Class 9 Computer Science Memory Organisation Notes

Memory Organisation

Computer organisation is the way in which the components are built in computers whereas Computer architecture is the science of integrating those components to achieve a level of functionality and performance this chapter we shall study a high level view of computer architecture that may be concerned with how the central processing unit (CPU) acts and how it uses computer memory.

Computer Architecture is the field of study of selecting and interconnecting hardware components to create computers that satisfy functional performance and cost goals. It refers to those attributes of the computer system that are visible to a programmer and have a direct effect on the execution of a program.

Computer Architecture concerns with machine Organization, interfaces, application, technology, measurement & simulation that Includes:

• Instruction set
• Data formats
• Principle of Operation
• Features (organization of programmable storage, registers used, interrupts mechanism, etc.)

In short, it is the combination of Instruction Set Architecture, Machine Organization and the related hardware.

Interconnection of Units

CPU sends data, instructions and information to the components inside the computer as well as to the peripherals and devices attached to it. Bus is a set of electronic signal pathways that allows information and signals to travel between components inside or outside of a computer.

The features and functionality of a bus are as follows

• A bus is a set of wires used for interconnection, where each wire can carry one bit of data.
• A computer bus can be divided into two types; internal bus and external bus.
• The internal bus connects components inside the motherboard like, CPU and system memory. It is also called the system bus.
• The external bus connects the different external devices; peripherals, expansion slots, I/O ports and drive connections to the rest of computer. It is also referred to as the expansion bus.
• The command to access the memory or the I/O device is carried by the control bus.
• The address of I/O device or memory is carried by the address bus. The data to be transferred is carried by the data bus.

Instruction Cycle

The instruction cycle represents the sequence of events that takes place as an instruction is read from memory and executed.

A simple instruction cycle consists of the following steps

• Fetching the instruction from the memory.
• Decoding the instruction for operation.
• Executing the instruction.
• Storing in memory.

Instructions Format

Computer understand instructions only in terms of 0s and 1s, which is called the machine language. A computer program is a set of instruction that describe the steps to be performed for carrying out a computational task. The processor must have two inputs; instructions and data.

The instruction tell the processor what actions are needed to be performed on the data. An instruction is divided into two parts; operation (op-code) and operand.

The op-code represents action that the processor must execute and the operand defines the parameters of the action and depends on the operation.

Memory Hierarchy

The hierarchical arrangement of storage in current computer architectures is called the memory hierarchy. It is designed to take advantage of memory locality in computer programs. Each level of the hierarchy is of higher speed and lower latency, and is of smaller size, than lower levels.

Modern computer system memory hierarchy (from top to bottom):

• Registers in the CPU
• Cache
• Main memory
• Disk cache
• Magnetic disk
• Optical disk / Magnetic tape

Memory Organisation

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. Each part is called cell. Each location or cell has a unique address, which varies from zero to memory size minus one.

Some Important Definitions –

ACCUMULATOR AND GENERAL AND SPECIAL PURPOSE REGISTERS:

A CPU contains a number of register to store data temporarily during the execution of a program. The number differ from processor to processor. Registers are classified as follows :

• General Purpose Registers : These registers store data and intermediate results during the execution of a program.
• Accumulators : It is most efficient in data movement, arithmetic and logical operation. It has some special features that the other general purpose registers do not have. After the execution of arithmetic and logical instructions, the result is placed in the accumulator. All data transfer between the CPU and device/port are performed through the accumulator.
• Special Purpose Registers : A CPU contains a number of special purpose registers for different purpose. These are:
  • Program Counter (PC) : The program counter keeps track of the address of the instruction which is to be executed next. So it holds the address of the memory location, which contains the next instruction to be fetched from the memory. Its contents is automatically incremented after an instruction has been fetched.
  • Stack Pointer (SP) : The stack is a sequence of memory location defined by the user. It is used to save the contents of a register if it is required during the execution of a program. the stack pointer holds the address of the last occupied memory location of the stack.
  • Instruction Register : It holds an instruction until it is decoded. Some computers have two types of instruction registers, and so they can fetch and save the next instruction while the execution of the previous instruction is going on.
  • Memory Address Register : The memory address register holds the address of the next memory location where the next instruction is to be executed. While the first instruction is being executed, the address of the next memory location is held by it. The computer’s CPU uses the address bus to communicate which memory address it wants to access, and the memory controller reads the address and then puts the data stored in that memory address back onto the address bus for the CPU to use.
  • Memory Data Address : The memory data register (MDR) is the register of a computer’s control unit that contains the data to be stored in the computer storage (e.g. RAM), or the data after a fetch from the computer storage. It acts like a buffer and holds anything that is copied from the memory ready for the processor to use it.

