ASSP/Variant Architecture

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.

The +Asic class

The heart of the ASSP/Variant is the Asic class, defined in ..\e32\include\kernel\arm\assp.h. This is a class that contains a number of pure virtual functions +that must be implemented.

Where there is an ASSP/Variant split, +the ASSP layer should derive a concrete implementation from the Asic class. The most convenient way of implementing the +Variant layer is to further derive from this ASSP class. Only the +Variant DLL is declared as ‘variant’ in the ROM – the ASSP is declared +as an extension. If there is no ASSP/Variant split, then the Variant +is simply a concrete implementation of the Asic class.

The ASSP layer can, itself, define additional functions +to be implemented in the Variant layer. For example, the ASSP layer +defines the pure virtual function VideoRamSize() which +the Variant layer provides to map the video frame buffer.

The template reference board port has the ASSP/Variant split. The +ASSP layer is implemented by the TemplateAssp class, +and the Variant layer is implemented by the Template class.

For reference: the template port ASSP layer implementation's .mmp file is at ...\template_assp\katemplate.mmp, and the Variant layer implementation's .mmp file is at ...\template_variant\vtemplate.mmp.

Note that one of the source files that forms the ASSP layer +must have the DECLARE_STANDARD_ASSP() declaration. +See ...\template_assp\assp.cpp.