Create porting_to_a_new_platform.md

docs/porting_to_a_new_platform.md

@@ -0,0 +1,42 @@
+# Porting to a new platform
+
+## Porting the run-time
+
+Porting will always start with the run-time, it is a prerequisite for any Ada
+application.
+
+Porting a run-time will be a different task depending on the CPU architecture
+and the run-time profile that you need.
+
+If your CPU architecture is already supported, for instance ARM Cortex-M4, the
+porting effort will be limited to the BSP: startup code, linker script, clock
+configuration and interrupt names.
+
+If the CPU architecture is not already supported, for instance AVR, there
+will be more work: context switch, handling interrupts, providing a clock and
+timing services, etc. It may also require some compiler development.
+
+There are three different run-time profiles (ZFP, Ravescar-SFP, Ravenscar-Full)
+with increasing set of features:
+
+ - ZFP (stands for Zero FootPrint): The minimum required to run an Ada program
+
+ - Ravenscar-SFP (stands for Small FootPrint): ZFP + the minimum required for Ada
+   Ravenscar tasking support (task, protected objects, interrupt handling,
+   clock and timming).
+
+ - Ravenscar-Full: Ravenscar-SFP + Ada exception propagation and standard
+   library packages like containers.
+
+If you are only interested in the ZFP profile (no tasking) then porting will be
+much simpler. Usually only the startup code an linker script need to be
+changed. Note that for the moment, the platform agnostic parts of
+`Ada_Drivers_Library` (middleware and components) works on all run-time
+profiles starting with ZFP.
+
+Please go to [AdaCore/bb-runtimes](https://github.com/AdaCore/bb-runtimes) for
+detailed instructions on how to port run-times.
+
+## Porting Ada_Drivers_Library
+
+This section is still in a "TODO" state.