Embedded systems play a crucial role in modern technology, powering everything from household appliances to industrial automation. When an embedded system is powered on, it does not immediately start executing the main application code. Instead, it runs a special piece of software known as startup code. This blog explores what startup code is, why it is essential, and how it works in an embedded system.
What is Startup Code?
Startup code is a small, low-level program that executes before the main application begins running on an embedded system. It is responsible for setting up the execution environment, initializing hardware, and preparing memory so that the main program can run smoothly. The startup code typically resides in the system’s boot memory (ROM or Flash) and executes immediately after a system reset or power-on event.
Why is Startup Code Important?
Startup code is essential because it ensures that the system is properly initialized before executing the main application. Some key responsibilities of startup code include:
- Processor Mode Configuration: Setting up the CPU to operate in the correct mode (e.g., user mode, privileged mode, or interrupt mode).
- Stack Initialization: Configuring the stack pointer to allocate space for function calls and local variables.
- Memory Initialization: Setting up RAM, copying initialized data from Flash to RAM, and clearing uninitialized variables.
- Clock and Peripheral Setup: Configuring system clocks, watchdog timers, and other essential peripherals.
- Exception Vector Table Setup: Setting up interrupt vectors and exception handlers.
- Jump to Main Function: Finally, transferring control to the
main()function of the application.
Components of Startup Code
Startup code consists of multiple sections that perform different functions. The key components include:
1. Reset Handler
The reset handler is the entry point of the startup code. It executes immediately after a reset and performs fundamental hardware initialization.
2. Stack Pointer Initialization
Most embedded systems require a stack for function calls and local variable storage. The startup code initializes the stack pointer to a predefined location in RAM.
3. Vector Table Configuration
The interrupt vector table contains addresses of interrupt service routines (ISR). The startup code ensures that the vector table is correctly placed in memory.
4. Data and BSS Segment Initialization
- Data Segment: Contains initialized global and static variables. The startup code copies them from Flash to RAM.
- BSS Segment: Stores uninitialized global and static variables. It is cleared to zero before execution.
5. Clock and System Configuration
Most embedded processors rely on an external clock source. The startup code configures the system clocks, PLL (Phase-Locked Loop), and other timing mechanisms.
6. Jump to main()
After setting up the execution environment, the startup code calls the main() function to begin program execution.
Example of Startup Code
Below is a simplified example of startup code for an ARM Cortex-M processor written in assembly language:
.section .text
.global _start
_start:
LDR R0, =_stack_top @ Load stack pointer address
MOV SP, R0 @ Initialize stack pointer
BL SystemInit @ Configure clocks and peripherals
BL __libc_init_array @ Initialize C runtime library
BL main @ Jump to main function
B . @ Infinite loop (in case main() returns)Customizing Startup Code
While many embedded development environments provide default startup code, developers often need to customize it to:
- Optimize boot time
- Add hardware-specific initializations
- Implement low-power or secure boot strategies
Conclusion
Startup code is a fundamental component of embedded systems, ensuring proper hardware and memory initialization before the main application executes. Understanding and customizing startup code can lead to more efficient and reliable embedded software development.
Are you working on an embedded project? Let us know your experience with startup code in the comments!