ISR Stack Usage on mbed

I’ve written some code to help exactly measure the ISR stack usage on ARM mbed:


Usage of the library has also been written up on the mbed website!


Here are some notes on how to fill the stack area with a canary value, and then measure the total ISR stack usage at runtime.

Here I am messing around with an NRF51_DK target, which has a total of 32KB of RAM.

The stack configuration is located in the startup_NRF51822.S file, which is located in the following folder for GCC ARM:

You can find out which assembly file is used for chip startup by running the mbed compile -c -v command, which then dumps a verbose amount of detail:

Compile [ 70.9%]: startup_NRF51822.S
[DEBUG] Compile: C:\tools\gcc-arm-none-eabi-4_9-2014q4-20141203-win32\bin\arm-none-eabi-gcc -x assembler-with-cpp -c -Wall -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -fmessage-length=0 -fno-exceptions -fno-builtin -ffunction-sections -fdata-sections -funsigned-char -MMD -fno-delete-null-pointer-checks -fomit-frame-pointer -Os -mcpu=cortex-m0 -mthumb -D__CORTEX_M0 -DTARGET_MCU_NRF51822 -DTARGET_MCU_NORDIC_32K -DSOFTDEVICE_PRESENT -DNRF51 -DTARGET_NRF51822 -DARM_MATH_CM0 -D__MBED_CMSIS_RTOS_CM -DS130 -DBLE_STACK_SUPPORT_REQD -D__CMSIS_RTOS -DTARGET_MCU_NRF51_32K @.\BUILD\NRF51_DK\GCC_ARM\.includes_7f5e7dfc3f0eeb6c86bfd931d07b1e3f.txt -include .\BUILD\NRF51_DK\GCC_ARM\mbed_config.h -o .\BUILD\NRF51_DK\GCC_ARM\mbed-os\targets\TARGET_NORDIC\TARGET_NRF5\TARGET_MCU_NRF51822_UNIFIED\device\TOOLCHAIN_GCC_ARM\startup_NRF51822.o .\mbed-os\targets\TARGET_NORDIC\TARGET_NRF5\TARGET_MCU_NRF51822_UNIFIED\device\TOOLCHAIN_GCC_ARM\startup_NRF51822.S
[DEBUG] Return: 0

In the Reset_Handler, there’s a segment that calls out and sets up a handful of items before the main program starts.

It is here that the ISR stack memory can be filled with some canary value:

This fills the stack area with 0xAFFEC7ED. The cool thing is that the BLX assembly instruction jumps to this routine w/o pushing anything to stack, so at this point the ARM stack pointer should be pointing to the absolute top of ISR stack memory. The return to caller is handled by the ARM Link Register:

Later, the total stack usage can be calculated by the routine:

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