Motorola DSP56305 Manuel d'utilisateur télécharger pdf (Page 86)

MOTOROLA Optimizing DSP56300/DSP56600 Applications 7-1

This section

describes ways to

optimize the size

and speed of the

code by efficiently

using the

instruction set.

Section 7

COMPACT OPCODE USE

The rich instruction set of the DSP56300 and DSP56600 gives a great

amount of flexibility to the DSP software engineer when writing the

DSP code. However, careful selection of the right opcode will help

the user to generate an optimized application. There are few aspects

of the instruction set that should be considered when choosing the

opcode to be used:

• Cycle count of an instruction

• Addressing modes

• Word count of an instruction

• Peripheral addressing

• Special instructions

The following paragraphs briefly describe important aspects of the

instruction set. Please consult with Appendix B of the

DSP56300

and

DSP56600 Family Manuals

for details on the exact cycle count

and word count of each instruction.

7.1 CYCLE COUNT OF AN INSTRUCTION

Most of the instructions are executed in one clock cycle. Among

them are most of the arithmetic instructions and the move

instructions. But some instructions need more clock cycles to

execute. The following paragraph will describe ways to minimize

the effect of these multi-cycle instructions on total code

performance.

7.1.1 Opening Small REP and DO Loops

The REP and DO instructions are a multi-cycle instructions that

needs several cycles to decode before the actual loop is executed.

Hence, a loop that should be iterated a small number of times will

take a long time to execute if initiated by the REP or DO

instructions. In the following example, a DO loop contains an

internal REP loop that should iterate ten times:

1 2 ... 81 82 83 84 85 86 87 88 89 90 91 ... 111 112

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Motorola DSP56305 Manuel d'utilisateur Page 86