MEMORY UNITS

This unit can store instruction, data and intermediate results. This unit supplies information to the other units of the computer when needed. A memory is just like the human brain. Computer memory is the storage space in computer where data is processed and instructions required for processing are stored. The memory is divided into large number of small parts. Each part is called cell. Each location or cell has a unique address which varies from zero to memory size minus one. For example if computer has 64k words, then this memory unit has 64 * 1024 =65536 memory location. The address of these locations varies from 0 to 65535.

 

Memory is primarily of three types

• Primary Memory/Main Memory.
• Secondary Memory ( Auxiliary Storage Devices).
• Cache Memory.

 

PRIMARY MEMORY (MAIN MEMORY)

Primary memory holds only those data and instructions on which computer is currently working. It has limited capacity and data get lost when power is switched off. It is generally made up of semiconductor device. These memories are not as fast as registers. The data and instruction required to be processed earlier reside in main memory.

 

Characteristics of Main Memory

1. These are semiconductor memories.
2. Usually volatile memory.
3. Data is lost in case power is switch off.
4. It is working memory of the computer.
5. Faster than secondary memories.
6. A computer cannot run without primary memory.

Main memory is divided into two categories, which are RAM and ROM.

Random Access Memory (RAM)

A RAM constitutes the internal memory of the CPU for storing data, program and program result. It is read/write memory. It is called random access memory (RAM).

Since access time in RAM is independent of the address to the word that is, each storage location inside the memory is as easy to reach as other location and takes the same amount of time. We can reach into the memory at random and extremely fast but can also be quite expensive.

RAM is volatile, i.e. data stored in it is lost when we switch off the computer or if there is a power failure. Hence a backup uninterruptible power system (UPS) is often used with computers. RAM is small, both in terms of its physical size and in the amount of data it can hold.

 

RAM is of two types:

• Static RAM (SRAM).
• Dynamic RAM (DRAM).

 

A) Static RAM (SRAM): The word static indicates that the memory retains its contents as long as power remains applied. However, data is lost when the power gets down due to volatile nature. SRAM chips use a matrix of 6-transistors and no capacitors. Transistors do not require power to prevent leakage, so SRAM need not have to be refreshed on a regular basis. Because of the extra space in the matrix, SRAM uses more chips than DRAM for the same amount of storage space, thus making the manufacturing costs higher. Static RAM is used as cache memory, needs to be very fast and small.

 

Characteristics of the Static RAM (SRAM)

1. It has long data lifetime.
2. There is no need to refresh.
3. Faster.
4. Used as cache memory.
5. Large size.
6. Expensive.
7. High power consumption.

 

B) Dynamic RAM (DRAM): DRAM, unlike SRAM, must be continually refreshed in order for it to maintain the data. This is done by placing the memory on a refresh circuit that rewrites the data several hundred times per second. DRAM is used for most system memory because it is cheap and small. All DRAMs are made up of memory cells. These cells are composed of one capacitor and one transistor.

 

Characteristics of the Dynamic RAM

1. It has short data lifetime.
2. Need to be refresh continuously.
3. Slower as compared to SRAM.
4. Used as RAM.
5. Lesser in size.
6. Less expensive
7. Less power consumption.

 

Read Only Memory (ROM)

ROM stands for Read Only Memory. The memory from which we can only read but cannot write on it. This type of memory is non-volatile. The information is stored permanently in such memories from the factory. A ROM, stores such instructions as are required to start computer when electricity is first turned on, this operation is referred to as bootstrap. ROM chip are not only used in the computer but also in other electronic items such as washing machine and microwave oven.

 

Advantages of ROM

1. Non-volatile in nature.
2. These cannot be accidentally changed.
3. Cheaper than RAMs.
4. Easy to test.
5. More Reliable than RAMs.
6. These are static and do not require refreshing.
7. Its contents are always known and can be verified.

