ASSP/Variant Architecture

A base port must provide a software layer called the ASSP/Variant.

The ASSP/Variant layer provides two main functions. First, it implements a small number of hardware-specific functions that are used by the Kernel. Second, it implements common peripheral control functions that other extensions and device drivers can use.

The most important of these functions is interrupt dispatching. During initialisation the ASSP/Variant must specify a dispatch function to be called for all hardware interrupts.

In general, the ASSP/Variant provides control functions for hardware which is shared between multiple devices. For example it is often not possible to do a read-modify-write on a GPIO port in order to change the state of an individual output line. This may be either because an output port register is write-only or because reading the port register reads the actual logic levels on the pins, not the last value written to the register, and the pin level typically lags the written value due to capacitive loading. In this case, the ASSP/Variant could provide a function to set and clear individual port bits, keeping a RAM copy of the last value written to the port register.

The simplest implementation is put all the code in a single DLL, called the Variant DLL (ecust.dll). For hardware architectures based on an ASSP, you can allow multiple types of phones that use the same ASSP to share the common code. To do this, the common ASSP code is implemented in a kernel extension, and the Variant DLL implements the code that is specific to the phone type.

In the Base Porting Guide, we refer to the ASSP layer and the Variant Layer, where the ASSP layer contains the source code tailored to a range of different microprocessors (e.g. ARM720/920/SA1/Xscale), and the Variant layer contains the source code associated with off-chip hardware (same CPU, different peripherals).

For example, the standard Symbian port for the template reference board is split into a core layer (in directory ...\template_assp\...) and code specific to the template board (in directory ...\template_variant\...). The .mmp file for the ASSP layer is ...\template_assp\katemplate.mmp, and the .mmp file for the Variant layer is ...\template_variant\vtemplate.mmp.