Clarify permissions when the level extension is not implemented

Update the infinite cap bit pattern and add a note to GCPERM.

src/cap-description.adoc

@@ -630,7 +630,7 @@ or 'root' capability.
 | SDP           | ones  | Grants all permissions
 | AP (MXLEN=32) | 0x8/0x9^1^  (see xref:cap_perms_encoding32[xrefstyle=short])
                         | Grants all permissions
-| AP (MXLEN=64) | 0xFF (see xref:cap_perms_encoding64[xrefstyle=short])
+| AP (MXLEN=64) | 0x3F/0xFF^3^ (see xref:cap_perms_encoding64[xrefstyle=short])
                         | Grants all permissions
 | CL            | one^2^| _Global_
 | CT            | zero  | Unsealed
@@ -652,6 +652,8 @@ or 'root' capability.
 
 ^2^ This field only exists if {cheri_levels_ext_name} is implemented.
 
+^3^ When {cheri_levels_ext_name} is not implemented the value is 0x3f since the bits reserved for `EL` and `SL` must be zero.
+
 [#section_cap_representable_check, reftext="Representable Range"]
 === Representable Range Check
 

src/insns/gcperm_32bit.adoc

@@ -33,6 +33,9 @@ permission bits in `rd` are set to 0.
 [#gcperm_bit_field]
 include::../img/acperm_bit_field.edn[]
 
+NOTE: When {cheri_levels_ext_name} is not implemented, the `CL`, `SL`, and `EL` fields always report 0.
+Therefore, the presence of the {cheri_levels_ext_name} can be detected by checking the <<GCPERM>> result on the <<infinite-cap>>.
+
 Exceptions::
 include::require_cre.adoc[]