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.