Make MPP/SPP WARL fields

1 files changed, 6 insertions, 3 deletions

diff --git a/src/machine.tex b/src/machine.tex
index fdff47f..e22c4f4 100644
--- a/src/machine.tex
+++ b/src/machine.tex
@@ -543,11 +543,14 @@ value {\em y}, {\em x}\,IE is set to {\em x}\,PIE; the privilege mode
 is changed to {\em y}; {\em x}\,PIE is set to 1; and {\em x}\,PP is
 set to U (or M if user-mode is not supported).
 
-{\em x}\,PP fields are \wlrl\ fields that need only be able to store
-supported privilege modes, including {\em x} and any implemented
-privilege mode lower than {\em x}.
+{\em x}\,PP fields are \warl\ fields that can hold only privilege mode {\em x}
+and any implemented privilege mode lower than {\em x}.  If privilege mode {\em
+x} is not implemented, then {\em x}\,PP must be hardwired to 0.
 
 \begin{commentary}
+M-mode software can determine whether a privilege mode is implemented
+by writing that mode to MPP then reading it back.
+
 If the machine provides only U and M modes, then only a single
 hardware storage bit is required to represent either 00 or 11 in MPP.
 \end{commentary}