FreeRTOS is a free and open-source real-time operating system (RTOS) that runs on most of the modern microcontrollers, including STM32. In 2017, Amazon took control of the FreeRTOS project and now offers regular maintenance and support.

An operating system (OS) is software that manages other software and hardware resources in a computer system. You may be familiar with the most popular universal operating systems such as Windows, macOS, Linux, iOS, and Android. Universal OS is often designed to focus on the user experience.

A module definition DEF file (*. def) is a text file containing one or more module statements describing various attributes of a DLL. The DEF file is especially useful when one needs to disable the name mangling of the exported functions and variables.

Communicating with the device drivers from the user-mode apps in Windows can be done by either sending standard I/O requests: Read, Write, or by sending dedicated I/O control codes (IOCTLs). 

Sending standard I/O requests are required when writing standard filesystem drivers. Custom I/O Control Codes can be used when communicating with non-standard (custom) drivers.