# Kernel Subsystem ## Overview The OpenHarmony kernel is a real-time OS kernel developed by Huawei for IoT devices. It is as lightweight as RTOS and as easy-to-use as Linux. The OpenHarmony kernel includes basic kernel functions such as process and thread scheduling, memory management, IPC mechanism, and timer management. The source code of the OpenHarmony kernel consists of two repositories: the [`kernel_liteos_a`](https://gitee.com/openharmony/kernel_liteos_a) repository for Cortex-A processors and the [`kernel_liteos_m`](https://gitee.com/openharmony/kernel_liteos_m) repository for Cortex-M processors. The directory structures of the two repositories are similar. The following describes the directory structure of kernel_liteos_a. ## Directory Structure **Table 1** Directory structure of the OpenHarmony kernel source code

Directory	Description
apps	User-space init and shell application program
arch	System architecture, such as ARM
bsd	Code of the driver and adaptation layer module related to the FreeBSD, such as the USB module
compat	Compatibility with the kernel POSIX interfaces
fs	File system module, which mainly derives from the NuttX open-source project
kernel	Kernel modules including the process, memory, and IPC modules
lib	Kernel library
net	Network module, which mainly derives from the lwip open-source project
platform	Code for supporting different systems on a chip (SOCs), such as Hi3516DV300
security	Code related to security features, including process permission management and virtual ID mapping management
syscall	System calls
tools	Building tool as well as related configuration and code

## Constraints Hi3518EV300 uses the JFFS2 file system by default, and Hi3516DV300 uses the VFAT file system by default. If other file systems need to be used, the configurations of the file systems must be added accordingly. ## Usage For details, see [Kernel Usage Guidelines](../kernel/Readme-EN.md). ## Repositories Involved [drivers_liteos](https://gitee.com/openharmony/drivers_liteos) [kernel_liteos_a](https://gitee.com/openharmony/kernel_liteos_a) [kernel_liteos_a_huawei_proprietary_fs_proc](https://gitee.com/openharmony/kernel_liteos_a_huawei_proprietary_fs_proc) [kernel_liteos_m](https://gitee.com/openharmony/kernel_liteos_m)