Anyone knows what the following fields in P6 SMM do and what their bit layout is and how they work?

Source: https://www.sandpile.org/x86/smm.htm
- A20M# (P6 7F18) : Is this required at all to be implemented (P6 only)
- SMM status (P6 7F1E) (what is this?)
- shutdown (P6 7F23)
- sreg_status0 (P6 7F28) (what is this?)
- sreg_status1 (P6 7F68) (what is this?)

I've currently implemented shutdown as a byte that when non-zero causes shutdown (and set to 01h if entered SMM from shutdown). Although I can't find anything about said byte?