The startup file for Cortex-M0 cores can be written in either assembly language or C. Since the startup file "startup_stm32f0xx.s" that came with the STM32F0Discovery Firmware package was already written in assembly language, I opted to simply use it. This is very likely to be the only source file that I will cover that will be written in assembly language. Everything else is written in C.
Also, since I'm using my own linker file (the Atollic Linker file that came in the STM32F0Discovery Firmware Package was copyrighted and didn't provide users permission to use it without using the Atollic Toolchain/ IDE) I had to modify the label _estack to _stack_start.
/********************************************************************************
* @file startup_stm32f0xx.s
* @author MCD Application Team
* @version V1.0.0
* @date 23-March-2012
* @brief STM32F0xx Devices vector table for RIDE7 toolchain.
* This module performs:
* - Set the initial SP
* - Set the initial PC == Reset_Handler,
* - Set the vector table entries with the exceptions ISR address
* - Branches to main in the C library (which eventually
* calls main()).
* After Reset the Cortex-M0 processor is in Thread mode,
* priority is Privileged, and the Stack is set to Main.
*****************************************************************************
*
* @attention
*
* <h2><center>© COPYRIGHT 2012 STMicroelectronics</center></h2>
*
* Licensed under MCD-ST Liberty SW License Agreement V2, (the "License");
* You may not use this file except in compliance with the License.
* You may obtain a copy of the License at:
*
* http://www.st.com/software_license_agreement_liberty_v2
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*
******************************************************************************
* Modification by Hussam Al-Hertani:
* _estack symbol modified to _stack_start symbol to match my linker file
******************************************************************************/
.syntax unified
.cpu cortex-m0
.fpu softvfp
.thumb
.global g_pfnVectors
.global Default_Handler
/* start address for the initialization values of the .data section.
defined in linker script */
.word _sidata
/* start address for the .data section. defined in linker script */
.word _sdata
/* end address for the .data section. defined in linker script */
.word _edata
/* start address for the .bss section. defined in linker script */
.word _sbss
/* end address for the .bss section. defined in linker script */
.word _ebss
.equ BootRAM, 0xF108F85F
/**
* @brief This is the code that gets called when the processor first
* starts execution following a reset event. Only the absolutely
* necessary set is performed, after which the application
* supplied main() routine is called.
* @param None
* @retval : None
*/
.section .text.Reset_Handler
.weak Reset_Handler
.type Reset_Handler, %function
Reset_Handler:
/* Copy the data segment initializers from flash to SRAM */
movs r1, #0
b LoopCopyDataInit
CopyDataInit:
ldr r3, =_sidata
ldr r3, [r3, r1]
str r3, [r0, r1]
adds r1, r1, #4
LoopCopyDataInit:
ldr r0, =_sdata
ldr r3, =_edata
adds r2, r0, r1
cmp r2, r3
bcc CopyDataInit
ldr r2, =_sbss
b LoopFillZerobss
/* Zero fill the bss segment. */
FillZerobss:
movs r3, #0
str r3, [r2]
adds r2, r2, #4
LoopFillZerobss:
ldr r3, = _ebss
cmp r2, r3
bcc FillZerobss
/* Call the clock system intitialization function.*/
bl SystemInit
/* Call the application's entry point.*/
bl main
bx lr
.size Reset_Handler, .-Reset_Handler
/**
* @brief This is the code that gets called when the processor receives an
* unexpected interrupt. This simply enters an infinite loop, preserving
* the system state for examination by a debugger.
*
* @param None
* @retval None
*/
.section .text.Default_Handler,"ax",%progbits
Default_Handler:
Infinite_Loop:
b Infinite_Loop
.size Default_Handler, .-Default_Handler
/*******************************************************************************
*
* The minimal vector table for a Cortex M0. Note that the proper constructs
* must be placed on this to ensure that it ends up at physical address
* 0x0000.0000.
