Course Name: Computer Application LECTURE Course Name: Computer Application Topic: Memory

Main/Primary/ Volatile/ Temporary Memory A memory is just like a human brain and it is used to store data and instructions. Computer memory is the storage space in computer where data is to be processed and instructions required for processing are stored. The memory is divided into large number of small parts called cells. Each location or cell has a unique address where data stored. It is the internal memory of the CPU for storing data, program and program result. It is read/write memory which stores data until the machine is working. As soon as the machine is switched off, data is erased. Normally when people talk about memory in relation to a PC, they are talking about RAM. RAM is small, both in terms of its physical size and in the amount of data it can hold.

Random Access Memory (RAM) Random Access Memory (RAM) is the main 'working' memory used by the computer. When the operating system loads from disk when you first switch on the computer, it is copied into RAM. Commonly , modern computers are supplied with over 2 GBs of RAM. As a rough rule, a Microsoft Windows based computer will operate faster if you install more RAM. Data and programs stored in RAM are volatile (i.e. the information is lost when you switch off the computer).

The Use of Primary Storage (RAM) Main Memory has several uses: - Input area – where the data is stored when it is read into CPU, awaiting processing. - Operating system – controls the operation of the computer. - Working storage – where calculations are performed and data is stored temporarily. - Output area where the information is stored prior to output. Both the input and output areas are buffer. Application program area where the user program is held.

Main/Primary/ Volatile/ Temporary Memory Characteristics of Main Memory Usually volatile memory. Data is lost in case power is switched off. It is working as memory of the computer. Faster than secondary memories. A computer cannot run without primary memory RAM is of two types SRAM DRAM

Types of RAM Static RAM (SRAM) The word static indicates that the memory retains its contents as long as power is being supplied. 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. So SRAM is used as cache memory and has very fast access. Characteristic of the Static RAM It has long life There is no need to refresh Faster Used as cache memory Large size Expensive High power consumption

Characteristics of the Dynamic RAM Dynamic RAM (DRAM) DRAM, unlike SRAM, must be continually refreshed in order 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 which are composed of one capacitor and one transistor. Characteristics of the Dynamic RAM It has short data lifetime Need to be refreshed continuously Slower as compared to SRAM Used as RAM Lesser in size Less expensive 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 during manufacture. A ROM, stores such instructions that are required to start a computer. This operation is referred to as bootstrap. ROM chips are not only used in the computer but also in other electronic items like washing machine and microwave oven.

ROM The 'Read Only Memory Basic Input Output System' is a chip located on the computer's system (mother) board, which contains software. This software performs a variety of tasks. When you first switch on the computer, the ROM-BIOS software performs a self-diagnostic test (POST- Power on Self Test) to check that the computer is working OK. This software then loads your operating system from the disk into the RAM.

Types of ROM Types of ROM are: PROM EPROM EEPROM 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 program. Inside the PROM chip there are small fuses which are burnt open during programming. It can be programmed only once and is not erasable.

EPROM Erasable and Programmable Read Only Memory The EPROM can be erased by exposing it to ultra-violet light for a duration of up to 40 minutes. 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 millisecond. 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.

ROM is non-volatile storage This means that when the power to the computer is turned off, the content of the ROM is not lost. ROM can store pre-set programs that are always put by computer manufactures. When you turn on a PC, you will usually see a reference on BIOS (Basic Input output System). This is part of the ROM chip containing all the programs needed to control the keyboard, monitor, disk drive and so on.

The advantages of ROM are as follows: Non-volatile in nature These cannot be accidentally changed Cheaper than RAMs Easy to test More reliable than RAMs These are static and do not require refreshing Its contents are always known and can be verified

Differentiate Between RAM &ROM ROM is non-volatile RAM is volatile ROM is cheaper than RAM RAM is very expensive The contents of ROM are always known and can be verified The contents are not known ROM can not be updated or corrected RAM can be updated and corrected ROM serves as permanent data storage RAM can serve as temporary data storage

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 Cache memory is faster than main memory. It consumes less access time as compared to main memory. It stores the program that can be executed within a short period of time. It stores data for temporary use. Disadvantages Cache memory has limited capacity. It is very expensive.

