Below is example code of how to call undocumented opcode. This technique has been used to detect if system CPU is very rare C&T Super386. While it is is FASM syntax porting it to any other assembler shloudn't be a problem as I'm not using any special FASM only features.
https://corexor.wordpress.com/2015/12/05/calling-ct-scall-safely/ (https://corexor.wordpress.com/2015/12/05/calling-ct-scall-safely/)
Have fun!
386 ?
that's SO last week
Just like DOS and 16-bit code ;) Anyway if you take a look at this CPU description it offers something similar to SMM mode but in same ways it is more advance than SMM and I guess most people heard about it just few weeks ago...