Board support package (BSP)

An RTOS knows how to switch tasks, but not which crystal is fitted, which pin drives the LED or where RAM begins. That knowledge lives in the BSP: startup code, clock setup, linker script, interrupt vectors and drivers for the board's peripherals. The term grew up in the commercial real-time world of the 1980s and 90s, when Wind River sold VxWorks and board vendors shipped the BSP that made it run on their hardware.

Each camp now has its own name for it. Yocto has BSP layers (meta-raspberrypi and friends), Linux describes the board in a devicetree, and Zephyr keeps a directory per board. The principle is unchanged: everything that differs between two boards with the same processor is kept out of the rest of the code. Swapping boards should mean rewriting the BSP, not the application — which holds true roughly as often as it is promised.