What is Memory?
Any system that processes digital data needs a facility for storing the unprocessed, partially processed, and completely processed data. A subsystem of such a digital processing system that can store all the mentioned data is called Memory.
Earlier the memory used to be of magnetic tape.
But nowadays we use semiconductor memory of various types and sizes.
In this lecture, we are going to learn about all the semiconductor memory which are used in Microprocessor and Microcontroller. But before starting to know what are the types of memory, we are going to know some advantages of Semiconductor Memory.
What are the Advantages of Semiconductor Memory?
The advantages of semiconductor memories are:
- Small size
- High speed
- Better reliability
- Low cost
- Ease of expansion of memory size
What are the types of Memory?
The memory devices can be classified, on the basis of various parameters. The parameters used as the basis of classification are as follows:
- Principle of operation
- Physical characteristics
- Mode of access
- The technology used for fabrication
In this section, we are going to learn about the types of memory based on the principle of operation and Physical characteristics.
Classification Based on Principle of Operation:
The classification of memories based on the principle of operation is shown in the below figure:
What is Sequential Memory?
- An example of sequential memory is a magnetic tape audio/video cassette.
- In sequential memories, the memory locations are organized in a sequence(one after the other).
- The reading/writing from such memories is a sequential process. Hence the time required to access a memory location is different for different locations.
- Sequential memories are further classified into two types:
- Shift registers
- Charge-coupled devices(CCD)
What is Random Access Memory (RWM or RAM)?
- Random access memory is also called read-write memory (RWM).
- The memory locations in this type of memory are organized in such a way that the access time required for accessing any location is the same. This is the advantage of using RAM over sequential memories.
- RAMs also can be further classified into two types:
- Static RAM
- Dynamic RAM
- RAMs can be fabricated using either bipolar technology or unipolar technology.
- RAM is a volatile memory so it loses the stored data when power is turned off.
1. Static RAM (SRAM):
- It is possible to implement the static RAM using the bipolar as well as MOS technology.
- It is possible to store data as long as power is applied to the chip.
- Static AM cells are basically flip flops that can stay in a given state (i.e. store a bit) as long as the power to the circuit is not interrupted.
2. Dynamic RAM (DRAM):
- In the dynamic RAM, the data is stored in the form of a charge on the capacitor.
- The single-bit Dynamic RAM is formed using only two components, one MOSFET and a capacitor.
- Hence it needs a very small area as compared to SRAM. hence DA consists of more memory cells as compared to the SRAM per unit area.
- One of the biggest disadvantages of DRAM is that the charge held by the capacitor needs to be refreshed after a few milliseconds. Hence DRAm needs extra circuitry for refreshing the charge.
Comparison of SRAM and DRAM:
- The comparison of DRAM and SRAM is shown in the below table:
|1.||Circuit configuration||Each SRAM cell is a flip flop.||Each DRAMunit consists of one MOSFET and a capacitor|
|2.||Number of components per cell||More||Less (Only two)|
|3.||Memory cells/unit area||Less than DRAM||More than SRAM|
|6.||Access time||Less. So these are faster memories||More. So these are slower memories.|
What is Read Only Memory (ROM)?
- These memories are designed only for reading the information that is already stored in them. The user cannot write any new information on them.
- These are similar to the prerecorded cassettes.
- A manufacturer or someone else can write ROMs but the writing process is much more complicated as compared to that of RAM.
- ROM is used to store some fixed information such as look-up tables, instructions, fixed data, etc.
- It is possible to organize ROMs in such a way that the reading time required for locations remains the same.
- The ROMs are further classified into the below categories as shown in the figure:
- The technologies used for ROM manufacturing are:
- Bipolar Technology
- MOS Technology
- Bipolar ROMs are faster and have higher driving capability whereas MOS ROMs require less area and consume less power. The improved MOS ROMs have higher speeds.
1. Mask Programmable ROM:
- In this, data patterns must be programmed as a part of the manufacturing process.
- Not reprogrammable
- Normally shifted for high volume usage due to low cost.
2. Programmable ROM (PROM):
- This device is electrically programmable. here user can program the chip. Normally PROM is a One-time Programmable (OTP).
- Not reprogrammable.
- Normally used for production purposes.
3. Erasable Programmable ROM (EPROM):
- In this, memory data can be written any number of times.
- The chip can be erased by exposing it to UV rays.
- This is used when one wants to develop a digital computer system..
4. Electrically Erasable Programmable ROM (E2PROM):
- These are very similar to EPROMs and use the MOS circuitry.
- The chip can be electrically erased.
- In this chip, it is possible to erase and program particular memory locations. This is called selective erasing.
- The main disadvantage of these ROMs is that they are very expensive and have very low storage density.
Comparison of RAM and ROM:
|1.||Operation involved||Reading and writing||Reading only|
|2.||Type of storage||Temporary||Permanent|
|3.||Types||SRAM, DRAM||PROM, EPROM, EEPROM|
|4.||Applications||calculators, Computers||computers, microprocessors|
what is Flash Memory?
- It is a special type of RAM.
- It is a non-volatile memory that is powered continuously.
- The easing and programming of the memory take place block by block.
- Due to this process, the flash memories are faster than EEPROMs which erase and write new data at the byte level.
- Flash memory cannot be used as random access memory because RAM is to be addressed at the byte level.
- The section of flash memory chips is organized in such a way that they can be erased instantaneously or in a flash, hence its name is “Flash Memory”.
- A special technique called Fower-Nordheim tunneling is used for erasing the chip.
- Important features of flash memory are high speed, low operating voltage, low power consumption, and durability.
- Typical applications of Flash memory are digital cameras, embedded controllers, cellular phones, etc.
Classification Based on Physical Characteristics:
- The classification based on the physical characteristics are:
- Erasable or Non-erasable Memories
- Volatile or non-volatile memories
Erasable or Nor-erasable Memories:
- An erasable memory is one that which the stored information can be erased and new information can be stored.
- The information stored in the non-erasable memory cannot be erased. For example, ROM is a non-erasable memory.
- The erasable memories can be further classified into two classes as follows:
- Location-by-location erasable memories.
- All locations are simultaneously erasable.
Location by location erasable memories:
- In this type, the desired memory locations can be erased one by one and then the new information is stored.
- EEROM, RAM, and CAM are examples of this type of erasable memory.
All Locations simultaneously erasable memories:
- These are the memories in which, the contents of all the locations of memory are erased simultaneously.
- For example, when we expose an EPROM to UV radiation, the content of all the locations gets erased simultaneously.
Volatile or Non-volatile Memories:
- If the information stored in a memory chip is lost when the electrical power is switched off, then the memory is called volatile memory.
- RAM is a volatile memory.
- If the information once stored in a memory chip does not change unless altered deliberately is called non-volatile memory.
- Such memories can hold the information even after switching off the power supply.
- ROMs are an example of non-volatile memory.