Memory is primarily of three types:

• Cache Memory
• Primary Memory/Main Memory
• Secondary Memory

1. Cache Memory

CPU is much faster than memory. The problem comes when the CPU issues a memory request, it will not get the data it need for many CPU cycles. The slower the memory the more cycles the CPU will have to wait. This problem can be overcome by introducing a small and very fast memory near the CPU. The small, fast memory is called 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.

Advantage :

• 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.

Disadvantage :

• Cache memory has limited capacity.
• It is very expensive

Buffer : It is a temporary physical storage used to hold data during execution of process from one place to another.

2. Primary Memory (Main Memory)

Primary memory holds only those data and instructions on which computer is currently working. It has limited capacity and data gets 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 instructions required to be processed earlier reside in main memory. It is divided into two subcategories: RAM and ROM.

Characteristic of Primary Main Memory

• These are semiconductor memories.
• It is known as main memory.
• Usually volatile memory.
• Data is lost in case power is switched off.
• It is working memory of the computer.
• Faster than secondary memories.
• A computer cannot run without primary memory.

COMPARISON BETWEEN RAM & ROM

RAM:

• Definition: Random Access Memory or RAM is a form of data storage that can be accessed randomly at any time, in any order and from any physical location, allowing quick access and manipulation.
• Stands for: Random Access Memory
• Use: RAM allows the computer to read data quickly to run applications. It allows reading and writing.
• Volatility: RAM is volatile i.e. its contents are lost when the device is powered off.
• Types: The two main types of RAM are static RAM and dynamic RAM.

ROM:

• Definition: Read-only memory or ROM is also a form of data storage that can not be easily altered or reprogrammed. Stores instructions that are not necessary for rebooting up to make the computer operate when it is switched off. They are hard wired.
• Stands for: Read-only memory.
• Use: ROM stores the program required to initially boot the computer. It only allows reading.
• Volatility: It is non-volatile i.e. its contents are retained even when the device is powered off.
• Types: The types of ROM include PROM, EPROM and EEPROM.

(i) RAM (RANDOM ACCESS MEMORY)

There are two types of Random Access Memory or RAM, each has its own advantages and disadvantages compared to the other. A. SRAM (Static RAM) B. DRAM (Dynamic RAM)

Comparison between SRAM and DRAM:

• Definition: SRAM essentially uses latches to store charge. DRAM makes use of capacitors to store bits in the form of charge.
• Speed: SRAM is Faster; DRAM is Slower.
• Size: SRAM is Bigger; DRAM is Smaller.
• Cost: SRAM is More expensive per bit; DRAM is Less expensive per bit.
• Capacity (same technology): SRAM is Less; DRAM is 5 to 10 times more than SRAM.
• Applications: SRAM is generally in smaller applications like CPU cache memory and hard drive buffers. DRAM is commonly used as the main memory in personal computers.
• Types: SRAM types include Asynchronous, Synchronous, and Pipeline Burst SRAM. DRAM types include Fast Page Mode, Extended Data Out, Burst EDO DRSSM, and Synchronous DRAM.
• Power Consumption: SRAM consumes Less; DRAM consumes More.

(ii) ROM (READ ONLY MEMORY)

ROM has further classified into three types. Each type has unique characteristics, but all types of ROM memory have two things in common they are:-

• Data stored in these chips is non-volatile i.e it is not lost when power is removed.
• Data stored in these chips is either unchangeable or requires a special operation to change.

