What is Scatter Loading in ARM: Explanation and Example

In ARM, scatter loading is a method to initialize different memory sections with specific data during program startup using a scatter-loading table. It tells the processor where to place various parts of the program or data in memory, allowing flexible and efficient memory layout.

How It Works

Scatter loading works like a detailed map for the ARM processor during program startup. Imagine you have different boxes of items that need to be placed in specific rooms of a house. The scatter-loading table acts as the map telling which box goes to which room.

In technical terms, the scatter-loading table lists memory regions and the data or code that should be loaded into each. When the program starts, the loader reads this table and copies the data from a source (like flash memory) to the correct destination in RAM or other memory areas.

This allows the program to organize code and data efficiently, placing critical code in fast memory and large data in slower memory, improving performance and memory use.

Example

This example shows a simple scatter-loading table snippet for an ARM linker script. It defines where the .text (code) and .data (initialized data) sections should be loaded.

ld

LR_IROM1 0x08000000 0x00080000  {    ; load region in flash
  ER_IROM1 0x08000000 0x00080000  {  ; execution region in flash
    *.o (RESET, +First)
    *(InRoot$$Sections)
    .ANY (+RO)
  }
  RW_IRAM1 0x20000000 0x00020000  {  ; RAM region
    .ANY (+RW +ZI)
  }
}

When to Use

Use scatter loading when you need precise control over where code and data are placed in memory. This is common in embedded systems where memory is limited and different types of memory have different speeds or properties.

For example, you might want to place critical interrupt code in fast on-chip RAM for quick response, while placing large buffers in slower external RAM. Scatter loading lets you specify these placements clearly.

It is also useful when combining multiple memory types or when booting from non-volatile memory and copying data to RAM at startup.

Key Points

• Scatter loading uses a table to map program sections to memory addresses.
• It enables flexible and efficient memory layout in ARM systems.
• Commonly used in embedded systems with multiple memory types.
• Helps optimize performance by placing code/data in appropriate memory.

Key Takeaways

Scatter loading directs where program parts are placed in memory during startup.

It improves memory use and performance by organizing code and data efficiently.

Essential for embedded ARM systems with varied memory types.

Defined using a scatter-loading table or linker script.

Helps place critical code in fast memory and large data in slower memory.