ss_ladies: Memory Organization

Memory Organisation

· Memory is made up of lots of storage locations.

· Each location can hold a word.

· A word is the term given to the number of bits that can be processed by the computer in a single operation.

· So the correct data can be found in main memory, each storage location has its own unique address. Hence, the data wouldn’t be sent to the incorrect destination. This concept is called addressability.

Memory Hierarchy

Main Memory

· Main memory is a memory unit that communicates directly to the CPU of the computer.

· It is a combination device of both RAM (random access memory) and ROM (read only memory).

· Main memory is a part of Central Processing Unit(CPU).

Characteristic of Memory

· Capacity, representing the global volume of information (in bits) that the memory can store

· Access time, corresponding to the time interval between the read/write request and the availability of the data

· Cycle time, representing the minimum time interval between two successive accesses

· Throughput, which defines the volume of information exchanged per unit of time, expressed in bits per second

· Non-volatility, which characterises the ability of a memory to store data when it is not being supplied with electricity

Types of Main Memories

· Main Memory are classified into two basic types:

i. Random Access Memory (RAM)

ii. Read Only Memory (ROM)

What is RAM?

· Random Access Memory (RAM) is a type of main memory which store the data temporarily when a program is running.

· RAM can be called as primary memory/primary storage/main storage/internal memory/core memory/core storage/internal storage

· It is sometimes called semiconductor memory because storage cells are made out of semi-conducting material.

· It takes the form of integrated circuits.

· Hold the data and instructions temporarily when the CPU is processing.

· Examples of what is stored in RAM:

§ the operating system

§ applications

§ the graphical user interface (GUI)

What is ROM?

· ROM is a type of main memory ROM is computer memory which retains its data, even when the computer is switched off.

· ROM can be called as auxiliary memory/physical memory/external memory

· Hold data necessary for starting up the computer permanently, whose contents cannot be changed by the user.

Difference between RAM and ROM

Types of RAM

There are two types of RAM:

i. Dynamic RAM (DRAM)

· Analog device

· Volatile in nature but they need regenerator to retain the data as long as they receive the power.

· DRAM chips contain a transistor that acts as a gate to a capacitor which purpose is to store the bits as charge in it

· The charge on the capacitor indicates a ‘1’ bit and no charge indicates a ‘0’ bit. The charge on the capacitor leaks away after a few milliseconds. Therefore, a dynamic RAM has to be refreshed periodically after every two milliseconds.

· DRAM uses its contents in a very short time even though the power supply is on.

· Consumes less power and has higher packing density. It is cheaper than the static RAM.

· DRAM’s be recommended for large sized memories.(i.e. main memory)

DRAM Structure

i. Static RAM (S-RAM)

· Digital device

· Bits store as flip-flops(on/off switches)

· Volatile in nature but they need no regenerator to retain the data as long as they receive the power.

· Consumes more power and is more expensive.

· The structure of SRAM chips are more complicated and hence require more space.

· Static RAM’s are faster than the dynamic RAM’s .Static RAM’s have an access time of approximately 85 to 90ns while the dynamic RAM’s may take 150 to 200ns to provide information.

· SRAM’s are recommended for medium sized memories.(i.e. cache memory)

SRAM structure

Differences between DRAM and SRAM

Types of ROM

There are five types of ROM:

i. Programmable Read Only Memory (PROM):

· A PROM program is used to record information in the PROM chip.

· The information once programmed into the PROM chip is permanent and cannot be changed or erased.

· The process of entering the information into the PROM chip is known as “burning the PROM.”

· It is need special equipment to program.

· The storing in ROMs will be very expensive when only a small number is required. Thus, PROMs provide a faster and less expensive approach for storing.

ii. Masked Read Only Memory (MROM)

· The masking and metallisation process permanently records the information.(ROM bits)

· It is not easy to perform this process as a large infrastructure is required, and therefore, it is usually the manufacturers who perform this process.

iii. Erasable Programmable Read Only Memory (EPROM)

· An erasable PROM.

·        It can be reprogrammed using an EPROM programmer.

·        It is only one transistor per bit.

·        The contents can be erased by expose it to high intensity ultraviolet light with a wavelength of 2537A (angstrom) for 30 minutes (approximately).

·        The process of changing the contents is not convenient, as the chip has to be removed from the board for exposure to the ultra-violet light source.

·        If the user wants to erase the contents of a single memory,the entire memory contents have to be erased.
The EPROM chip is cheap, reliable and widely available

iv. Electrically Erasable Programmable Read Only Memory (EEPROM)

· An electrically erasable PROM.

· Using electrical signals can alter the information and that is why the chip need not be removed from the board.

· EEPROM chip has over the EPROM is that even single memory can be altered, i.e., the entire memory need not be erased and reprogrammed unless required.

· Changing the contents does not require additional equipment.

· The change in the contents of the EEPROM chip is made in milliseconds, which is much less than the erasing time for EPROM.

· It is less dense than EPROM.