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Basic OS Development Concepts for RaspberryPi
This article covers some fundamental concepts related to developing an operating system for the Raspberry Pi, including Board Support Packages (BSPs), reexporting in Rust, assembly code for the AArch64 architecture, and linker scripts for the GNU linker (ld).
A Board Support Package (BSP) is a crucial component in embedded systems development. It contains software that allows the operating system to interface with the hardware of a specific board. This includes drivers for devices that are featured on the respective board, such as GPIO (General Purpose Input/Output) pins, UART (Universal Asynchronous Receiver/Transmitter), SPI (Serial Peripheral Interface), I2C (Inter-Integrated Circuit), and more.
In the context of a Raspberry Pi, the BSP would include the necessary drivers to interact with the board's specific set of hardware, such as its specific ARM processor, its memory layout, its USB ports, its network interface, etc.
Reexporting in Rust is a way to make a particular item available in a different module without having to use its full path. This is done using the pub use statement. In the context of the BSP code, there is no reexporting. This means that the items defined in the bsp module are not made available in other modules using pub use. This design choice can help to maintain a clear separation between the BSP code and the rest of the kernel code.
The AArch64 architecture is used by the ARM processors in Raspberry Pi 3 and 4. The assembly code for this architecture is part of the booting process of an operating system. The _start label marks the entry point of the program. This is where execution begins when the system is booted. The wfe instruction stands for "Wait For Event". This is a power-saving instruction that puts the processor in a low-power state until an event happens (like an interrupt), at which point it wakes up again.
Linker scripts are used to control the memory layout of the final executable. The ENTRY directive specifies the entry point of the program. The PHDRS block defines a program header, which is a part of the final executable file that describes how it should be loaded into memory. The SECTIONS block describes how different sections of the input files should be arranged in the output file.
In summary, these concepts are fundamental to understanding and developing an operating system for the Raspberry Pi. They provide the basis for interfacing with the hardware, structuring the code, and controlling the execution of the program.