Startup sequence and reset vector in Embedded C

Introduction

The startup sequence prepares the microcontroller to run your program. The reset vector tells the processor where to start running code after a reset.

When you want to understand how your embedded program begins running after power on.

When debugging why your microcontroller does not start your main program.

When writing low-level code that runs before main(), like initializing hardware.

When customizing the reset behavior or adding your own startup code.

When learning how embedded systems boot up step-by-step.

Syntax

Embedded C

void Reset_Handler(void) {
    // Initialization code here
    main();
}

// Vector table example
void (* const vector_table[])(void) __attribute__((section(".isr_vector"))) = {
    Reset_Handler, // Reset vector
    // Other interrupt handlers
};

The reset vector is a pointer to the Reset_Handler function.

The startup sequence runs before main() and sets up memory and hardware.

Examples

This Reset_Handler clears uninitialized variables and copies initialized variables before calling main.

Embedded C

void Reset_Handler(void) {
    // Clear .bss section
    // Copy .data section from flash to RAM
    main();
}

The vector table is placed in a special memory section so the processor knows where to find interrupt handlers.

Embedded C

void (* const vector_table[])(void) __attribute__((section(".isr_vector"))) = {
    Reset_Handler, // Reset vector
    NMI_Handler,
    HardFault_Handler,
    // ... other handlers
};

Sample Program

This program simulates the startup sequence by copying initialized data and clearing uninitialized data before running main.

Embedded C

#include <stdint.h>

// Simulated memory sections
uint32_t data_init[] = {1, 2, 3};
uint32_t data[3];
uint32_t bss[3];

void main(void) {
    // Main program starts here
    for (int i = 0; i < 3; i++) {
        // Print data and bss values
        // In embedded, you might toggle LEDs or send data
    }
}

void Reset_Handler(void) {
    // Copy initialized data from flash to RAM
    for (int i = 0; i < 3; i++) {
        data[i] = data_init[i];
    }
    // Clear bss section
    for (int i = 0; i < 3; i++) {
        bss[i] = 0;
    }
    main();
}

// Vector table with reset vector
void (* const vector_table[])(void) __attribute__((section(".isr_vector"))) = {
    Reset_Handler
};

// Simulate reset by calling Reset_Handler
int main_wrapper() {
    Reset_Handler();
    return 0;
}

Important Notes

The reset vector is usually the first entry in the vector table.

The startup code runs before main() and prepares memory and hardware.

In real embedded systems, startup code is often written in assembly or provided by the compiler.

Summary

The reset vector tells the processor where to start after reset.

The startup sequence prepares memory and hardware before main() runs.

Understanding this helps you debug and customize embedded programs.

Practice

1. What is the primary role of the reset vector in an embedded C program?

easy

A. It manages power consumption modes.

B. It initializes all global variables to zero.

C. It handles interrupts during program execution.

D. It tells the microcontroller where to start executing code after a reset.

Solution

1. Step 1: Understand the reset vector concept

   The reset vector is a special address in memory that points to the first instruction executed after a reset.

2. Step 2: Identify its role in program start

   When the microcontroller resets, it jumps to the reset vector to begin running the startup code.

3. Final Answer:

   It tells the microcontroller where to start executing code after a reset. -> Option D

4. Quick Check:

   Reset vector = start address after reset [OK]

Hint: Reset vector points to start code after reset [OK]

Common Mistakes:

• Confusing reset vector with interrupt vector
• Thinking reset vector initializes variables
• Assuming reset vector manages power modes

2. Which of the following is the correct way to declare the reset vector in embedded C?

easy

A. int ResetVector = 0x0000;

B. void ResetVector() { main(); }

C. void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler;

D. char *ResetVector = "Reset_Handler";

Solution

1. Step 1: Understand reset vector declaration

   The reset vector is usually a pointer to a function placed in a special memory section.

2. Step 2: Check syntax correctness

   void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler; correctly declares a constant function pointer to Reset_Handler placed in the ".reset_vector" section using GCC attribute syntax.

3. Final Answer:

   void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler; -> Option C

4. Quick Check:

   Function pointer in reset vector section = correct declaration [OK]

Hint: Reset vector is a function pointer in special section [OK]

Common Mistakes:

• Declaring reset vector as int or char pointer
• Defining reset vector as a function instead of pointer
• Missing section attribute for placement

3. Given the following code snippet, what will be the output after reset?

void Reset_Handler(void) {
    int x = 5;
    x += 10;
    while(1) {}
}

void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler;

medium

A. The program will enter an infinite loop after setting x to 15.

B. The program will run main() and print 15.

C. The program will cause a runtime error due to missing main.

D. The program will reset repeatedly.

Solution

1. Step 1: Analyze Reset_Handler behavior

   Reset_Handler sets x to 5, adds 10 making x = 15, then enters an infinite loop.

