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Recently the Hard Disk Driver (HDD) manufacturers have begun using the Dynamic Random Access Memory (DRAM) as cache memory to improve the HDD performance. In this paper we use an additional NAND Flash memory as a second level cache memory for that purpose. The NAND Flash memories have some advantages over both the HDD and the DRAM cache, such as faster access time, smaller size and larger capacity. First, we design a Solid State Disk (SSD) by using the NAND Flash memory. The SSD is connected to a PC system by means of the Serial Advanced Technology Attachment (SATA) bus. Then, by using software, we combine the HDD and the SSD into one storage device of the PC. The Operating System (OS) will save files to the different storage devices according to the sizes and uses of the files. When the user wants to open the frequently used programs, the system reads data from the SSD in order to avoid any HDD mechanical access head delay. We utilize this mixed architecture to improve the performance of the whole PC system.