Computer Memory
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The document provides an overview of computer memory, detailing the importance of primary (RAM and ROM) and secondary memory (e.g., hard drives, CDs) in data storage and processing. It describes various types of memory, including cache, RAM (static and dynamic), and ROM, as well as their characteristics, advantages, and disadvantages. Additionally, it explains different secondary storage devices such as magnetic tapes and disks, highlighting their roles in long-term data retention and cost-effectiveness compared to primary memory.
Computer Memory
- 1. Introduction to Computer System Primary Memory & Secondary Memory Prepared By S.K. Barnwal Assistant Professor Arka Jain University Research Scholar (NIT, Jamshedpur)
- 2. Memory Computer memory is just like a human brain used to store data and instructions. It is the storage space in the 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, which varies from zero to memory size minus one. For example, if the computer has 64k words, then this memory unit has 64 * 1024 = 65536 memory locations. The address of these locations varies from 0 to 65535. In actual memory is the most essential element of a computing system because without it computer can’t perform simple tasks. Computer memory is of two basic type – Primary memory(RAM and ROM) and Secondary memory(hard drive, CD, etc.). Random Access Memory (RAM) is primary-volatile memory and Read Only Memory (ROM) is primary-non-volatile memory.
- 3. Memory Memory unit is the amount of data that can be stored in the storage unit. This storage capacity is expressed in terms of Bytes. • 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. • Word: A computer word, like a byte, is a group of fixed number of bits processed as a unit, which varies from computer to computer but is fixed for each computer. • 1 KB = 1024 Bytes • 1 MB = 1024 KB • 1 GB = 1024 MB • 1 TB = 1024 GB Memory is primarily of three types − • Cache Memory • Primary Memory/Main Memory • Secondary Memory
- 4. Memory Cache Memory: Cache memory is a very high speed semiconductor memory which can speed up the CPU. It acts as a buffer between the CPU and the main memory. It is used to hold those parts of data and program which are most frequently used by the CPU. The parts of data and programs are transferred from the disk to cache memory by the operating system, from where the CPU can access them. Advantages • The advantages of cache memory are as follows − • 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 • The disadvantages of cache memory are as follows − • Cache memory has limited capacity. • It is very expensive.
- 5. Memory Primary Memory (Main Memory): Primary memory holds only those data and instructions on which the computer is currently working. It has a limited capacity and data is 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 resides in the main memory. It is divided into two subcategories RAM and ROM. Characteristics of Main Memory • These are semiconductor memories. • It is known as the main memory. • Usually volatile memory. • Data is lost in case power is switched off. • It is the working memory of the computer. • Faster than secondary memories. • A computer cannot run without the primary memory.
- 6. Memory Secondary Memory: This type of memory is also known as external memory or non-volatile. It is slower than the 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. The contents of secondary memories are first transferred to the main memory, and then the CPU can access it. For example, disk, CD-ROM, DVD, etc. Characteristics of Secondary Memory • These are magnetic and optical memories. • It is known as the backup memory. • It is a non-volatile memory. • Data is permanently stored even if power is switched off. • It is used for storage of data in a computer. • Computer may run without the secondary memory. • Slower than primary memories.
- 7. Memory Hierarchy Speed Increases Memory Space Decreases Speed Decreases Memory Space Increases
- 8. RAM RAM (Random Access Memory): It is the internal memory of the CPU for storing data, program, and program result. It is a read/write memory which stores data until the machine is working. As soon as the machine is switched off, data is erased. Access time in RAM is independent of the address, that is, each storage location inside the memory is as easy to reach as other locations and takes the same amount of time. Data in the RAM can be accessed randomly but it is very expensive. It is volatile, data stored in it is lost when switch off the computer or if there is a power failure. Hence, a backup UPS (Uninterruptible Power System) 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)
- 9. 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 be refreshed on a regular basis. Characteristic of Static RAM • Long life • No need to refresh • Faster • Used as cache memory • Large size • Expensive • High power consumption
- 10. RAM Dynamic RAM (DRAM): It is 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 as it is cheap and small. Characteristics of Dynamic RAM • Short data lifetime • Needs to be refreshed continuously • Slower as compared to SRAM • Used as RAM • Smaller in size • Less expensive • Less power consumption
- 11. ROM Read Only Memory (ROM): The memory from which data can only read but cannot write on it. This type of memory is non-volatile in nature. 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. Types of Read Only Memory (ROM) – • PROM (Programmable read-only memory) – It can be programmed by user. Once programmed, the data and instructions in it cannot be changed. • EPROM (Erasable Programmable read only memory) – It can be reprogrammed. To erase data from it, expose it to ultra violet light. To reprogram it, erase all the previous data. • EEPROM (Electrically erasable programmable read only memory) – The data can be erased by applying electric field, no need of ultra violet light. We can erase only portions of the chip.