2. Step 2: Understand program flow after reset

   After reset, the microcontroller jumps to Reset_Handler and stays in the infinite loop; no main() is called.

3. Final Answer:

   The program will enter an infinite loop after setting x to 15. -> Option A

4. Quick Check:

   Reset_Handler runs then loops forever = infinite loop [OK]

Hint: Reset_Handler runs then loops forever, no main called [OK]

Common Mistakes:

• Assuming main() is called automatically
• Expecting output or print statements
• Confusing variable x scope and effect

4. Identify the error in this startup code snippet:

void Reset_Handler(void) {
    // Initialize data
    extern int _sidata, _sdata, _edata;
    int *src = &_sidata;
    int *dst = &_sdata;
    while(dst < &_edata) {
        *dst++ = *src++;
    }
    main();
}

void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler;

medium

A. Reset_Handler should not call main() directly.

B. The pointers _sidata, _sdata, _edata should be declared as extern char, not int.

C. The while loop condition should use <= instead of <.

D. ResetVector should not be a function pointer.

Solution

1. Step 1: Check pointer types for memory copy

   Memory sections like _sidata, _sdata, _edata represent byte addresses and should be pointers to char or unsigned char, not int.

2. Step 2: Understand pointer arithmetic impact

   Using int pointers causes incorrect copying because pointer arithmetic advances by sizeof(int), skipping bytes.

3. Final Answer:

   The pointers _sidata, _sdata, _edata should be declared as extern char, not int. -> Option B

4. Quick Check:

   Memory pointers for data init = char* not int* [OK]

Hint: Use char pointers for memory copy in startup [OK]

Common Mistakes:

• Using int pointers for byte-wise memory copy
• Changing loop condition incorrectly
• Thinking main() call is wrong here

5. You want to create a startup sequence that initializes .data and .bss sections and then jumps to main(). Which of the following code snippets correctly implements this in embedded C?

hard

A. void Reset_Handler(void) {
extern char _sidata, _sdata, _edata, _sbss, _ebss;
char *src = &_sidata;
char *dst = &_sdata;
while(dst < &_edata) *dst++ = *src++;
dst = &_sbss;
while(dst < &_ebss) *dst++ = 0;
main();
}
void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler;

B. void Reset_Handler(void) {
int *src = &_sidata;
int *dst = &_sdata;
while(dst <= &_edata) *dst++ = *src++;
main();
}
void ResetVector = Reset_Handler;

C. void Reset_Handler(void) {
main();
}
void (* ResetVector)(void) = Reset_Handler;

D. void Reset_Handler(void) {
extern char _sidata, _sdata, _edata;
char *src = &_sdata;
char *dst = &_sidata;
while(src < &_edata) *dst++ = *src++;
main();
}
void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler;

Solution

1. Step 1: Verify .data section initialization

   void Reset_Handler(void) { extern char _sidata, _sdata, _edata, _sbss, _ebss; char *src = &_sidata; char *dst = &_sdata; while(dst < &_edata) *dst++ = *src++; dst = &_sbss; while(dst < &_ebss) *dst++ = 0; main(); } void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler; correctly copies initialized data from flash (_sidata) to RAM (_sdata to _edata) using char pointers and proper loop.

2. Step 2: Verify .bss section zero initialization

   void Reset_Handler(void) { extern char _sidata, _sdata, _edata, _sbss, _ebss; char *src = &_sidata; char *dst = &_sdata; while(dst < &_edata) *dst++ = *src++; dst = &_sbss; while(dst < &_ebss) *dst++ = 0; main(); } void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler; zeroes the .bss section (_sbss to _ebss) by setting bytes to 0 before calling main().

3. Step 3: Check reset vector declaration

   void Reset_Handler(void) { extern char _sidata, _sdata, _edata, _sbss, _ebss; char *src = &_sidata; char *dst = &_sdata; while(dst < &_edata) *dst++ = *src++; dst = &_sbss; while(dst < &_ebss) *dst++ = 0; main(); } void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler; declares ResetVector as a constant function pointer placed in the reset vector section, which is correct.

4. Final Answer:

   Option A correctly implements startup sequence with .data and .bss initialization and reset vector. -> Option A

5. Quick Check:

   Proper .data/.bss init + correct reset vector = void Reset_Handler(void) { extern char _sidata, _sdata, _edata, _sbss, _ebss; char *src = &_sidata; char *dst = &_sdata; while(dst < &_edata) *dst++ = *src++; dst = &_sbss; while(dst < &_ebss) *dst++ = 0; main(); } void (* const ResetVector)(void) __attribute__((section(".reset_vector"))) = Reset_Handler; [OK]

Hint: Initialize .data from flash, zero .bss, then call main() [OK]

Common Mistakes:

• Using int pointers instead of char pointers
• Swapping source and destination pointers
• Missing .bss zero initialization
• Incorrect reset vector declaration