Motorola DSP56305 User Manual Page 45
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3-16 Optimizing DSP56300/DSP56600 Applications MOTOROLA
Program Control
PC Relative Instructions
There are two main advantages for using PC relative addressing
over absolute addressing:
1. Code Relocation—Code written with PC relative code could
be relocated, reused or imported to different program
addresses without the need to update the program labels.
2. Code Compactness—Most address references are generally
to locations not many memory words away in the program.
Therefore, the PC displacement will usually be a small
number, that may fit in the address field of a 1-word opcode.
Absolute addressing generally will not fit, and requires a
second word to specify it.
Compare the following 2 examples:
;first example: using absolute addressing
cmp x0,a
jne _CONT
inc b
_CONT
rts
;second example: using PC relative addressing
cmp x0,a
bne <_CONT1
inc b
_CONT1
rts
After assembly, the labels _CONT and _CONT1 have definite
values. In the first example, if _CONT > 4095, then the Assembler
must use the 2-word opcode, placing the value of _CONT as the
second word. _CONT1, however, has the value of 2, therefore fitting
into the 1-word opcode version of the instruction Bcc (Branch on
Condition). Furthermore, the value of _CONT1 remains the same
regardless of the location of the code in the program space. The
Short Addressing mode force operator (“<” in the Bne argument)
Note: 1. The LRA opcode can only add a displacement to the PC. The Assembler
translates the absolute address to displacement from the Location Counter, so
the two modes (absolute address/ displacement register) are the same from the
machine’s point of view.
Table 3-5 Instructions with Program Memory Arguments
Function
Address
Argument
Mnemonic
The Address Argument
Encoded in the
opcode
(total 1 w)
2nd
word
Register Data
Memory
<ea>
- DSP56300/DSP56600 1
- Digital Signal 1
- Processors 1
- TABLE OF CONTENTS 2
- LIST OF FIGURES 6
- LIST OF TABLES 7
- INTRODUCTION 8
- 1.2 DSP56600 CORE FAMILY 9
- Table 1-1 11
- Introduction 12
- Description and Use 13
- Application Note Structure 14
- 1.4.3 Appendixes 15
- DATA OPERATIONS 16
- DSP56300 Family Manual 21
- 2.3 THE MAX INSTRUCTION 22
- 2.4 USING THE BARREL SHIFTER 23
- Data Operations 24
- Using the barrel shifter 24
- DSP56300 26
- DSP56600 Family 26
- Double precision arithmetic 27
- INSTRUCTIONS 28
- PROGRAM CONTROL 30
- Program Control 31
- Hardware Loops 31
- 3.2 THE HARDWARE STACK 32
- DSP56600 Family Manuals 35
- 3.3 USING THE STACK EXTENSION 36
- Using the Stack Extension 37
- EXTENSION 39
- Conditional DALU Instructions 40
- 3.6 PC RELATIVE INSTRUCTIONS 42
- DSP56600 Family Manual 43
- 3.7 USING FAST INTERRUPTS 46
- Using Fast Interrupts 48
- USING THE DMA 50
- Using the DMA 51
- Using Slow, Low-Cost Memories 54
- 4.4 SERVICING A PERIPHERAL 55
- Servicing a Peripheral 56
- Section 5 62
- 5.1 THE INSTRUCTION CACHE 62
- 5.1.1 Cache Sectors 64
- The Instruction Cache 65
- 5.1.3 Cache Burst Mode 67
- 5.2 MEMORY SWITCH 70
- Memory Switch 71
- 5.3 USING THE BOOTSTRAP ROM 72
- Using the Bootstrap ROM 73
- PIPELINE INTERLOCKS 74
- Pipeline Interlocks 75
- Data ALU Pipeline Interlocks 75
- DSP56000 Family Manual 76
- INTERLOCKS 80
- Interlocks 80
- 6.3 STACK EXTENSION DELAYS 81
- Interlocks? 83
- COMPACT OPCODE USE 86
- Instructions 87
- Compact Opcode Use 89
- Cycle Count of an Instruction 89
- 7.2 ADDRESSING MODES 90
- 7.2.2 Short Addressing Mode 91
- 7.2.3 Short Immediate Mode 91
- 7.2.5 Register Addressing 92
- 7.2.6 Word Count 92
- 7.3 PERIPHERAL ADDRESSING 92
- 7.4 SPECIAL INSTRUCTIONS 93
- 7.4.1 Dual Data Spaces 93
- 7.4.3 Clearing Registers 94
- Special Instructions 95
- SAVING POWER 96
- A.1.2 Stop Standby Mode 97
- A.1.3 Low-Power Clock Divider 97
- Saving Power 98
- Disabling Functional Blocks 98
- DEBUG AND TEST SUPPORT 100
- B.2 JTAG PORT FEATURES 101
- B.3 ADDRESS TRACING 102
- Debug and Test Support 103
- Address Tracing 103
- USING THE PROFILER 104
- C.3 THE PROFILING REPORT 105
- C.3.1 Basic Report 105
- C.3.2 Symbol Report 106
- Using the Profiler 107
- C.3.4 Code Coverage Report 108
- C.3.5 Basic Subroutine Report 109
- C.3.8 Subroutine Call Report 111
- C.4 USING THE PROFILE REPORT 111
- How to reach us: 112
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