Memory devices can be classified in several ways: 1. By location as registers, main memory, and secondary memory. Registers are inside the CPU while main memory is external but faster than secondary memory like hard disks. 2. By access as sequential (location must be accessed in order) vs random access memory (RAM) which allows random access. 3. As static (maintains data without refresh) vs dynamic RAM which must be periodically refreshed. 4. As volatile (loses data on power off) vs non-volatile like ROM and magnetic storage. Read-only memory (ROM) is non-volatile and only allows reading. It is used to permanently store information. Various RO

1. Memory Devices
Mr.A.Rameshbabu AP/ECE
Ramco Institute of Technology
Academic Year 2018 – 2019 (Odd)

2. MEMORY UNIT

3. Classification of Memories
Memories are classified as
1. Register, Main Memory and Secondary Memory
2. Sequential Access Memory and Random Access Memory
3. Static and Dynamic Memory
4. Volatile and Non-volatile Memory
5. Magnetic and semiconductor Memory

4. 1.Register, Main Memory and
Secondary Memory
• These memories are scattered through out the computer, those
based on the location and usage are called registers, main memory
and secondary memory.
Register
• It is available with in the CPU to store data temporarily during
arithmetic and logical operations like addition, subtraction ,
AND,OR,etc.
• They have very low access time, as they are available inside the
CPU.
Main Memory
• Main memories of a computer, usually of semiconductor type , are
available external to the CPU to store program and data during
execution of a program

5. • In the main memory, each memory location is identified by an
unique address and is accessed for read/write operation in a lesser
speed than registers.
• As the storage capacity of the main memory is inadequate,
secondary or auxiliary memories are added to enhance storage
capabilities.
• This secondary memory operates at a lesser speed when
compared to registers and main memory.
• Normally, secondary memories are of magnetic memory tape
(magnetic tape, magnetic drum, floppy disk and hard disk) that
are used to store large quantities of data.

6. 2.Sequential Access Memory and Random
Access Memory
Sequential Access Memory
Particular memory location is accessed sequentially. (i.e) the
ith memory location is accessed only after sequencing through
previous (i-1) memory locations.
Therefore, the access time of a sequential memory varies
depending on the location to be accessed.
Example - magnetic tape memory
Random Access Memory
Any location can be accessed in a random manner and thus
has equal access time for all memory locations.
Example – semiconductor RAM

7. 3.Static and Dynamic Memory
Dynamic memory
• Content changes with time
• It uses the capacitance of transistor as the storage device
• Only one transistor is needed to store one bit of information
• The capacitor must be refreshed periodically without being
discharged in order to prevent loss of information
• Example - Charge coupled devices
Static memory
• Content does not change with time
• Static memory devices require no refreshing, and hold data as
long as d.c. power is applied
• Example - Register and MOS cell

8. 4.Volatile and Non-volatile Memory
• A Volatile memory loses its stored data when power to the
memory circuit is removed
• A non-volatile memory retains stored data permanently even after
the power supply is turned OFF.
• Magnetic core memory and Read Only Memory(ROM) are
examples of non-volatile memory devices.

9. 5.Magnetic and semiconductor memory
• These memories are classified based on the material used for
construction.
• The magnetic memories are constructed using magnetic material.
• Example – magnetic tape, floppy disk and compact disks
• Magnetic recording is the process of storing data magnetically on
the surface of a tape, disk or drum
• Semiconductor memories are constructed out of Semiconductor
material using LSI and VLSI technologies.
• Example – RAM,ROM

10. ROM – READ ONLY MEMORY

11. Read Only Memory
• ROM is a semiconductor memory device used to store the information
permanently.
• It performs only read operation and does not have a write capability.
• A ROM is programmed for particular purpose during the
manufacturing process and the user cannot alter this function.
• Non-volatile - If un-powered, its content retains
• The ROM is a combinational Logic circuit.
• It includes both the decoder and the OR gates within a single IC
package.
• In order to minimize the number of address lines, decoders are used.

12. Read Only Memory
• Number of bits per word is equal to the number of output lines

14. 32 x 8 ROM

16. • Types of ROMs
– Mask Programmed ROM
– Programmable ROM (PROM)
– Erasable Programmable ROM (EPROM)
– Electrically Erasable PROM (EEPROM)
– Electrically Alterable PROM (EAPROM)

17. • Mask Programmed ROM
– Programmed during manufacturing time
• Programmable Read-Only Memory (PROM)
– Blown fuses to produce ‘0’ & intact fuses to produce ‘1’
• Erasable Programmable ROM (EPROM)
– Erase all data by Ultra Violet light
• Electrically Erasable PROM (EEPROM)
– Erase the required data using an electrical signal

18. Programmable ROM (PROM)

19. EPROM – Erasable Programmable ROM

20. Advantages of ROM
• They are non-volatile.
• They are cheaper than RAM.
• They are static and do not refreshing.
• They are more reliable than RAM as their circuit is simple.
• They are available in longer sizes than RAM.
• They are easier to interface than RAM

21. RAM – RANDOM ACCESS
MEMORY

22. Random Access Memory

23. RAM – Internal Construction
 BINARY CELL (BC)

24.  BINARY CELL LOGIC DIAGRAM

25. 4x4 RAM

26. READ / WRITE OPERATION

27. WRITE OPERATION

28. READ OPERATION

29. COINCIDENT DECODING

30. ADDRESS MULTIPLEXING

31. Reference
M. Morris Mano, “Digital Design”, 4th
Edition, Prentice Hall of India Pvt. Ltd.,
2008 / Pearson Education (Singapore)
Pvt. Ltd., New Delhi, 2003.