What to do with the mode bit when no Zcherihyrbid

[jamie-melling]

I can't see anywhere in the spec what to do with the mode bit when there is no Zcherihybrid.

If you have a purecap only machine and a capability has the Mode-bit set, should this be treated as an invalid permission or as a reserved bit set? I think invalid permission might make more sense for RV32 systems as the mode bit is part of the AP encoding.
However, for RV64, reserved bit makes more sense but it would be a bad idea to have an inconsistency between RV64 and RV32.

While searching the spec, the only thing I could find was

When MXLEN=64, the Mode is encoded separately; a new CHERI Execution Mode Encoding field
is added to the capability format as shown in Table 5. The CHERI Execution Mode Encoding is
only valid for code capabilities, otherwise the field is reserved.

I think it might be better to reword the last sentence as

The CHERI Execution Mode Encoding is only valid for code capabilities, otherwise, the combination Mode bit set for non-executable capabilities is reserved for future use.