[Doc] Update documentation for the write_io_mapping command

docs/source/manual/file_formats/index.rst

@@ -21,3 +21,5 @@ OpenFPGA widely uses XML format for interchangable files
    bitstream_setting
 
    fabric_key
+
+   io_mapping_file

docs/source/manual/file_formats/io_mapping_file.rst

@@ -0,0 +1,33 @@
+.. _file_format_io_mapping_file:
+
+I/O Mapping File (.xml)
+-----------------------
+
+The I/O mapping file aims to show 
+
+- What nets have been mapped to each I/O
+- What is the directionality of each mapped I/O 
+
+An example of design constraints is shown as follows.
+
+.. code-block:: xml
+
+  <io_mapping>
+      <io name="gfpga_pad_GPIO_PAD[6:6]" net="a" dir="input"/>
+      <io name="gfpga_pad_GPIO_PAD[1:1]" net="b" dir="input"/>
+      <io name="gfpga_pad_GPIO_PAD[9:9]" net="out_c" dir="output"/>
+  </io_mapping>
+
+.. option:: name="<string>"
+
+  The pin name of the FPGA fabric which has been mapped, which should be a valid pin defined in OpenFPGA architecture description.
+
+  .. note:: You should be find the exact pin in the top-level module of FPGA fabric if you output the Verilog netlists.
+
+.. option:: net="<string>"
+
+  The net name which is actually mapped to a pin, which should be consistent with net definition in your ``.blif`` file.
+
+.. option:: dir="<string>"
+
+  The direction of an I/O, which can be either ``input`` or ``output``.

docs/source/manual/openfpga_shell/openfpga_commands/fpga_bitstream_commands.rst

@@ -72,3 +72,19 @@ write_fabric_bitstream
   .. option:: --verbose
 
     Show verbose log
+
+write_io_mapping
+~~~~~~~~~~~~~~~~
+
+  Output the I/O mapping information to a file
+
+  .. option:: --file <string> or -f <string>
+
+    Specify the file name where the I/O mapping will be outputted to.
+    See file formats in :ref:`file_format_io_mapping_file`.
+
+  .. option:: --verbose
+
+    Show verbose log
+
+