Types of memories in microprocessor system

Here present the article about the types of memories in microprocessor system.

• There are two basic kinds of memory used in microprocessor systems - memories are classified as two types volatile memories and non- volatile memories.
• Volatile memories is random access memory(RAM) and non-volatile memories is read only memory(ROM).
• It is a volatile memory as a retain their stat as long as power is applied or data loses when the power is turned off.
• Other hand non- volatile momory can hold data even if power is turned off.

⏩Random access memory(RAM):

• The memory in which the time taken to transfer information to or from any desired random location is always the same is called random access memory.
• There are two types of RAMs

1. Static RAM
2. Dynamic RAM

1. Static RAM

• Memories that consists of circuits capable of retaining their state as long as power is applied are know as static RAM memories.
• These are random access memory (RAM) and hence combinely called static RAM memories.

Advantages of SRAM:

1.  Fast memory (less access time) 
2.  Refreshing circuit is not required.

Disadvantages of SRAM: 

1. Low package density 
2.  Costly

2. Dynamic RAM

• Dynamic RAM stores the data as a charge on the capacitor and they refreshing of charge on the capacitor after every few milliseconds to hold the data even if power is ON.

Advantages of DRAM: 

1. High package density 
2. Low cost

Disadvantages of DRAM: 

1.  Required refreshing circuit to maintain or refresh charge on the capacitor, every after few milliseconds.

Comparison between SRAM and DRAM

⏩Read only memory(ROM):

• It is the memory that performs the read only operation. 
• We can't write data in read only memories.
• It is non - volatile memory. 
• Generally ROM is used to store the binery codes for the sequence of instructions you want the computer to carry out and data such as look up tables. This is because this type of information does not change.
• There are four types of ROM :

1. Masked ROM 
2. PROM
3. EPROM
4. EEPROM

1. masked ROM:

• the program or data are permanently installed at the time of manufacturing as per requirement.
•  The data cannot be altered. 
• The process of permanent recording is expensive but economic for large quantities.

2. PROM(Programmable Read Only Memory)

• PROM Is Programmable Read Only Memory.
• This memory has nichrome or polysilicon wires arrenged in a matrix these wires can be functionally viewed as diode or fuses.
• This memory can be programmed by user with a special PROM programmer that selectively burns the fuses according to the bit pattern to be stored the process is known as burning the PROM.

3. EPROM(Erasable programmable read only memory)

• The memory stores a bit by charging the floating gate of an FET.
• Information is stored by using an EPROM programmer.
• Which applies high voltage to charge the gate. 
• All the information can be erased by exposing the chip to ultraviolet light through its quarts window and the chip can be reprogrammed because the chip can be reused many times.
• This memory is ideally sulted for product development experiment projects and collage laboratories.
• The disadvantage of EPROM are 

1. It must be taken out of the circuit to erase
2. The entire chip must be erased 
3. The erasing process takes is to 20 minutes.

4. EEPROM(Electrically Erasable programmable read only memory)

• This memory is functionally simillar to EPROM except than that information can be altered by using electrical signals at the register level rather than erasing all the information.
• This has on advantage in field and remote control application. 
• In microprocessor system software update is a common occurence if EEPROM are used in the system.
• They can be updated from a central computer by using a remote link via telephone lines.
• This memory includes a chip Erase mode where by the entire chip can be earased in 10 minutes vs 1 sec. To 20 min. To earase an EPROM however this memory is expensive compared to EPROM or flash memory.

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