Depending on the chosen OS, there are different techniques to abstract/describe the hardware/SoC on which the OS is running, such that the kernel of the OS can work with an abstract description of the hardware components – peripherals CPU cores, hardware accelerators, interrupt lines, etc. For Unix Like Operating Systems such as VxWorks, Linux, etc. there is the tree structure data format – Device Tree. It describes the hardware in a binary blob-compilable format. Also, the Kernel loads it at boot time. Although not so popular in applications with RTOSs, Device Tree starts to gain popularity with Real-Time Operating Systems. Other common hardware abstraction methods for hardware in RTOS applications include writing standardized C routines that have good documentation. 

The specification/documentation of the various APIs acts as a to-do list for developers that implement the hardware abstraction layers. We can use it later on, in the upper kernel layers/device drivers. Many silicon vendors implement a widespread specification is the Common Microcontroller Software Interface Standard (CMSIS).