Memory: Memory is an essential component of the microcomputer system. It is used to store both instructions and data. It is used to store both instructions and data. Memory is made up of registers and the number of bits stored in a register is called memory word .Memory word is  identified by an address .

1.Primary Memory:  It is the memory used by microprocessor to execute programs. The microprocessor can access only those items that are stored in this memory. Hence, all data and program must be within primary memory prior to its execution. Primary memory is much larger than processer memory that is included in the microprocessor chip.     

    A. Static RAM (SRAM): This memory is made up of flip flops and it stores bit as voltage. A single flip flop stores binary data either 1 or 0. Each flip flop is called storage cell. Each cell requires six transistors. Therefore, the memory chip has low density but high speed. This memory is more expensive and consumes more power. 

    B. Dynamic RAM (DRAM): This memory is made up of MOS transistor gates and it stores the bit as charge. The advantage of DRAM are it has high density, low power consumption and cheaper than SRAM. But the bit information leaks therefore needs to be rewritten again every few milliseconds. It is called refreshing the memory and requires extra circuitry to do this. It is slower than SRAM.  

Read Only Memory (ROM): ROM contains a permanent pattern of data that cannot be changed. It is non volatile that is no power source is required to maintain the bit values in memory. ROM are basically of 5 types.

  1. Masked ROM: A bit pattern is permanently recorded by the manufactures during production.
  2. Programmable ROM: In this ROM, a bit pattern may be written into only once and the writing process is performed electrically. That may be performed by a supplier or customer. 
  3. Erasable PROM (EPROM): This memory stores a bit in the form of charge by using EPROM programmer which applies high voltage to charge the gate. It is reusable.  
  4. Electrically Erasable PROM(EEPROM): It is functionally same as EPROM except that information can be altered by using electrical signal at the register level rather than erasing all the information. It is expensive compared to EPROM and flash and can be erased in 10 Ms. 
  5. Flash Memory: It is variation of EPROM. The difference is that EPROM can be erased in register level but flash memory must be erased in register level but flash memory must be erased in its entirety or at block level.

2.Secondary memory: The devices that provide backup storage are called secondary memory. It includes serial access type such as magnetic disks and random access type such as magnetic disks. It is non volatile memory. Eg Hard Disk, Pen drive , SSD etc.