Q. What is typical storage hierarchy?

A typical storage hierarchy is displayed in Figure above. Though Figure shows only block diagram however it includes storage hierarchy:   

Register

Cache memory

Main memory

Secondary Storage and 

Mass Storage. 

As we move up hierarchy we encounter storage elements which have faster access time and higher cost per bit stored as well as slower access time as a result of moving down hierarchy. So cache memory normally has the fastest access time and the smallest storage capacity as well as the highest cost per bit stored. Primary memory (main memory) falls next in storage hierarchy list. On-line, direct-access secondary storage devices like magnetic hard disks make up level of hierarchy just below main memory. Off-line, direct-access and sequential access secondary storage devices like magnetic tape, ZIP disk, Floppy disk and WORM disk etc. fall next in storage hierarchy. Mass storage devices generally referred to as archival storage are at the bottom of storage hierarchy. They are cost-effective for storage of very large quantities of data when fast access time is not essential.

Please note two significant points here: 

• Size of memory increases as we move down hierarchy.
• Quantum of data which is transferred between two consecutive memory layers at a time also increases as we go from a higher to lower side. For illustration from main memory to Cache transfer one or a few memory words are accessed at a time while in a hard disk to main memory transfer, a block of about 1 Megabyte is transferred in a single access.