System initialization phase: detailed explanation of the last step in the Linux startup process

The system initialization phase is the last phase in the operating system startup process, and it is also the phase when the operating system actually starts running. In Linux systems, the system initialization phase includes processes such as loading the kernel, initializing the kernel, and starting the first user space process. This article will explain in detail the specific steps of the Linux system initialization phase, and attach relevant code examples.

1. Loading the kernel
   The first step in the system initialization phase is to load the kernel. Under the action of the boot loader, the kernel file (usually the vmlinuz file located in the /boot directory) is loaded into the memory, and the kernel entry address is set.

In the Boot Loader configuration file (such as the GRUB configuration file), the path to the kernel file and the startup parameters passed to the kernel are specified. Once the kernel is loaded, control is given to the kernel.

# 示例GRUB配置文件
title Linux
root (hd0,0)
kernel /boot/vmlinuz root=/dev/sda1

2. Initializing the kernel
   After the kernel is loaded, the system will perform the kernel initialization process. At this stage, the kernel will perform a series of operations, including initializing kernel data structures, enabling hardware devices, establishing memory mapping, etc.

The kernel will initialize each subsystem by calling the start_kernel() function, set the interrupt vector table, initialize the scheduler, load the driver, etc.

// 示例内核初始化代码片段
void start_kernel(void)
{
    /* 初始化内核数据结构 */
    setup_arch();
    
    /* 启用硬件设备 */
    setup_hardware();
    
    /* 建立内存映射 */
    setup_memory();
    
    /* 初始化进程调度器 */
    sched_init();
    
    /* 加载驱动程序 */
    driver_init();
    
    /* ... */
}

3. Start the first user space process
   After the kernel initialization is completed, the system will start the first user space process, usually the init process. The init process is the first process in user space and is responsible for initializing the system environment, starting other user space processes, etc.

In Linux systems, the init process is implemented by the /sbin/init executable file. It reads the configuration file (usually /etc/inittab) to start system services and maintain System run level.

// 示例init进程的简单实现
int main() {
    read_inittab();
    start_services();
    
    // 等待系统关闭信号
    while(1) {
        if (received_shutdown_signal()) {
            shutdown_system();
        }
    }
    return 0;
}

Through the above three steps, the system completed the initialization phase, successfully started and entered the user space. In actual operation, the startup process of the Linux system also involves more complex content, such as device management, file system mounting, etc. The smooth progress of the system initialization phase is the basis for the normal operation of the system, and is of great significance for understanding the operating mechanism of the operating system and troubleshooting.

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