Emerging portable devices relay on DRAM/flash memory system to satisfy requirements on fast and large data storage and low-energy consumption. This paper presents a novel approach to reduce energy of memory system, which unlike others, lowers energy of refresh operation in DRAM. The approach is based on two key ideas: (1) DRAM-based flash cache that keeps dirty pages to reduce the number of accesses to flash memory; and (2) OS-controlled page allocation/aging to stop the refresh operations in banks, whose pages are clean and not accessed for a long time. Simulations show that by using this technique we can decrease the overall energy consumption of DRAM/flash memory on video applications by 8-26% while reducing the DRAM refresh energy by 59-74%.