decodetree: Allow !function with no input bits

[mirror_qemu.git] / docs / devel / decodetree.rst

diff --git a/docs/devel/decodetree.rst b/docs/devel/decodetree.rst
index 44ac621ea87ca2b227fbf7a359516521f01f62af..ce7f52308ffecce95508869d267eb9e6fa06c35a 100644 (file)
--- a/docs/devel/decodetree.rst
+++ b/docs/devel/decodetree.rst
@@ -23,7 +23,7 @@ Fields
 
 Syntax::
 
-  field_def     := '%' identifier ( unnamed_field )+ ( !function=identifier )?
+  field_def     := '%' identifier ( unnamed_field )* ( !function=identifier )?
   unnamed_field := number ':' ( 's' ) number
 
 For *unnamed_field*, the first number is the least-significant bit position
@@ -34,6 +34,12 @@ present, they are concatenated.  In this way one can define disjoint fields.
 If ``!function`` is specified, the concatenated result is passed through the
 named function, taking and returning an integral value.
 
+One may use ``!function`` with zero ``unnamed_fields``.  This case is called
+a *parameter*, and the named function is only passed the ``DisasContext``
+and returns an integral value extracted from there.
+
+A field with no ``unnamed_fields`` and no ``!function`` is in error.
+
 FIXME: the fields of the structure into which this result will be stored
 is restricted to ``int``.  Which means that we cannot expand 64-bit items.