Combining C code with assembly code in DSP applications

By Eran Belaish, CEVA
DSP DesignLine -- (12/10/07, 03:00:00 AM EST)

As DSP processors become more powerful and compiler optimization techniques improve, the once common trend of writing DSP applications solely in assembly has withered away. Today, almost every DSP application is comprised of a combination of both C code and assembly code. In critical functions, where performance is of the essence, DSP engineers continue to use highly optimized assembly code. Less critical functions, however, are now written in C, allowing easier maintenance and better portability. This combination of C and assembly code requires special tools and methodologies in the tool box of every DSP engineer.

It is well known that assembly coding has the advantage of better performance, while C coding is considered much easier and faster to write. To understand why this is so, let's take a closer look at the advantages and disadvantages of assembly coding compared to C coding:

Assembly Coding Pros:

• Assembly code can take advantage of a processor's unique instructions as well as various specialized hardware resources. On the other hand, C code is generic, and must support various hardware platforms. Thus, it is difficult for C to support platform-specific code.
• The assembly programmer is usually very familiar with the application and can make assumptions that are unavailable to the compiler.
• The assembly programmer can use human creativity; the compiler, advanced as it may be, is merely an automatic program.

• The assembly programmer has to handle time-consuming machine-level issues such as register allocation and instruction scheduling. With C code, these issues are taken care of by the compiler.
• Assembly coding requires specialized knowledge of the DSP architecture and its instruction set, whereas C coding only requires knowledge of the C language—which is rather common.
• With assembly code, it is extremely difficult and time consuming to port applications from one platform to another. Porting is relatively easy for C applications.

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