*******************************************************************************/
.section .isr_vector,"a",%progbits
.type g_pfnVectors, %object
.size g_pfnVectors, .-g_pfnVectors
g_pfnVectors:
.word _stack_start
.word Reset_Handler
.word NMI_Handler
.word HardFault_Handler
.word 0
.word 0
.word 0
.word 0
.word 0
.word 0
.word 0
.word SVC_Handler
.word 0
.word 0
.word PendSV_Handler
.word SysTick_Handler
.word WWDG_IRQHandler
.word PVD_IRQHandler
.word RTC_IRQHandler
.word FLASH_IRQHandler
.word RCC_IRQHandler
.word EXTI0_1_IRQHandler
.word EXTI2_3_IRQHandler
.word EXTI4_15_IRQHandler
.word TS_IRQHandler
.word DMA1_Channel1_IRQHandler
.word DMA1_Channel2_3_IRQHandler
.word DMA1_Channel4_5_IRQHandler
.word ADC1_COMP_IRQHandler
.word TIM1_BRK_UP_TRG_COM_IRQHandler
.word TIM1_CC_IRQHandler
.word TIM2_IRQHandler
.word TIM3_IRQHandler
.word TIM6_DAC_IRQHandler
.word 0
.word TIM14_IRQHandler
.word TIM15_IRQHandler
.word TIM16_IRQHandler
.word TIM17_IRQHandler
.word I2C1_IRQHandler
.word I2C2_IRQHandler
.word SPI1_IRQHandler
.word SPI2_IRQHandler
.word USART1_IRQHandler
.word USART2_IRQHandler
.word 0
.word CEC_IRQHandler
.word 0
.word BootRAM /* @0x108. This is for boot in RAM mode for
STM32F0xx devices. */
/*******************************************************************************
*
* Provide weak aliases for each Exception handler to the Default_Handler.
* As they are weak aliases, any function with the same name will override
* this definition.
*
*******************************************************************************/
.weak NMI_Handler
.thumb_set NMI_Handler,Default_Handler
.weak HardFault_Handler
.thumb_set HardFault_Handler,Default_Handler
.weak SVC_Handler
.thumb_set SVC_Handler,Default_Handler
.weak PendSV_Handler
.thumb_set PendSV_Handler,Default_Handler
.weak SysTick_Handler
.thumb_set SysTick_Handler,Default_Handler
.weak WWDG_IRQHandler
.thumb_set WWDG_IRQHandler,Default_Handler
.weak PVD_IRQHandler
.thumb_set PVD_IRQHandler,Default_Handler
.weak RTC_IRQHandler
.thumb_set RTC_IRQHandler,Default_Handler
.weak FLASH_IRQHandler
.thumb_set FLASH_IRQHandler,Default_Handler
.weak RCC_IRQHandler
.thumb_set RCC_IRQHandler,Default_Handler
.weak EXTI0_1_IRQHandler
.thumb_set EXTI0_1_IRQHandler,Default_Handler
.weak EXTI2_3_IRQHandler
.thumb_set EXTI2_3_IRQHandler,Default_Handler
.weak EXTI4_15_IRQHandler
.thumb_set EXTI4_15_IRQHandler,Default_Handler
.weak TS_IRQHandler
.thumb_set TS_IRQHandler,Default_Handler
.weak DMA1_Channel1_IRQHandler
.thumb_set DMA1_Channel1_IRQHandler,Default_Handler
.weak DMA1_Channel2_3_IRQHandler
.thumb_set DMA1_Channel2_3_IRQHandler,Default_Handler
.weak DMA1_Channel4_5_IRQHandler
.thumb_set DMA1_Channel4_5_IRQHandler,Default_Handler
.weak ADC1_COMP_IRQHandler
.thumb_set ADC1_COMP_IRQHandler,Default_Handler
.weak TIM1_BRK_UP_TRG_COM_IRQHandler
.thumb_set TIM1_BRK_UP_TRG_COM_IRQHandler,Default_Handler
.weak TIM1_CC_IRQHandler
.thumb_set TIM1_CC_IRQHandler,Default_Handler
.weak TIM2_IRQHandler
.thumb_set TIM2_IRQHandler,Default_Handler
.weak TIM3_IRQHandler
.thumb_set TIM3_IRQHandler,Default_Handler
.weak TIM6_DAC_IRQHandler
.thumb_set TIM6_DAC_IRQHandler,Default_Handler
.weak TIM14_IRQHandler
.thumb_set TIM14_IRQHandler,Default_Handler
.weak TIM15_IRQHandler
.thumb_set TIM15_IRQHandler,Default_Handler
.weak TIM16_IRQHandler
.thumb_set TIM16_IRQHandler,Default_Handler
.weak TIM17_IRQHandler
.thumb_set TIM17_IRQHandler,Default_Handler
.weak I2C1_IRQHandler
.thumb_set I2C1_IRQHandler,Default_Handler
.weak I2C2_IRQHandler
.thumb_set I2C2_IRQHandler,Default_Handler
