__IO uint32_t

This is a data type definition for a 32-bit unsigned integer that is volatile and can be modified by both the CPU and external devices. The "__IO" keyword is a compiler-specific way of indicating that the variable is "volatile", which means that its value can change at any time, even without an explicit instruction to do so, and must be read and written directly from/to memory. This type of variable is commonly used in embedded systems and device drivers, where direct hardware access is required.