• Memory unit is an essential component in any digital computer as it is needed for storing programs and data.
• A computer is equipped with a hierarchy of memory subsystems, some internal to the system (directly accessible by the processor), and some external (accessible by the processor via an I/O module)
• External memory consists of peripheral storage devices, such as disk and tape, that are accessible to the CPU via I/O controllers. External memory can also be referred as secondary memory or auxiliary memory.
• Internal memory is equated with main memory. But there are other forms of internal memory like CPU requires its own local memory in the form of registers. Internal memory is also called as main memory/Primary memory.
Main memory can be classified as
-- RAM (Random Access Memory)
-- RAM is working memory. Data can be read or written in RAM with the help of address location and when the data is no longer needed we can use the storage location for writing again.
-- Contents of volatile memory are vanished when power supply is switched off.
2) Non Volatile :
-- ROM (Read Only Memory)
-- It is useful to have instructions that are used often , permanently stored inside the computer. Programs and data on the ROM are not lost if the computer is powered down
There are three key characteristics of memory:
• ACCESS TIME
The relationship between them is as follows:
ü Greater capacity, smaller cost per bit
ü Greater capacity, greater access time
ü Smaller access time, greater cost per bit
The overall goal of using a memory hierarchy is to obtain the highest possible average access speed while minimizing total cost of entire memory system.