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3.1.10 Interface Inheritance

3.1.11 Summary

Interfaces and Modules

module's header file defines a concrete interface; the functions defined in the header uniquely identify a

specific (or concrete) implementation within a system. A special type of interface header is used to define

abstract interfaces; abstract interfaces define functions that are implemented by more than one module in

a system. An abstract interface header is identical to a normal module interface header except that it

declares a structure of function pointers named XYZ_Fxns. A module ABC is said to implement an

abstract interface XYZ if it declares and initializes a static structure of type XYZ_Fxns named ABC_XYZ.

The TMS320 DSP Algorithm Standard API Reference (SPRU360) contains all of the abstract interface

definitions for eXpressDSP-compliant algorithms. All eXpressDSP-compliant algorithm modules, for

example, must implement the IALG interface. Appendix A of the TMS320 DSP Algorithm Standard API

Reference document contains an example of a module that implements the IALG interface.

By convention, all abstract interface headers begin with the letter 'i'. To insure no chance for confusion, we

drop the adjective "concrete" and "abstract" when referring to a module's interfaces.

Although all eXpressDSP-compliant algorithms implement the IALG interface, it is important to note that

almost all of the TMS320 DSP Algorithm Standard modules must implement a more specific algorithm

interface; i.e., they must implement all of the IALG functions as well as methods specific to the algorithm.

For example, a G.729 enCoder algorithm must not only implement IALG; it must also implement an

"enCode" function that is specific to the G.729 algorithm.

In this common case — where we want to define a new interface that requires additional methods beyond

those defined by IALG — we define a new interface that "derives from" or "inherits from" the IALG

interface. Interface inheritance is implemented by simply defining the new interface's "Fxns" structure so

that its first field is the "Fxns" structure from which the interface is inherited. Thus, any pointer to the new

interface's "Fxns" structure can be treated as a pointer to the inherited interface's "Fxns" structure.

In the case of the G.729 enCoder algorithm, this simply means that the first field of the G729E_Fxns

structure is an IALG_Fxns structure. This ensures that any G.729 enCoder implementation can be treated

as a "generic" eXpressDSP-compliant algorithm.

All interfaces (including those not currently part of the TMS320 DSP Algorithm Standard) that extend IALG

should employ the same technique. The abstract IFIR interface example defined in the TMS320 DSP

Algorithm Standard API Reference illustrates this technique.

The previous sections described the structure shared by all modules. Recall that modules are the most

basic software component of an eXpressDSP-compliant system. The following table summarizes the

common design elements for a module named XYZ.

Element Description Required

XYZ_init() XYZ_exit() Module initialization and finalization yes

functions

xyz.h Module's interface definition yes

Structure type of all module configuration Only if module has global

XYZ_Config

parameters. configuration parameters

Global structure of all module configuration Only if module has global

XYZ

parameters. configuration parameters

Structure type defining all functions Only if the interface is an

XYZ_Fxns

necessary to implement the XYZ interface. abstract interface definition

The next table summarizes the common elements of all modules that manage one or more instance

objects.

32 Algorithm Component Model SPRU352G – June 2005 – Revised February 2007

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Texas Instruments TMS320 DSP User Manual Page 32

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