Corrected instruction set extension chapter with info on custom WFE i…

…nstruction

Signed-off-by: Arjan Bink <[email protected]>

docs/user_manual/source/instruction_set_extensions.rst

@@ -3,4 +3,25 @@
 CORE-V Instruction Set Extensions
 =================================
 
-|corev| does not support any custom ISA Extensions internal to the core. Custom instructions can be added external to the core via the eXtension interface described in :ref:`x_ext`.
+Custom instructions
+-------------------
+
+|corev| supports the custom instruction(s) listed in :numref:`Custom instructions`.
+
+.. table:: Custom instructions
+  :name: Custom instructions
+  :widths: 10 10 80
+  :class: no-scrollbar-table
+
+  +-------------------------+-------------+--------------------------------------------------+
+  | Custom instruction      | Encoding    | Description                                      |
+  +=========================+=============+==================================================+
+  | ``wfe``                 | 0x8C00_0073 | Wait For Event, see :ref:`wfe`.                  |
+  +-------------------------+-------------+--------------------------------------------------+
+
+Further custom instructions can be added external to the core via the eXtension interface described in :ref:`x_ext`.
+
+Custom CSRs
+-----------
+
+|corev| does not contain custom CSRs.