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+Library at ROM
+==============
+
+This document provides an overview of the "library at ROM" implementation in
+Trusted Firmware-A (TF-A).
+
+Introduction
+~~~~~~~~~~~~
+
+The "library at ROM" feature allows platforms to build a library of functions to
+be placed in ROM. This reduces SRAM usage by utilising the available space in
+ROM. The "library at ROM" contains a jump table with the list of functions that
+are placed in ROM. The capabilities of the "library at ROM" are:
+
+1. Functions can be from one or several libraries.
+
+2. Functions can be patched after they have been programmed into ROM.
+
+3. Platform-specific libraries can be placed in ROM.
+
+4. Functions can be accessed by one or more BL images.
+
+Index file
+~~~~~~~~~~
+
+.. image:: ../resources/diagrams/romlib_design.png
+    :width: 600
+
+Library at ROM is described by an index file with the list of functions to be
+placed in ROM. The index file is platform specific and its format is:
+
+::
+
+    lib function    [patch]
+
+    lib      -- Name of the library the function belongs to
+    function -- Name of the function to be placed in library at ROM
+    [patch]  -- Option to patch the function
+
+It is also possible to insert reserved spaces in the list by using the keyword
+"reserved" rather than the "lib" and "function" names as shown below:
+
+::
+
+    reserved
+
+The reserved spaces can be used to add more functions in the future without
+affecting the order and location of functions already existing in the jump
+table. Also, for additional flexibility and modularity, the index file can
+include other index files.
+
+For an index file example, refer to ``lib/romlib/jmptbl.i``.
+
+Wrapper functions
+~~~~~~~~~~~~~~~~~
+
+.. image:: ../resources/diagrams/romlib_wrapper.png
+    :width: 600
+
+When invoking a function of the "library at ROM", the calling sequence is as
+follows:
+
+BL image --> wrapper function --> jump table entry --> library at ROM
+
+The index file is used to create a jump table which is placed in ROM. Then, the
+wrappers refer to the jump table to call the "library at ROM" functions. The
+wrappers essentially contain a branch instruction to the jump table entry
+corresponding to the original function. Finally, the original function in the BL
+image(s) is replaced with the wrapper function.
+
+The "library at ROM" contains a necessary init function that initialises the
+global variables defined by the functions inside "library at ROM".
+
+Script
+~~~~~~
+
+There is a ``romlib_generate.py`` Python script that generates the necessary
+files for the "library at ROM" to work. It implements multiple functions:
+
+1. ``romlib_generate.py gentbl [args]`` - Generates the jump table by parsing
+   the index file.
+
+2. ``romlib_generator.py genvar [args]`` - Generates the jump table global
+   variable (**not** the jump table itself) with the absolute address in ROM.
+   This global variable is, basically, a pointer to the jump table.
+
+3. ``romlib_generator.py genwrappers [args]`` - Generates a wrapper function for
+   each entry in the index file except for the ones that contain the keyword
+   ``patch``. The generated wrapper file is called ``<fn_name>.s``.
+
+4. ``romlib_generator.py pre [args]`` - Preprocesses the index file which means
+   it resolves all the include commands in the file recursively. It can also
+   generate a dependency file of the included index files which can be directly
+   used in makefiles.
+
+Each ``romlib_generate.py`` function has its own manual which is accessible by
+runing ``romlib_generator.py [function] --help``.
+
+``romlib_generate.py`` requires Python 3 environment.
+
+
+Patching of functions in library at ROM
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The ``romlib_generator.py genwrappers`` does not generate wrappers for the
+entries in the index file that contain the keyword ``patch``. Thus, it allows
+calling the function from the actual library by breaking the link to the
+"library at ROM" version of this function.
+
+The calling sequence for a patched function is as follows:
+
+BL image --> function
+
+Memory impact
+~~~~~~~~~~~~~
+
+Using library at ROM will modify the memory layout of the BL images:
+
+- The ROM library needs a page aligned RAM section to hold the RW data. This
+  section is defined by the ROMLIB_RW_BASE and ROMLIB_RW_END macros.
+  On Arm platforms a section of 1 page (0x1000) is allocated at the top of SRAM.
+  This will have for effect to shift down all the BL images by 1 page.
+
+- Depending on the functions moved to the ROM library, the size of the BL images
+  will be reduced.
+  For example: moving MbedTLS function into the ROM library reduces BL1 and
+  BL2, but not BL31.
+
+- This change in BL images size can be taken into consideration to optimize the
+  memory layout when defining the BLx_BASE macros.
+
+Build library at ROM
+~~~~~~~~~~~~~~~~~~~~~
+
+The environment variable ``CROSS_COMPILE`` must be set appropriately. Refer to
+:ref:`Performing an Initial Build` for more information about setting this
+variable.
+
+In the below example the usage of ROMLIB together with mbed TLS is demonstrated
+to showcase the benefits of library at ROM - it's not mandatory.
+
+.. code:: shell
+
+    make PLAT=fvp                                                   \
+    MBEDTLS_DIR=</path/to/mbedtls/>                                 \
+    TRUSTED_BOARD_BOOT=1 GENERATE_COT=1                             \
+    ARM_ROTPK_LOCATION=devel_rsa                                    \
+    ROT_KEY=plat/arm/board/common/rotpk/arm_rotprivk_rsa.pem        \
+    BL33=</path/to/bl33.bin>                                        \
+    USE_ROMLIB=1                                                    \
+    all fip
+
+--------------
+
+*Copyright (c) 2019, Arm Limited. All rights reserved.*