Quality RTOS & Embedded Software

 Real time embedded FreeRTOS RSS feed 
Quick Start Supported MCUs PDF Books Trace Tools Ecosystem


problem with GCC ARM_CM3 port

Posted by gbiagioni on December 7, 2015

The implementation of prvPortStartFirstTask( void ) in FreeRTOS/Source/portable/GCC/ARM_CM3/port.c begins with the following assembly instructions

" ldr r0, =0xE000ED08 n" /* Use the NVIC offset register to locate the stack. / " ldr r0, [r0] n" " ldr r0, [r0] n" " msr msp, r0 n" / Set the msp back to the start of the stack. */

This strips the stack back to the start of stack as defined in the interrupt vector table. This has the undesirable effect of losing automatic variables that were on the stack at the time the first task is started. This is bad. For example, if tasks are created before scheduler startup and are passed addresses of automatic variables, they will likely see clobbered data.

Is there some reason that the stack is stripped back in this way?

problem with GCC ARM_CM3 port

Posted by rtel on December 7, 2015

See the following discussion: http://www.freertos.org/FreeRTOSSupportForumArchive/January2015/freertosMainstackpointerresetwhenstartingthescheduler_e5a776c1j.html

I think strictly speaking this is against the C standard - however considered desirable when using very small microcontrollers. I think the first Cortex-M port was done on a device with 2K of RAM.

[ Back to the top ]    [ About FreeRTOS ]    [ Privacy ]    [ Sitemap ]    [ ]

Copyright (C) Amazon Web Services, Inc. or its affiliates. All rights reserved.

Latest News

NXP tweet showing LPC5500 (ARMv8-M Cortex-M33) running FreeRTOS.

Version 10.1.1 of the FreeRTOS kernel is available for immediate download. MIT licensed.

View a recording of the "OTA Update Security and Reliability" webinar, presented by TI and AWS.


FreeRTOS and other embedded software careers at AWS.

FreeRTOS Partners

ARM Connected RTOS partner for all ARM microcontroller cores

Espressif ESP32

IAR Partner

Microchip Premier RTOS Partner

RTOS partner of NXP for all NXP ARM microcontrollers


STMicro RTOS partner supporting ARM7, ARM Cortex-M3, ARM Cortex-M4 and ARM Cortex-M0

Texas Instruments MCU Developer Network RTOS partner for ARM and MSP430 microcontrollers

OpenRTOS and SafeRTOS

Xilinx Microblaze and Zynq partner