Like embedded hardware, embedded software architecture is highly flexible. Simple embedded software (such as electronic toys, calculators, and so on) may be only a few thousand lines of code and perform simple input and output functions. On the other hand, complex embedded systems (such as smartphones, robots, and so on) need more complex software architecture, similar to desktop computers and servers.
Complex embedded systems provide more powerful functions, need more convenient interfaces for users, and require the support of more powerful hardware. With the improvement of hardware integration and processing capabilities, the hardware bottleneck has gradually loosened and even broken, so embedded system software now tends to be fully functional and diversified.
An embedded software system is composed of four layers, from bottom to top:
- Hardware abstraction layer
- Operating system layer
- System service layer
- Application layer
Hardware Abstraction Layer
The hardware abstraction layer (HAL), as a part of the OS, is a software abstraction layer between the embedded system hardware and OS. It includes the boot loader, board support package (BSP), device drivers, and other components. Similar to the BIOS in PCs, the boot loader is a program that runs before the OS kernel executes. It completes the initialization of the hardware, establishes the image of memory space, and consequently enables the hardware and software environment to reach an appropriate state for the final scheduling of the system kernel.
Operating System Layer
An OS is a software system for uniformly managing hardware resources. It abstracts many hardware functions and provides them to applications in the form of services. Scheduling, file synchronization, and networking are the most common services provided by the OS. Operating systems are widely used in most desktop and embedded systems. In embedded systems, the OS has its unique characteristics: stability, customization, modularity, and real-time processing.
The common embedded OS contains embedded Linux, Windows CE, VxWorks, MeeGo, Tizen, Android, Ubuntu, and some operating systems used in specific fields. Embedded Linux is a general Linux kernel tailored, customized, and modified for mobile and embedded products. In particular, VxWorks is used in the Mars probes by NASA.
System Service Layer
The system service layer is the service interface that the OS provides to the application. Using this interface, applications can access various services provided by the OS. To some extent, it plays the role of a link between the OS and applications. This layer generally includes the file system, graphical user interface (GUI), task manager, and so on. A GUI library provides the application with various GUI programming interfaces, which enables the application to interact with users through application windows, menus, dialogue boxes, and other graphic forms instead of a command line.
The application, located at the top level of the software hierarchy, implements the system functionality and business logic. From a functional perspective, all levels of modules in the application aim to perform system functions. From a system perspective, each application is a separate OS process. Typically, applications run in the less-privileged processor mode and use the API system schedule provided by the OS to interact with the OS.