• A. Programmable Read-Only Memory (PROM) : This form of ROM is initially blank. The user or manufacturer can write data/program on it by using special devices. However, once the program or data is written in PROM chip, it cannot be changed. If there is an error in writing instructions or data in PROM, the error cannot be erased. PROM chip becomes unusable.
• B. Erasable Programmable Read-Only Memory (EPROM) : This type of ROM can have its contents erased by ultraviolet light and then reprogrammed by user/manufacturer. This procedure can be carried out many times; however, the constant erasing and rewriting will eventually render the chip useless.
• C. Electrically Erasable Programmable Read-Only Memory (EEPROM) : This type of ROM works in a similar way to Flash memory in that it can its contents can be ‘flashed’ for erasure ad then written to without having to remove the chip from its environment. EEPROMs are used to store a computer system’s BIOS, and can be updated without returning the unit to the factory. In many cases, BIOS updates can be carried out by computer users wishing a BIOS update.

Flash Memory

Flash memory is an example of quite a recent type of storage technology known as solid state devices. This type of portable storage has become very popular because of its low price and high storage capacity compared to its rivals, e.g. floppy disk.

Solid state devices are regarded as being robust and reliable because they have no moving parts with the data stored in semiconductor chips. This technology already exists in the form of flash memory used to store the Basic Input/Output System (BIOS) of a motherboard.

Unlike ROM, flash memory can be read form and written to and unlike RAM does not require power to retain its data. Although these devices typically cannot hold as much data as hard disks, CD-ROMs and DVDs, the storage capacity is continually increasing.

Virtual Memory

Virtual memory is a technique that allows the execution of processes which are not completely available in memory. The main visible advantage of this scheme is that programs can be larger than physical memory. Virtual memory is the separation of user logical memory from physical memory.

This separation allows an extremely large virtual memory to be provided for programmers when only a smaller physical memory is available. Following are the situations, when entire program is not required to be loaded fully in main memory. User written error handling routines are used only when an error occured in the data or computation. Certain options and features of a program may be used rarely. Many tables are assigned a fixed amount of address space even though only a small amount of the table is actually used.

3. Secondary Memory (Auxillary Memory)

The size of the main memory is very small if large data need to be stored in it. Further, the main memory is volatile in nature i.e. the contents are lost when power supply is stopped. To overcome these another memory is used in a computer system called secondary memory or the auxiliary memory. This is large as well as non-volatile in nature. 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/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.

Characteristics of Secondary Memory

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

Storage Devices

Storage devices are also called storage media. It is a hardware device that can hold information. Two main storage devices are used in computers. The primary storage device also known as RAM and the secondary storage device such as a computer hard drive. Secondary storage can be either internal or external storage. Storage device is required by the computer to save any settings or additional information.

Secondary Memory devices includes:

