A method for accessing a memory is provided. The method includes entering a memory accessing mode for updating a top of low memory (TOLM) value stored in a TOLM register in a chipset of a system with a highest memory address when a memory accessing command is received. The memory accessing command requests the utilization of a memory block in a memory of the system corresponding to an address space occupied by a memory-mapped input output (MMIO) function. The system then accesses the corresponding memory block in the memory according to the address space recorded in the memory accessing command. After the access is completed, the memory accessing mode is closed and the original TOLM value is written back to the TOLM register. Therefore, the present invention can access the “MMIO memory block” to prevent a waste of the memory.