add recommendation on unused elements

diff --git a/simple_v_extension/specification.mdwn b/simple_v_extension/specification.mdwn
index e27a446c6ffe1a7f4ba1ba7fa89be7d7d4080ff0..5e0c1b311253362595fe1932450a6fbdbfe52b70 100644 (file)
--- a/simple_v_extension/specification.mdwn
+++ b/simple_v_extension/specification.mdwn
@@ -804,4 +804,13 @@ No specific hints are yet defined in Simple-V
 
 # Under consideration <a name="issues"></a>
 
-placeholder
+for element-grouping, if there is unused space within a register
+(3 16-bit elements in a 64-bit register for example), recommend:
+
+* For the unused elements in an integer register, the used element
+  closest to the MSB is sign-extended on write and the unused elements
+  are ignored on read.
+* The unused elements in a floating-point register are treated as-if
+  they are set to all ones on write and are ignored on read, matching the
+  existing standard for storing smaller FP values in larger registers.
+