- 12. Classification of Secondary Storage Devices As primary memory, is expensive as well as limited. The faster primary memory is also volatile. If we need to store large amount of data or programs permanently, we need a cheaper and permanent memory. Such memory is called secondary memory that provides permanent storage of data and in bulk quantity. The Secondary storage media can be fixed or removable. Fixed Storage media is an internal storage medium like hard disk that is fixed inside the computer. Storage medium that are portable and can be taken outside the computer are termed as removable storage media. Applications: • Permanent Storage: Primary Memory (RAM) is volatile, i.e. it loses all information when the electricity is turned off, so in order to secure the data permanently in the device, Secondary storage devices are needed. • Portability: Storage medium, like the CDs, flash drives can be used to transfer the data from one device to another.
- 13. Secondary Storage Devices Magnetic Tape, Magnetic Disk, Optical Disk, Magneto Optical disk: These are the storage devices provides a way to store data for a long duration. These disks differ in many characteristics; firstly magnetic disk works by using magnetic material over the disk whereas in optical disk polycarbonate plastic is used in its construction and laser is used to store and retrieve the data. The magnetic and optical disk comes under the category of secondary storage devices. The need to devise these devices has emerged because the previous semiconductor storage devices have very limited capabilities, for example, the cost of storing the information in such devices is very high.
- 14. Secondary Storage Devices Magnetic Tape, Magnetic Disk, Optical Disk, Magneto Optical Disk
- 15. Secondary Storage Devices Magnetic Tape memory: In magnetic tape only one side of the ribbon is used for storing data. It is sequential memory which contains thin plastic ribbon to store data and coated by magnetic oxide. Data read/write speed is slower because of sequential access. It is highly reliable which requires magnetic tape drive writing and reading data. The width of the ribbon varies from 4mm to 1 Inch and it has storage capacity 100 MB to 200 GB. Advantages : • These are inexpensive, i.e., low cost memories. • It provides backup or archival storage. • It can be used for large files. • It can be used for copying from disk files. • It is a reusable memory. • It is compact and easy to store on racks. Disadvantages : • Sequential access not allowed, • It requires caring to store, i.e., vulnerable humidity, dust free, and suitable environment. • stored data cannot be easily updated or modified
- 16. Secondary Storage Devices Magnetic Disk: It is a type of secondary memory which is a flat disc covered with magnetic coating to hold information. It is used to store various programs and files. The polarized information in one direction is represented by 1, and vice versa. The direction is indicated by 0. It can store large amounts of data, but data access rate is slower than main memory because of secondary memory. Data can be modified or can be deleted easily in the magnetic disk memory. It also allows random access of data. • Advantages :- These are economical memory The easy and direct access of data possible. It can store large amounts of data. It has better data transfer rate than magnetic tapes. It has less prone to corruption of data as compared to tapes. • Disadvantages :- These are less expensive than RAM but more expensive than magnetic tape. It need clean and dust free environment to store. These are not suitable for sequential access.
- 17. Secondary Storage Devices Optical Disc: It is an electronic data storage medium that can be written to and read from using a low-powered laser beam. Originally developed in the late 1960s, the first optical disc, created by James T. Russell, stored data as micron-wide dots of light and dark. In this a laser read the dots and the data was converted to an electrical signal. Then the data was converted to audio or visual output could hold much more data than similarly sized magnetic storage media. Magneto-optical Disc: It is a hybrid storage medium. In reading, spots with different directions of magnetization give different polarization in the reflected light of a low-power laser beam. In writing, every spot on the disk is first heated by a strong laser beam and then cooled under a magnetic field, magnetizing every spot in one direction, to store all 0s. The writing process then reverses the direction of the magnetic field to store 1s where desired.