How a Microcontroller starts

Accompanying code for the video How a Microcontroller starts - watch the video for more details.

Tools

• NUCLEO-F042K6
• VS Code with the Cortex-Debug extension

sudo apt install gcc-arm-none-eabi
sudo apt install openocd gdb-multiarch

Commands

Disassemble and size

Utilities for inspecting size and disassembly.

arm-none-eabi-size myprogram.elf
arm-none-eabi-objdump -D myprogram.elf | less

Compile myprogram.c

Compile minimal C program.

arm-none-eabi-gcc -mcpu=cortex-m0 -mthumb -std=c11 -g -O0 -c myprogram_simple.c -o myprogram_simple.o

Link WITH default libc (Newlib), startfiles, and linker script

Link with default behaviour of toolchain including default standard C library, startfiles and linker script. Insert empty stubs (nosys.specs) for Newlib's system calls.

arm-none-eabi-gcc --specs=nosys.specs myprogram_simple.o -o myprogram.elf

Link WITHOUT libc, startfiles, and default linker script

Discard the default libc, startfiles, and linker script.

arm-none-eabi-gcc -nolibc -nostartfiles -T empty.ld -Wl,--verbose myprogram_simple.o -o myprogram.elf

Link with custom startup file and linker script

Compile a custom startup file and link with custom linker script.

arm-none-eabi-gcc -mcpu=cortex-m0 -mthumb -std=c11 -g -O0 -c myprogram_with_vars.c -o myprogram_with_vars.o
arm-none-eabi-gcc -mcpu=cortex-m0 -mthumb -std=c11 -g -O0 -c startup_stm32f042k6.c -o startup_stm32f042k6.o
arm-none-eabi-gcc -nolibc -nostartfiles -T stm32f042k6.ld myprogram_with_vars.o startup_stm32f042k6.o -o myprogram.elf

Link with libc (Newlib)

arm-none-eabi-gcc -mcpu=cortex-m0 -mthumb -std=c11 -g -O0 -c myprogram_with_libc.c -o myprogram_with_libc.o
arm-none-eabi-gcc -mcpu=cortex-m0 -mthumb -std=c11 -g -O0 -c startup_stm32f042k6_with_libc.c -o startup_stm32f042k6_with_libc.o
arm-none-eabi-gcc -mcpu=cortex-m0 -mthumb -nostartfiles --specs=nosys.specs --specs=nano.specs -T stm32f042k6.ld myprogram_with_libc.o startup_stm32f042k6_with_libc.o -o myprogram.elf