 

Types of ROM

1. MROM (Masked ROM): The very first ROMs were hard-wired devices that contained a pre-programmed set of data or instructions. These kinds of ROMs are known as masked ROMs. It is inexpensive ROM.
2. PROM (Programmable Read only Memory): PROM is read-only memory that can be modified only once by a user. The user buys a blank PROM and enters the desired contents using a PROM programmer. Inside the PROM chip there are small fuses which are burnt open during programming. It can be programmed only once and is not erasable.
3. EPROM (Erasable and Programmable Read Only Memory): The EPROM can be erased by exposing it to ultra-violet light for duration of up to 40 minutes. Usually, EPROM eraser achieves this function. During programming an electrical charge is trapped in an insulated gate region. The charge is retained for more than ten years because the charge has no leakage path. For erasing this charge, ultra-violet light is passed through a quartz crystal window (lid). This exposure to ultraviolet light dissipates the charge. During normal use the quartz lid is sealed with a sticker.
4. EEPROM (Electrically Erasable and Programmable Read Only Memory): The EEPROM is programmed and erased electrically. It can be erased and reprogrammed about ten thousand times. Both erasing and programming take about 4 to 10 ms (milli-second). In EEPROM, any location can be selectively erased and programmed. EEPROMs can be erased one byte at a time, rather than erasing the entire chip. Hence, the process of re-programming is flexible but slow.

 

SECONDARY MEMORY (AUXILIARY STORAGE DEVICE)

This type of memory is also known as external memory or non-volatile. It is slower than main memory. These are used for storing data and information permanently. CPU directly does not access these memories instead they are accessed via input-output routines. Contents of secondary memories are first transferred to main memory, and then CPU can access it.

 

Characteristics of Secondary Memory

1. These are magnetic and optical memories.
2. It is known as backup memory.
3. It is non-volatile memory.
4. Data is permanently stored even if power is switched off.
5. It is used for storage of the data in the computer.
6. Computer may run without secondary memory.
7. Slower than primary memories.

 

Types of Secondary Storage

A) Floppy Disk: A floppy disk, or diskette, is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles. They are read and written by a floppy disk drive (FDD). Floppy disks, initially as 8-inch (200 mm) media and later in 5.25-inch (133 mm) and 3.5inch (90 mm) sizes.

 

Internal parts of a 3½ inch floppy disk

1. A hole that indicates a high-capacity disk.
2. The hub that engages with the drive motor.
3. A shutter that protects the surface when removed from the drive.
4. The plastic housing.
5. A polyester sheet reducing friction against the disk media as it rotates within the housing.
6. The magnetic coated plastic disk.
7. A schematic representation of one sector of data on the disk; the tracks and sectors are not visible on actual disks.
8. The write protection tab (unlabeled) is upper left.

 

B) Hard Disk: A hard disk drive (HDD) is a data storage device used for storing and retrieving digital information using rapidly rotating discs (platters) coated with magnetic material. An HDD retains its data even when powered off. Data is read in a random-access manner, meaning individual blocks of data can be stored or retrieved in any order rather than just sequentially.

 

C) Flash Drive: USB flash drives are often used for the same purposes for which floppy disks or CD-ROMs were used, i.e ., for storage, back-up and transfer of computer files. They are smaller, faster, have thousands of times more capacity, and are more durable and reliable because they have no moving parts.

1. USB Standard, Male A-plug.
2. USB mass storage controller device
3. Test points.
4. Flash memory chip.
5. Crystal oscillator
6. LED (Optional)
7. Write-protect switch (Optional).
8. Space for second flash memory chip.

 

D) Compact Disk (CD): A compact disc is a small, portable, round medium made of molded polymer for electronically recording, storing, and playing back audio, video, text, and other information in digital form. Standard CDs have a diameter of 120 millimetres and can hold up to 80 minutes of uncompressed audio or 737 MB of data. The Mini CD has various diameters ranging from 60 to 80 millimetres; they are sometimes used for CD singles, storing up to 24 minutes of audio or delivering device drivers.