• A. Magnetic Disks : Magnetic disks play two roles in computer systems:
  • Long-term, nonvolatile storage for files, even when no programs are running
  • A level of the memory hierarchy below main memory used as a backing store for virtual memory during program execution
  A magnetic disk consists of a collection of platters (generally 1 to 12), rotating on a spindle at 3,600 to 15,000 revolutions per minute (RPM). These platters are metal or glass disks covered with magnetic recording material on both sides. The disk surface is divided into concentric circles, called tracks. Each track in turn is divided into sectors. A sector is the smallest unit that can be read or written.
• B. Optical Disks : Optical disks are another type of secondary memory. Many types of optical disks are available in the market like CD(Compact disks), DVD (Digital versatile disks) etc. CD-R are write once CDs i.e. data can be written to them only once. CD-RW on the other hand are rewritable CDs i.e. data can be written and erased many times.
• C. Magnetic Tape : Magnetic tape is a long and narrow strip of plastic that thin magnetic material is coated on. Nearly all recording tape is of this type, whether used for recording audio or video or computer data storage.
• D. Floppy diskette : A Floppy Disk Drive is a computer disk drive that helps a user to save data to removable diskettes. 8 inch disk drives were first made available in 1971, which were later replaced with \( 3 \frac{1}{2} \) inch floppy disk drives.
• E. CD-ROM disc : Compact Disc-Read Only Memory, CD-ROM drives or optical drives are CD players inside computers that can have speeds in the range from 1x and beyond, and have the capability of playing audio CDs and computer data CDs.
• F. CD-R and CD-RW disc : It is also called as a CD writer, CD-WO (Write once), WORM (Write Once Read Many) drive. CD-R is used for CD-Recordable and is a writable disc and drive that is capable of having information written to the disc once and then having that disc read many times after that.
• G. Recordable DVD (Digital video Disk) drives: These DVD drives are alternatively referred to as a DVD writer, recordable DVD drives. The recordable DVD drives are capable of creating DVD discs. Standards include DVD-R, DVD-RW, DVD + R, DVD + RW, DVD + R DL (DVD + R9), and DVD-RAM.
• H. Jump drive and USB flash drive : USB flash drive is a portable drive that is as small as the size of your thumb. Other such portable drives include data stick, pen drive, keychain drive, thumb drive. It connects to the computer USB port. Flash drives are available in different sizes such as 256MB, 512MB, 1GB, 5GB, and 16GB.
• I. Hard Drive : It is represented sometimes by its short name as HD or HDD. It is the computer’s main storage area that permanently stores the data on to the computer. It consists of one or more hard drive platters inside of air sealed casing. They connect to the mother board using ATA, SCSI or a SATA cable.
• J. Memory card : These are referred to as a flash memory card or a memory card. The memory card type of storage media is used in digital cameras, digital camcorders, handheld computers, printers, MP3 players and cell phones. MicroSD flash memory card is one such example.
• K. Memory stick : Sony Memory Stick was first invented by Sony in October 1998. It is a flash memory card used with Sony digital cameras and other Sony products. The capacity range from 4MB to 256GB. They can have maximum capacity of 2TB.
• L. Zip Diskette : Zip Diskette are made out of a much harder plastic and are much bigger than the standard floppy diskette. Zip drive is a medium capacity removable floppy disk storage system.
• M. Blue ray disk : Blu-ray (not Blue-ray) also known as Blu-ray Disc (BD), is the name of a new optical disc format jointly developed by the Blu-ray Disc Association (BDA). It enables recording, rewriting and playback of high-definition video (HD). It can hold up to 25GB on a single-layer disc and 50GB on a dual-layer disc.

Tape cassette

A tape cassette is magnetically thin coated piece that is wrapped with plastic around wheels. These are capable of storing data. Tape is far cheaper than other storage mediums but is much slower than the jump drives, memory sticks etc.

Memory Management Table

Secondary Memory Device storage Method and capacity:

• Floppy disk (5.25 inch): Magnetic; 1.2 MB
• Hard Disk: Magnetic; upto 1 TB
• CD-ROM: optical; 640 MB to 680 MB
• DVD-ROM: optical; 4.7 GB to 17 GB
• Pen-Drive: solid state; 1 GB to 512 GB
• Magnetic Tape: Magnetic; upto 1 TB

Approximate/Actual Values:

• Bit (b): 0 or 1
• Byte (B): 8 bits
• Kilobytes (KB): 1000 bytes (Approx); 1024 bytes (Actual)
• Megabte (MB): 1 million bytes (Approx); 1024 KB (Actual)
• Gigabyte (GB): 1 billion bytes (Approx); 1024 MB (Actual)
• Terabyte (TB): 1 trillion bytes (Approx); 1024 GB (Actual)
• Petabyte (PB): \( 10^{15} \) Bytes (Approx); 1024 TB (Actual)
• Exa Byte (EB): \( 10^{18} \) Bytes (Approx); 1024 PB (Actual)
• Zetta Byte (ZB): \( 10^{21} \) Bytes (Approx); 1024 EB (Actual)
• Yotta Byte (YB): 1024 ZB (Actual)
• Bronta Byte (BB): 1024 YB (Actual)

Important Measurement :

1 Bit = Binary digit

The term ‘Bit’ is short for Binary digit.

1 Nibble = 4 Bits

8 bits = 1 Byte = 2 Nibble