Memory Hierarchy in Computer Architecture
Memory Unit is an essential component of any digital computer because it is needed for storing programs and data. the memory hierarchy system consists of all storage devices employed in a computer system from the slow but high capacity secondary memory to relatively faster main memory.
From the top to bottom the cost of memory will decrease and the accessing speed also decreases. But from bottom to top the storage capacity will decrease and accessing speed will increase.
Registers are faster of all memories, a register is a group of flip-flops with each flip-flop capable of storing one bit of information. An n-bit register has a group of n flip flops and it is capable of storing any binary information of n bits.
Cache is a special very high-speed memory. It is the intermediate Memory between CPU and main memory. The cache is used for storing segments of programs currently being executed by the CPU and frequently needed in the present calculations.
The memory unit that communicates directly with CPU is called Main Memory. The main memory is a relatively large and fast memory used to store program and data during the processor execution. The principle technology used for the main memory is based on Semi-Conductor integrated circuits (RAM Chips). It is a volatile memory.
Devices that provide backup store are called “Auxiliary Memory” or “Secondary Memory“. The most common Auxiliary Memory used in the computer system is magnetic disks. They are used for storing system programs, large data files and other backup information.
Magnetic Tapes are slow devices compare to magnetic disks, Generally tapes are used for backup storage.
In recent years, optical disks (CD-ROMs) are become available. They have much much higner recording density than magnetic disks. A CD is prepared by using a high power infrared laser to burn 0.8 micron diameter holes in a coated glass master disk. The burned area is called pits and the unburned area between the ‘pits’ are called ‘lands’.
At the bottom of the hierarchy the relaively slow magnetic tapes used to store removable files. Next are the optical memory, magnetic disks used as backup storage. The main memory occupies a central position being able to communicate directly with the CPU and secondary memory devices through I/O processor, cache memory is used to synchronize the speed between the CPU and main memory.