Understanding the ARM Cortex-M Reset Sequence

Upon any hardware or power-on reset, an ARM Cortex-M processor follows a precise sequence to bring the system into a known, stable state before executing application code. First, the Main Stack Pointer (MSP) is initialized from the value at address 0x0000 0000, ensuring the stack is correctly set up. Next, the Program Counter (PC) is loaded with the reset handler’s address from 0x0000 0004, transferring control to the startup code. The reset handler—typically provided by the MCU’s startup file—performs critical early initialization: setting up the data and BSS sections, configuring clocks, and initializing the C runtime environment before calling main(). This flow guarantees that by the time user code executes, all core hardware and memory regions are correctly configured.

The Three Key Steps of Reset

1. MSP Initialization

   • On reset, the processor reads the 32-bit value at address 0x0000 0000 into the Main Stack Pointer (MSP), establishing the stack base for exception handling and function call returns.

2. PC Loading

   • It then reads the next 32-bit word at 0x0000 0004—pointing to the address of the reset handler—into the PC, causing the CPU to branch there.

3. Executing the Reset Handler

   • The reset handler (typically in the startup assembly file) runs early initialization routines—relocating and zero-clearing memory sections, setting up system clocks, and initializing the C library—before finally calling the main() function of the application.
     
     

Why the Reset Sequence Matters

• Memory Safety: MSP setup prevents stack corruption by pointing to valid SRAM.

• Predictable Startup: By fetching the reset handler from a fixed vector table, the system always begins execution at a known location.

• C Runtime Readiness: Automatic data/BSS initialization and standard library setup ensure that global variables and library functions behave correctly once main() runs.

Where It Lives: The Vector Table and Startup File

• The vector table resides at the start of flash (0x0000 0000) and holds the initial MSP and PC values.

• The startup file (often named startup_stm32f4xx.s or similar) defines the reset handler and exception vectors. It implements low-level assembly for early setup and invokes higher-level C routines.
  
  

Summary

Understanding the Cortex-M reset sequence—from MSP load to reset handler execution—is fundamental for embedded developers. It underpins reliable system boot, correct memory initialization, and seamless transition into application code.

Written By: Musaab Taha

This article was improved with the assistance of AI.