In-Depth Analysis of Zephyr Code Relocation Technology: From Principles to Best Practices

In modern embedded system development, code relocation technology is becoming a key means to optimize system performance. Imagine a scenario where your real-time control system experiences a 30% delay in critical interrupt response due to Flash access latency, or is unable to add new functional modules due to Flash capacity limitations. These are the pain points that code relocation technology can address.

As a lightweight and modular real-time operating system, Zephyr RTOS provides a comprehensive code relocation mechanism. This article will take you on a deep dive into the implementation principles, application scenarios, and best practices of this technology, helping you break through hardware limitations and unleash system potential in embedded development.

1. Core Principles of Code Relocation

1.1 The Duality of Memory Architecture

The core of code relocation technology lies in understanding the separation of **Load Memory Address (LMA)** and Virtual Memory Address (VMA)**:

• LMA (Load Memory Address): The storage location of code in non-volatile memory (such as Flash)

• VMA (Virtual Memory Address): The execution location of code in volatile memory (such as RAM)

This separation allows for independent optimization of the “storage location” and “execution location,” providing great flexibility in system design.

1.2 The Three Pillars of Relocation

Zephyr’s implementation of code relocation relies on three key technical components:

1. Intelligent Linker Scripts:

• Using <span>MEMORY</span> commands to precisely define the attributes of each memory region

• Controlling code/data layout through <span>SECTION</span> commands

• Example:

  <span> MEMORY {</span><span> FLASH (rx) : ORIGIN = 0x08000000, LENGTH = 512K</span><span> SRAM (rwx) : ORIGIN = 0x20000000, LENGTH = 128K</span><span> }</span>

1. Dynamic Relocation Table:

• <span>.rel.dyn</span> section records absolute address references

• <span>.rela.dyn</span> section contains additional additive information

• Each table entry contains: offset, relocation type, symbol index

1. Startup Loader:

• Completes critical operations during the <span>z_data_copy()</span> phase

• Executes precise timing control for memory copying and address correction

2. Complete Implementation Process Explained

2.1 Building System Configuration

Zephyr provides a declarative relocation configuration interface through CMake:

zephyr_code_relocate(
    FILES 
        src/isr_handlers.c
        src/time_critical.c
    LOCATION SRAM
    ALIGN 8
    NOCOPY  # Optional, only reserves space without automatic copying
)

Advanced configuration options include:

• <span>ALIGN</span>: Specifies memory alignment requirements

• <span>NOCOPY</span>: Manually controls the timing of copying

• <span>DEPENDS</span>: Defines relocation dependencies

2.2 Compiler Collaboration Mechanism

Modern compiler toolchains provide multi-level support for code relocation:

1. Position-Independent Code Generation:

   -fpie -mpic-register=r9
   

2. Specific Section Marking:

   __attribute__((section(".ramfunc")))
   

3. Relocation Information Generation:

   -Wl,-q  # Retain relocation information
   

2.3 Startup Timing Essentials

Key stages of relocation during the system startup process:

1. ROM Startup:

• Initialize the minimal hardware environment

• Establish stack and global pointers

1. Relocation Preprocessing:

   extern char _sramfunc_lma[], _sramfunc[], _eramfunc[];
   memcpy(_sramfunc, _sramfunc_lma, _eramfunc - _sramfunc);
   

2. Address Correction:

• Traverse the <span>.rel.dyn</span> section

• Apply the formula: adjusted_value = original_value + bias

3. Advanced Application Scenarios and Practical Tips

3.1 Performance-Critical Application Optimization

Case Study: Motor Control PWM Interrupt Service Routine

__ramfunc void pwm_isr(void)
{
    // 1. Read sensor data
    // 2. Execute PID calculation
    // 3. Update PWM duty cycle
    __DSB();  // Ensure memory operations are complete
}

Optimization Results:

• Interrupt latency reduced from 1.2μs to 0.3μs

• Execution time jitter reduced by 80%

3.2 Memory-Constrained System Design

Strategy:

1. Keep low-frequency access code in Flash

2. Only relocate hot functions to RAM

3. Use <span>__ramfunc</span> in conjunction with <span>CONFIG_ISR_STACK_SIZE</span> for adjustments

Memory Savings:

• Typical savings of 30-50% in RAM usage

• Extended battery-powered device runtime by 15-20%

3.3 Dynamic Firmware Update Solutions

Implement secure relocation with MCUboot:

1. Preparation Phase:

   west sign -t imgtool --align 8 --pad
   

2. Verification Phase:

   boot_is_img_confirmed()
   

3. Switching Phase:

   boot_write_img_confirmed()
   

4. Debugging and Performance Analysis in Practice

4.1 Diagnostic Toolchain

1. Memory Layout Analysis:

   west build -t ram_report
   

   Output Example:

   Region    Start      End        Size
   FLASH     0x0800xxxx 0x0801xxxx 64KB
   SRAM      0x2000xxxx 0x2001xxxx 32KB
   

2. Runtime Verification:

   printk("Function %p is in %s memory\n", 
         func_ptr, 
         (func_ptr >= SRAM_START) ? "RAM" : "Flash");
   

4.2 Performance Benchmark Testing

Use ARM DWT counters for precise measurement:

uint32_t start = DWT->CYCCNT;
critical_function();
uint32_t cycles = DWT->CYCCNT - start;

Typical result comparison:

Execution Location Average Cycle Count Worst CaseFlash 850 1200RAM 320 350

5. Best Practices and Common Pitfalls

5.1 Design Guidelines

1. Granularity Control:

• Relocating individual functions is better than entire files

• Prioritize small functions that are executed frequently

1. Dependency Management:

   zephyr_library_sources_ifdef(
      CONFIG_ENABLE_RELOCATION
      src/critical.c
   )
   

2. Safety Considerations:

• Enable MPU protection for relocated areas

• Use CRC checks to verify relocation integrity

5.2 Typical Problem Solutions

Problem 1: Function pointers become invalid after relocation

Solution:

// Use a unified relocation-aware pointer macro
#define RELOC_PTR(ptr) ((typeof(ptr))((uintptr_t)(ptr) + relocation_bias))

Problem 2: Cache consistency in multi-core systems

Solution:

SCB_CleanDCache_by_Addr((uint32_t*)func_ptr, func_size);

6. Future Evolution and Extended Applications

6.1 Machine Learning Acceleration

Relocate neural network inference engines to dedicated memory:

zephyr_code_relocate(
    FILES src/nn_engine.c
    LOCATION AI_ACCELERATOR
)

6.2 Heterogeneous Computing Support

/ {
    compute_subsystem {
        compatible = "openamp,compute";
        memory-region = <&compute_ram>;
    };
};

Conclusion: Mastering Relocation to Unleash Hardware Potential

Through this in-depth exploration, we systematically analyzed the implementation mechanisms and application methods of Zephyr code relocation technology. This technology is not just simple memory relocation; it is a key optimization tool in embedded system design. In practical projects, it is recommended to:

1. Start with small-scale experiments to gradually validate effects

2. Establish performance benchmarks to quantify optimization benefits

3. Customize solutions based on specific hardware characteristics

As the complexity of IoT devices continues to rise, mastering code relocation technology will become one of the core competencies for embedded developers. We hope this article provides substantial help and inspiration for your development work.