explain concept of computer memory? differentiate primary and secondary memory in detail. explain different secondary memory.

Main memory is directly or indirectly connected to the
central processing unit via a memory bus. It is actually two buses
(not on the diagram): an  href="http://en.wikipedia.org/wiki/Address_bus">address bus and a  title="Bus (computing)" href="http://en.wikipedia.org/wiki/Bus_(computing)">data
bus. The MAIN MEMORY acts as the liaison between the primary and the
secondary, the primary' s main function is first sends a number through an address bus,
a number called  href="http://en.wikipedia.org/wiki/Memory_address">memory address, that
indicates the desired location of data. Then it reads or writes the data itself using
the data bus. Additionally, a  href="http://en.wikipedia.org/wiki/Memory_management_unit">memory management
unit(MMU) is a small device between CPU and RAM recalculating the actual
memory address, for example to provide an abstraction of  href="http://en.wikipedia.org/wiki/Virtual_memory">virtual memory or other
tasks.

Secondary storage (also
known as external memory or auxiliary storage), differs from primary storage in that it
is not directly accessible by the CPU. The computer usually uses its  title="Input/output"
href="http://en.wikipedia.org/wiki/Input/output">input/outputchannels to
access secondary storage and transfers the desired data using  href="http://en.wikipedia.org/wiki/Data_buffer">intermediate area in
primary storage. Secondary storage does not lose the data when the device is powered
down—it is non-volatile. Per unit, it is typically also two orders of magnitude less
expensive than primary storage. Consequently, modern computer systems typically have two
orders of magnitude more secondary storage than primary storage and data is kept for a
longer time there.