Motorola once sc140 Manual Do Utilizador

Introduction to the SC140 Tools

21

7   

Control Code: The True Bit Exercise

The True bit exercise shows how the compiler uses the True bit and how you can help the compiler to 
improve the performance. The True bit is set/cleared by compare or test instructions. The use of the True 
bit as a control flag together with DSP-specific code makes the SC140 very powerful for applications 
including both control and DSP code. The True bit can affect conditional branching as well as conditional 
execution of groups of instructions.

Conditional branching includes:

•

BT/BF: Branch relative if True bit is True/False.

•

BTD:BFD: Branch delayed relative if True bit is True/False.

•

JT/JF: Jump if True bit is True/False.

•

JTD/JFD: Jump delayed if True bit is True/False.

Conditional execution of instructions includes:

•

IFT/IFF: IF True bit is True/False.

•

IFA: IF always, which is unconditionally executed with IFT/IFF.

The conditional execution set combinations are very flexible and are represented in  Figure 10, which 
represents the maximum number of ALUs (that is, two) and one Arithmetic Address Unit (AAU) per 
subset. The C compiler automatically generates the conditional execution set, and some examples are 
provided to highlight potential code optimization.

Figure 10.   Control Instructions Using the True Bit

Number of memory moves (bandwidth)

2N

N/2

Code size

Small

Large

Table 3.   Inner Loop Characteristics of Multi-sample and Single-sample Techniques. (Continued)