+.. _subregister-indices:
+
+Subregister Indices
+~~~~~~~~~~~~~~~~~~~
+
+The register machine operands can reference a portion of a register by using
+the subregister indices. The example below shows an instance of the ``COPY``
+pseudo instruction that uses the X86 ``sub_8bit`` subregister index to copy 8
+lower bits from the 32-bit virtual register 0 to the 8-bit virtual register 1:
+
+.. code-block:: llvm
+
+ %1 = COPY %0:sub_8bit
+
+The names of the subregister indices are target specific, and are typically
+defined in the target's ``*RegisterInfo.td`` file.
+