Verifying Memory Constraints with Linker Assertions

You will implement linker script assertions to trigger build errors if the compiled code exceeds available physical memory limits.

In this lesson, you will learn how to proactively stop the build process when your project’s memory footprint exceeds the physical hardware limits of your target device.

The Problem: Silent Overflows

In embedded systems, we define our hardware layout using the MEMORY command in a linker script. However, the linker doesn’t automatically raise an error if a section (like .text or .data) grows so large that it overlaps into another region or spills outside the defined memory boundary. Without a safeguard, you might only discover an overflow during runtime when the processor fetches invalid instructions or data, leading to unpredictable system crashes.

The Solution: Linker Assertions

The linker script provides an ASSERT() command that evaluates a logical expression during the final linking phase. If the condition is false, the linker halts the build and outputs a diagnostic message. This is typically used at the end of the linker script to check that the current location counter . has not exceeded a specific memory address boundary.

Consider this example where we ensure our combined .text and .data sections do not exceed a 64KB limit:

SECTIONS
{
    .text : { *(.text) } > FLASH
    .data : { *(.data) } > RAM
    
    /* Calculate the end of used memory */
    _end_of_code = .;
    
    /* Verify we haven't crossed the 64KB limit */
    ASSERT(_end_of_code <= 0x10000, "Error: Code size exceeds 64KB limit!")
}

In this snippet, _end_of_code captures the final address after all sections have been placed. The ASSERT command checks this address against our known physical limit (0x10000). If the code size grows beyond this point, the linker stops immediately, preventing the creation of a broken binary.

You can also use this to check for stack or heap overlaps, or to ensure that critical data structures are aligned within specific address ranges. By incorporating these checks, you transform "undefined behavior" into "predictable build failure," allowing you to address resource constraints early in the development cycle rather than in the field.

Key Takeaway: Use the ASSERT command in your linker script to turn memory overflows into immediate build errors, ensuring your code always fits within the physical constraints of your target hardware.