Your firmware is close to the flash and RAM limits of the MCU. How do you reduce memory usage?

What the interviewer wants to hear

• Measure first with the map file, size and nm
• Compiler flags: -Os, LTO, function/data sections with gc-sections
• Remove heavy library features like float printf
• Make tables const so they stay in flash
• Right-size stacks, buffers and heap using measured usage

Model answer

I start by measuring, not guessing: the linker map file and arm-none-eabi-nm --size-sort show which functions and data are largest, and tools like puncover make it visual. For flash, I build with -Os, enable link-time optimisation, and use -ffunction-sections and -fdata-sections with --gc-sections so unused code is dropped. Library choices often matter most: a printf with float support or a big HAL driver can cost tens of kilobytes, so I use newlib-nano or a small printf, and register-level code for simple peripherals. For RAM, I make lookup tables and strings const so they stay in flash, measure task stacks with high-water marks and shrink them, size buffers from real data rates, and remove the heap if nothing needs it. I track memory size in CI so growth is caught in code review.

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