TODO

- when mfpi is executed and both previous mode and current mode
  are user, the instruction functions as though it were mfpd
  source : EK-KDJ1B-UG_KDJ11-B_Nov86.pdf

- yellow stack protection traps description
  source : EK-KDJ1B-UG_KDJ11-B_Nov86.pdf, 1-3
  red stack protection traps description
  source : EK-KDJ1B-UG_KDJ11-B_Nov86.pdf, 1-10
  Yellow trap is interpreted differently by simh; simh also
  triggers yellow trap on pushes by mfpx? this in not mentioned
  in EK-KDJ1B-UG_KDJ11-B_Nov86.pdf?
- stacklimit explained in some more detail, but again
  differently, in description of 11/70 in PDP11_Handbook1979.pdf

- tstset/wrtlock insns
  tstset : 0072dd, 'Reads/locks destination word and stores
  it in r0. Writes/unlocks (r0) v 1 into destination. If mode
  is 0, traps to 10'
  wrtlock : 0073dd, 'Writes contents of r0 into destination
  during bus lock. If mode is 0, traps to 10'
  source : EK-KDJ1B-UG_KDJ11-B_Nov86.pdf, 9-16