Memory is the second major component present in any computer system.
The memory system is built up as a hierarchy of levels as shown in the figure given below.
Now-a-day, in modern computers, the capacity of all the layers of the memory increased such as main memory's capacity mostly lies between 2 MB to 16 MB and the magnetic disk's capacity mostly lies between 500 GB to 2 TB, etc.
Here, from the above figures, the registers internal to the Central Processing Unit is the top layer or level of the memory. They are made up of the same material as the Central Processing Unit. Therefore, they are as fast as Central Processing Unit.
The next layer of the memory is the cache memory. The cache memory is mostly controlled by the hardware.
Note - Due to its high cost, cache memory is limited in size.
Now, the next layer of the memory is the main memory. Main memory is also called as Random Access Memory (RAM).
The next layer of the memory is the magnetic disk. Magnetic disk is also called as hard disk.
And, the last layer or final layer of the memory is the magnetic tape.
Almost every computers have some amount of non-volatile Random Access Memory.
Unlike Random Access Memory (RAM), non-volatile memory doesn't loose its content when the power is turned off.
Note - ROM is programmed at the factory and can't be changed when the programming of the ROM is successfully done.
Read Only Memory (ROM) is fast and inexpensive.
On computers, the bootstrap loader used to start the computer is contained in Read Only Memory (ROM).
Electrically Erasable Read Only Memory (EEPROM) and Flash Random Access Memory (FRAM) are also non-volatile memory, but it can be erased and rewritten.
CMOS is another type of memory. It is a volatile memory.
Computers uses CMOS memory just to hold the current time/date.
The CMOS memory and the clock circuit that increments the time in it are powered by a small batter. Therefore, you will see the time is correctly updated, even when the computer is unplugged or when you install the new operating system.
The check and mapping result in converting an address by the program called a virtual address into an address used by the memory, called a physical address.
Memory Management Unit (MMU) is the device that performs the check and mapping.
MMU is physically located on the Central Processing Unit chip or closer to it.
MMU is logically located between the CPU and the memory.