A memory controlling device of a computing device including a CPU, a memory, and a flash-based storage device is provided. The memory controlling device includes an address manager and an interface. The address manager aggregates a memory space of the memory and a storage space of the storage device into an expanded memory space, and handles a memory request for the expanded memory space from the CPU by using the memory space of the memory as a cache for the storage space of the storage device. The interface is used to access the memory and the storage device.