Present memory systems often include static random-access memory (SRAM), dynamic random-access memory (DRAM), and non-volatile storage, such as a hard disk or solid-state memory (e.g., flash memory). Typically SRAM is a memory that is at a lower level in the memory hierarchy and receives memory access requests from the processing element(s). If the requested data is not present in SRAM, the DRAM memory is queried for presence of the requested data. If the data is not present in DRAM, the data is retrieved from non-volatile storage, which contains all of the program instructions and data required for the execution of the program.
Since SRAM is fast relative to DRAM, SRAM is often used to implement caches. Since DRAM is less expensive than SRAM and requires fewer transistors, DRAM is often used to implement the main memory in a memory system hierarchy.