Storage unit and memory system

A memory system comprising:

• a host;
• a main memory module;
• a storage unit accessed by the host;
• and
• a system bus connecting the host, the main memory module, and the storage unit,
• wherein the storage unit includes
• a random access memory device to be accessed using an address in units of word,
• a storage device to be accessed using an address in units of sector, and
• a storage controller that controls accesses to the random access memory device and the storage device according to the addresses designated via the system bus, and
• wherein the storage controller includes
• a first interface function for access to data stored on the storage device designated using the sector address according to an instruction from the host via the system bus,
• a second interface function by the host for direct access to data on the random access memory device using the word address designated via the system bus,
• a function of using the random access memory device as a first disk cache and determining data to be saved in the random access memory device in response to the access by the first interface function,
• a function of transferring the data designated using the sector address by repeating register access, and
• a function of transferring the data designated using the sector address by a bus master function as continuous word-sized data between the main memory module and itself through the system bus.