What is Cache Memory? Helps speed computer processes by storing frequently used instructions and data Also called memory cache, cache store, or RAM cache L1 cache built in processor L2 and L3 cache not built in processor L2 advanced transfer cache most common

Memory Units Memory unit is: the amount of data that can be stored in the storage unit. that in which storage capacity is expressed in terms of Bytes. Following are the main memory storage units: Bit Binary Digit A binary digit is logical 0 and 1 representing a passive or an active state of a component in an electric circuit. Nibble A group of 4 bits is called nibble. Byte: A group of 8 bits is called byte. A byte is the smallest unit which can represent a data item or a character.

How is a character sent from keyboard to computer? Step 1: Press letter T Step 2: Electronic signal for letter T sent to system unit Step 3: Signal changed to its ASCII code (01010100) and stored in memory Step 4: After processing, binary code for letter T is converted to image on output device

Memory Units Few Higher storage units are: Bits 1b = (0/1) Nible 1 N = 4 Bits Byte (B) 1 B = 8 bits Kilo byte (KB) 1 KB = 1024 Bytes Megabyte (MB) 1 MB = 1024 KB Giga Byte (GB) 1 GB = 1024 MB Tera Byte (TB) 1 TB = 1024 GB Peta Byte (PB) 1 PB = 1024 TB Exa Byte (EB) 1 EB = 1024 PB Petta Byte (ZB) 1 PB =1024 EB Yotta Byte (YB) 1 YB = 1024 PB

Secondary Memory This type of memory is also known as external memory or non-volatile. Used in a computer system to overcome the limitations of primary storage Has virtually unlimited capacity because the cost per bit of storage is very low Has an operating speed far slower than that of the primary storage Used to store large volumes of data on a permanent basis and Also known as auxiliary memory It is slower than main memory. These are used for storing data/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. For example : disk, CD-ROM, DVD etc. .

Sequential access storage devices Data can only be retrieved in the same sequence in which it is stored Access time varies according to the storage location of the information being accessed Suitable for sequential processing applications where most, if not all, of the data records need to be processed one after another Magnetic tape is a typical example of such a storage device

Direct Access Storage Devices Devices where any storage location may be selected and accessed at random Permits access to individual information in a more direct or immediate manner Approximately equal access time is required for accessing information from any storage location Suitable for direct processing applications such as online ticket booking systems, on-line banking systems Magnetic, optical, and magneto-optical disks are typical examples of such a storage device

Optical Disks CD (Compact Disk) It is read only memory device Store up to 650 MB of data Portable and can be used on any computer that has a CD-ROM drive CD-R (Recordable) It is also read memory device but it can record the data. Stands for Write Once Read Many. Data can be written only once on them, but can be read many times. CD-WR (Writable, Recordable)/Rewritable Data to be recorded can be written on its surface in multiple recording sessions Can be erased and written afresh DVD (Digital Video / Versatile Disk) – Store up to 4 GB Data and maximum data can be stored on DVD is upto 17 GB DVD-R Recordable DVD-WR (Writable, Recordable)/Rewritable

Universal Serial Bus (USB) Flash drives plug into the USB port and when viewed, via the Windows Explorer, look just like any other drive. They come in a range of sizes and are a very cheap way of transferring data between computers. Memory Cards A memory card (also called a flash memory card) is a card containing memory chips that are often used in devices such as digital cameras, telephones, music players, video game consoles, GPS systems and similar devices where there is a need to store data in a compact form, often using a battery power source. There are a number of different types of memory cards with different storage capacities. Many new PCs have built-in slots for different types of memory cards.

Floppy Disk Floppy disks are also known as diskettes. They are very slow compared to hard disks or CD / DVD disks, and hold relatively small amounts of data (1.44 Mbytes). Sometimes people will backup important data from their hard disk to floppy disks. However, as diskettes are notoriously unreliable this is not the best way of backing up valuable data (but is better than nothing). Modern computers are not normally supplied with this type of drive. Floppy disks have almost entirely been replaced by CD or DVD disks.

Network drive and online storage Within an office it is normal for the computers to be connected together via a network. This allows you to store your data centrally, on a network server. This network server should be backed by the IT support staff on a daily basis. This means that your data is safely backed up for you. Alternatively you may create and store your data on your own PC or laptop and periodically copy it across the network to be stored safely on a central network server. In many companies network software automatically backs up selected folders on each computer to the central server.