Custom instructions is what Tensilica Xtensa "configurable cores" about. That's different from the custom instructions you are talking about in that the technol

Custom instructions is what Tensilica Xtensa "configurable cores" about. That's different from the custom instructions you are talking about in that the technology is all about pre-fab stage, that is possible to add custom instructions on levels lower than microcode, but still it's hardcoded into the "customized" CPU. The main problem with the technology is still C compiler support for added custom instructions.