore, a millicode instruction is perceived by the architecture as a single instruction. This allows millicode to do things that cannot be simulated in ordinary code. For example, it would be impossible to write a simulation of the PLO instruction. -- Edward E Jaffe Phoenix Software International, Inc 831 Parkview Drive North El Segundo, CA 90245 http://www.phoenixsoftware.com/
