In this lesson, you will learn how to use linker directives to force data structures and code sections to specific memory boundaries required by your microcontroller's hardware.
Why Alignment Matters
In embedded systems, hardware peripherals and CPU architectures often have strict requirements for how data is placed in memory. For instance, a DMA controller might require a data buffer to start on a 32-byte boundary to support burst transfers, or a specific cache line might require 16-byte alignment to prevent data corruption. If your data structure starts at an "unaligned" address, the hardware might trigger a bus fault, perform slower unaligned accesses, or fail to cache the data correctly.
Controlling Alignment with the Linker
While your C compiler handles basic alignment, you often need to move data to specific sections to satisfy hardware constraints. You do this using the ALIGN command within your linker script.
When defining a memory region, you can ensure that the starting address of a section is a multiple of a specific power of two. Here is how you might force a section meant for DMA buffers to be aligned to a 32-byte boundary:
.dma_buffer_section : ALIGN(32)
{
*(.dma_data)
} > RAM
In your C code, you must ensure the compiler doesn't "re-pack" your variables in a way that ignores this alignment. You can use compiler-specific attributes to enforce this alongside your linker script:
uint8_t dma_buffer[1024] __attribute__((section(".dma_data"), aligned(32)));
How it Works
The ALIGN(32) directive tells the linker to "skip" forward in the current memory region until the address is a multiple of 32. If the previous section ended at address 0x20000007, the linker will automatically insert padding bytes until it reaches 0x20000020.
By combining these linker directives with C attributes, you create a "contract" between your software and the hardware: you guarantee that the data lives exactly where the peripheral controller expects it to be, preventing bus errors and ensuring optimal data transfer performance.
Key Takeaway: Use linker ALIGN directives and C alignment attributes to force data structures onto specific memory boundaries required by hardware peripherals or cache architectures, preventing alignment-related bus faults and performance bottlenecks.