 

E) Digital Video Disk (DVD): It is an optical disc technology with a 4.7 gigabyte storage capacity on a single – sided, one layered disk, which is enough for a 133 -minute movie. It can be single or double sided, and can have two layers on each side. Two layered DVD will hold up to 17 gigabytes of video, audio, or other information. DVD uses the MPEG-2 file and compression standard. DVD is an optical disc storage format, invented by Phillips, Sony, Toshiba and Panasonic in 1995. It offers higher storage capacity than compact discs while having the same dimensions.

 

F) Cache Memory: Cache memory is a very high speed semiconductor memory which can speed up CPU. It acts as a buffer between the CPU and main memory. It is used to hold those parts of data and program which are most frequently used by CPU. The parts of data and programs are transferred from disk to cache memory by operating system, from where CPU can access them.

 

Advantages of Cache Memory 

1. Cache memory is faster than main memory.
2. It consumes less access time as compared to main memory.
3. It stores the program that can be executed within a short period of time.
4. It stores data for temporary use.

 

Disadvantages of Cache Memory 

1. Cache memory has limited capacity.
2. It is very expensive.

 

DIFFERENCES BETWEEN PRIMARY MEMORY AND SECONDARY MEMORY

 

PRIMARY MEMORY

1. These are semiconductor memories.
2. They are characterized as volatile as in random access memories (RAM) or non-volatile as in memories (ROM).
3. They contain program and data that is currently being used by microprocessor.
4. These memories are fast enough to interact with the microprocessor.
5. Also known as Main Memory.
6. Data is lost in case power is switch off.
7. A computer cannot run without primary memory.
8. It is working memory of the computer.

 

SECONDARY MEMORY

1. These are magnetic and optical memories.
2. They are non-volatile.
3. They are used for bulk storage.
4. Slower than primary memories.
5. Also known as Backup Memory or Auxiliary Memory. E.g. Tapes, Floppies, Hard Discs, CD ROMs, DVDs
6. Data are permanently stored even if power is switched off.
7. Computer may run without secondary memory.
8. It is used for storage of the data out of computer.

 

DIFFERENCES BETWEEN PRIMARY STORAGE DEVICES AND SECONDARY STORAGE DEVICES

 

PRIMARY STORAGE DEVICES

1. These devices are temporary
2. These devices are expensive and smaller.
3. These devices have less storage capacity.
4. These devices refer to RAM.
5. Data transfer through these devices to computer are faster.
6. Primary memory storages are connected through data buses to CPU.
7. Primary memory is the main memory (Hard disk, RAM) where the operating system resides.

 

SECONDARY STORAGE DEVICES

1. These devices are permanent.
2. These devices are cheaper and larger.
3. These devices have high storage capacity.
4. These devices refer to FDD.
5. Data transfer through these devices to computer via cables, are therefore slow.
6. Secondary storages are connect through data cables to CPU.
7. Secondary memory can be external devices like CD, floppy magnetic discs etc. secondary storage cannot be directly accessed by the CPU and is also external memory storage.

 

UNITS OF STORAGE

Bit (Binary Digit)

A binary digit is logical 0 & 1 representing a passive or an active state of a component in an electric circuit.

 

Nibble

A group of 4 bits is called nibble. Nibbles are important in hexadecimal and binary coded decimal (BCD).

 

Byte

A group of 8 bits is called byte. A byte is the smallest unit which can represent a data item or a character.

 

Word

A computer word like a byte, is a group of fixed number of bits processed as a unit which varies from computer but is fixed for each computer. The length of a computer word is called word-size or word length and it may be as small as 8 bits or may be as long as 96 bits. A computer stores the information in the form of the computer words.

 

UNIT CONVERSION

1 nibble = 4 bits.

1 byte = 8 bits.

1 kilobyte (KB) = 1024 Bytes.

1 Megabyte (MB) = 1024 KB.

1 Gigabyte (GB) = 1024 MB.

1 Terabyte (TB) = 1024 GB.

1 Petabyte (PB) = 1024 TB.