doc: Convert semihosting readme to rST
This converts the semihosting readme to rST. I have tried to make only
cosmetic changes, but I did fix up the first link (which was broken).
Signed-off-by: Sean Anderson <sean.anderson@seco.com>
diff --git a/doc/usage/semihosting.rst b/doc/usage/semihosting.rst
new file mode 100644
index 0000000..ed16e4d
--- /dev/null
+++ b/doc/usage/semihosting.rst
@@ -0,0 +1,43 @@
+.. SPDX-License-Identifier: GPL-2.0+
+.. Copyright 2014 Broadcom Corporation.
+
+Semihosting
+===========
+
+Semihosting is ARM's way of having a real or virtual target communicate
+with a host or host debugger for basic operations such as file I/O,
+console I/O, etc. Please see `Arm's semihosting documentation
+<https://developer.arm.com/documentation/100863/latest/>`_ for more
+information.
+
+For developing on armv8 virtual fastmodel platforms, semihosting is a
+valuable tool since it allows access to image/configuration files before
+eMMC or other NV media are available.
+
+There are two main ARM virtual Fixed Virtual Platform (FVP) models,
+`Versatile Express (VE) FVP and BASE FVP
+<http://www.arm.com/products/tools/models/fast-models/foundation-model.php>`_.
+The initial vexpress64 u-boot board created here runs on the VE virtual
+platform using the license-free Foundation_v8 simulator. Fortunately,
+the Foundation_v8 simulator also supports the BASE_FVP model which
+companies can purchase licenses for and contain much more functionality.
+So we can, in U-Boot, run either model by either using the VE FVP (default),
+or turning on ``CONFIG_BASE_FVP`` for the more full featured model.
+
+Rather than create a new armv8 board similar to ``armltd/vexpress64``, add
+semihosting calls to the existing one, enabled with ``CONFIG_SEMIHOSTING``
+and ``CONFIG_BASE_FVP`` both set. Also reuse the existing board config file
+vexpress_aemv8.h but differentiate the two models by the presence or
+absence of ``CONFIG_BASE_FVP``. This change is tested and works on both the
+Foundation and Base fastmodel simulators.
+
+Loading files
+-------------
+
+The semihosting code adds a "hostfs"::
+
+ smhload <image> <address> [env var]
+
+That will load an image from the host filesystem into RAM at the specified
+address and optionally store the load end address in the specified
+environment variable.