#13 Startup Code Part-1: What is startup code and how the CPU gets from reset to main?

Summary

In this lesson on embedded systems programming, Miro Samek explains the startup code that runs before the main function, focusing on the IAR library’s standard startup code. The session covers key concepts such as data section initialization, stack pointer setup, and the vector table. Samek walks through the process of debugging the startup sequence, highlighting the role of the ARM Cortex-M processor's vector table and exception handling. The lesson emphasizes the importance of understanding the system’s startup behavior, with practical insights on debugging and the significance of proper initialization in embedded applications.

Takeaways

• 😀 The lesson focuses on startup code in embedded systems, which runs before the main function.
• 😀 The IAR library provides standard startup code that initializes various data sections of your application.
• 😀 The vector table, found at the start of the reset process, plays a crucial role in processor initialization.
• 😀 The startup code initializes the stack pointer (SP) and handles the setup of the floating-point unit (FPU).
• 😀 The __low_level_init function allows for early hardware initialization, such as adjusting CPU clock speed.
• 😀 The linker map file provides detailed information about program sections, including code and data sizes.
• 😀 The .bss section holds uninitialized data, which is zeroed out during startup by the startup code.
• 😀 The .data section contains initialized data, which the startup code copies from the 'Initializer bytes' section in ROM to RAM.
• 😀 Understanding the differences between .data, .bss, and .text sections is critical for embedded systems programming.
• 😀 The ARM Cortex-M processor automatically loads values into the stack pointer and program counter at the start of the reset process.
• 😀 The generic vector table in IAR's startup code does not include interrupt vectors specific to your microcontroller, which need to be manually added.

Q & A

What is the primary focus of this lesson?

The lesson focuses on the startup code that runs before the main function in embedded systems programming, specifically looking at the IAR library's standard startup code and how it initializes various data sections.

What steps does the lesson suggest to begin the debugging process?

The lesson suggests making a copy of the previous project, renaming it, and opening it in the IAR toolset. Additionally, the 'Run to main' option should be unchecked, and the correct debug driver and hardware settings for the Tiva-C LaunchPad should be configured.

What does the '__iar_program_start' function do in the startup code?

__iar_program_start is the first function called in the startup sequence before main, and it is responsible for initiating the necessary hardware and software initialization, such as setting up the stack pointer (SP) and calling functions like '__iar_init_vfp' for hardware floating-point unit initialization.

What is the purpose of the '__low_level_init' function?

The '__low_level_init' function is designed to perform hardware-specific initialization that needs to occur early in the startup process, such as increasing the CPU clock speed to speed up subsequent operations.

How does the startup code handle uninitialized and initialized data?

The startup code clears uninitialized data in the .bss section by setting it to zero, and it copies initialized data from the 'Initializer bytes' section in ROM to the .data section in RAM, ensuring that all variables are correctly initialized before main is called.

What information can be found in the linker map file?

The linker map file provides detailed information about the memory layout of the program, including the size of different program sections like read-only code, read-only data, read-write data, and stack, as well as the placement of these sections in ROM and RAM.

What is the role of the '.data' and '.bss' sections in the linker map file?

The '.data' section holds initialized data that is copied from ROM to RAM, while the '.bss' section holds uninitialized data that is cleared to zero during startup. These sections are crucial for managing memory during the initialization process.

Why is it important to enable the generation of the linker map file?

Generating the linker map file allows embedded programmers to understand how their program is organized in memory, which is vital for optimizing code size, ensuring correct data placement, and troubleshooting memory-related issues.

What does the Vector Table contain, and why is it important?

The Vector Table contains the reset value for the stack pointer (SP) and the start address for the program counter (PC), along with the exception and interrupt vectors. It is important because it determines the initial execution flow of the processor after a reset.

What is the significance of the ARM Cortex-M's handling of the stack pointer (SP) and program counter (PC) after reset?

After a reset, the ARM Cortex-M processor automatically loads the stack pointer (SP) from address 0 and the program counter (PC) from address 4, with the least significant bit of the PC set to 1 to indicate Thumb mode. This mechanism is crucial for correctly starting the execution of the embedded system.