Motorola DSP56305 Instrukcja Użytkownika Pobierz pdf (Strona 91)

7-6 Optimizing DSP56300/DSP56600 Applications MOTOROLA

Compact Opcode Use

Addressing Modes

7.2.2 Short Addressing Mode

The lower portion (first 64 locations 0–63) of data memory can be

accessed by special short addressing modes that can specify the

location as part of the opcode, contrary to other locations where a

second instruction word is required. Example:

move X:5,x0

This instruction executes in 1 clock cycle. This makes it possible to

use the lower portion of the data memory as general purpose

registers without a significant increase in code length.

7.2.3 Short Immediate Mode

There are some MOVE instructions that permit the specification of

immediate data numbers in a small range so that a second word is

not required. Example:

move #5,r0

This instruction executes in one clock cycle. This makes it possible

to initialize registers without executing 2-word, 2-cycle instructions.

7.2.4 Short Immediate Operands

There are also some arithmetic instructions that defines a short

immediate operand. Example:

move #>$8000,a ;the “>” is the ‘force long’

move #>$0075,r0 ;assembler directive

xor #>$0003,a

This example can be optimized by replacing all the instructions by

their short immediate operand versions:

move #<$80,a

move #<$75,r0

xor #<$03,a

1 2 ... 86 87 88 89 90 91 92 93 94 95 96 ... 111 112

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Motorola DSP56305 Instrukcja Użytkownika Strona 91