.weak SPI1_IRQHandler
.thumb_set SPI1_IRQHandler,Default_Handler
.weak SPI2_IRQHandler
.thumb_set SPI2_IRQHandler,Default_Handler
.weak USART1_IRQHandler
.thumb_set USART1_IRQHandler,Default_Handler
.weak USART2_IRQHandler
.thumb_set USART2_IRQHandler,Default_Handler
.weak CEC_IRQHandler
.thumb_set CEC_IRQHandler,Default_Handler
/************************ (C) COPYRIGHT STMicroelectronics *****END OF FILE****/
The first section of code that I'd like to analyze starts at line 71 and is the "Reset_Handler" code. This code was defined by our linker to be the entry point for our startup code as is saved at the very top of the .text section.
The First chunk of code that runs in the Reset_Handler function is the "LoopCopyDataInit" subroutine ( called on line 83). This subroutine copies the iniitalization data for the variables in .data section from the (LMA).data section in Flash to the (VMA) .data section in RAM. This ensures that all static and global variables in RAM are properly initialized to the values determined in the source code.
After the .data section is initialized another subroutine "LoopFillZerobss" is called on line 90. This subroutine ensures that all the uninitialized global and static variables of the .bss section are initialized to zero.
The next subroutine that is called is the "SystemInit" subroutine on line 102. This is actually a C function that can be found in system_stm32f0xx.c source file. It basically puts the MCU in a "uninitialized" state and then sets the clock tree to Flash wait states to specific values, essentially setting the CPU's clock speed. We will be looking at that function in more detail in a future blog entry.
The final subroutine that is called is the "main" subroutine at line 104. This is the main function in the source file "main.c" and is the heart of our source code.
The next chunk of code that I'd like to look at is the interrupt vector table. The interrupt vector table is the component of the .isr_vector section as can be seen on line 127. It starts on line 133 under the g_pfnVectors label. Whenever an event occurs on an enabled interrupt source, code execution will jump to the prescribed interrupt source's vector address, where it will hopefully find the interrupt service routine / function that services that interrupt or event. The interrupt vector table starts on line 133 all the way to line 178. each ".word" entry corresponds to a interrupt vector address. Each of these addresses is associated wiith an interrupt subroutine/function for example the interrupt vector address on line 133 is that of the Reset interrupt and is associated with the "Reset_Handler" routine discussed above.
The next chunk of code (lines 191-291) assigns a weak alias "Default_Handler" to all of the interrupt subroutines in the interrupt vector table found on lines 134-178. So that if the interrupt handler is not defined anywhere in the project, the interrupt vector address is assigned to the "Default_Handler" weak alias handler routine. If an appropriate interrupt handler is defined however, it will take precedence to this weak alias. The "Default_Handler" subroutine is defined on line 117 and is basically an infinite loop.
One interesting observation can been found on line 132. The first location of the .isr_vector section and thus the interrupt vector table must always contain the inital stack pointer address. This is typically the last location of the RAM block and is the starting location of the stack defined by the label _stack_start.