In this lesson, you will learn how to transition from a feature-rich ELF executable to a clean, hardware-ready raw binary image using objcopy.
Why ELF is Not Enough for Flash
Up to this point, you have been working with ELF (Executable and Linkable Format) files. ELFs are excellent for development because they contain essential metadata: symbol tables for debugging, section headers for the linker, and relocation information. However, your target microcontroller’s flash memory does not understand ELF headers. If you were to program the raw ELF file directly onto a chip, the processor would attempt to "execute" the metadata as if it were machine code, leading to an immediate hard fault.
The Role of objcopy
The GNU objcopy utility acts as a translator. It takes your ELF file and strips away the "wrapping"—the debug symbols and non-loadable sections—to leave behind only the raw machine code bytes that need to be mapped into the device's memory addresses.
To generate a raw binary image (.bin), you typically execute a command similar to this:
arm-none-eabi-objcopy -O binary firmware.elf firmware.bin
Understanding the Conversion
When you run this command, objcopy parses the ELF headers to identify sections marked as LOAD (typically your .text and .data sections). It ignores debug sections like .debug_info or .symtab. The output is a flat stream of bytes that perfectly mirrors the layout of your target flash memory.
If your application has large gaps in its memory map (for example, code at address 0x08000000 and configuration data at 0x0800F000), objcopy will fill the intervening space with padding bytes (usually 0xFF or 0x00), ensuring the binary remains a contiguous block that can be programmed directly via your bootloader or external flash programmer.
By automating this step within your Makefile, you ensure that every build produces a lean, deployable image ready for production or field updates.
Key Takeaway: Use objcopy -O binary to strip high-level ELF metadata and generate a concise, address-aligned payload that your microcontroller can execute immediately upon reset.