Memory Organisation
Category : Banking
Memory Organisation
Computer organization 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:
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
INTRUCTION 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
INTRUCTIONS FORMAT
Computer understand instructions only in terms of Os 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 mast 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.
Following diagram shows memory hierarchy in a modem computer system
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 GENRAL 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. 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 register do not have. After the execution of arithmetic and logical instruction, the result is placed in the accumulator. All data transfer between the CPU and device/ port are performed through the accumulator.
Special purpose register: A CPU contains a number of special purpose registers for different purpose. These are
(i) PROGRAM COUNTER (PC): The programmer 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.
(ii) STRACK 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 hold the address of the last occupied memory location of the stack.
(iii) INTRUCTION REGISTER: It holds an instruction until it is decoded. Some computers have two type of instruction register, and so they can fetch and save the next instruction while the execution of the previous instruction is going on.
(iv) 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 want 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 CPU to use.
Memory-memory is made up of large number of cells.
(v) 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.
Classification of Computer Memory:
Memory is primarily of two types:
Buffer: It is a temporary physical storage used to hold data during execution of process from one place to another.
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
COMPARISON BETWEEN RAM & ROM
Description |
RAM |
ROM |
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, |
Read-only memory or ROM is also a form of data storage that cannot 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 |
Random Access Memory |
Read-only memory. |
Use |
RAM allows the computer to read data quickly to run applications. It allows reading and writing. |
ROM stores the program required to initially boot the computer. It only allows reading. |
Volatility |
RAM is volatile i.e. its contents are lost when the device is powered off. |
It is non-volatile i.e. its contents are retained even when the device is powered off. |
Types |
The two main types of RAM are static RAM and dynamic RAM. |
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.
COMPARISON BETWEEN SRAM (STATIC RANDOMACCESS MEMORY) AND DRAM (DYNAMIC
RANDOM ACCESS MEMORY):
|
SRAM |
DRAM |
Definition |
It is a type of RAM. SRAM essentially uses latches to store charge. |
It is also a type of RAM. DRAM makes use of capacitors to store bits in the form of charge. |
Speed |
Faster |
Slower |
Size |
Bigger |
Smaller |
Cost |
More expensive per bit |
Less expensive per bit |
Capacity (same technology)
|
Less |
5 to 10 times more than SRAM |
Applications |
Generally in smaller applications like CPU cache memory and hard drive buffers |
Commonly used as the main memory in personal computers |
Types |
Asynchronous SRAM Synchronous SRAM Pipeline burst SRAM |
Fast Page Mode DRAM Extended Data Out DRAM Burst EDO DRSSM Synchronous DRAM |
Power |
Less |
Less |
(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:-
CACHE MEMORY
CPU is much faster than memory. The problem comes when the CPU issues a memory request, it will not get the data is need for many CPU cycles. The slower the memory the 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 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 use to hold those parts of data and program which are most frequently used by CPU. The parts of data and program are transferred from disk to cache memory by operating system, from where CPU can access them.
Advantage
Disadvantage
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 rival, e.g. floppy disk.
Solid state devices are regarded as being robust 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, flase memory can be read from 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-ROM 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 the data or computation. Certain option 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.
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
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.
Following are the different types of computer Storage Devices/Secondary memories -
Secondary Memory devices includes:
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, so 10 platters have 20 recording surfaces. 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.
Magnetic tape recording uses magnetic tape which moves on a recording head. Electrical signals are fed to the recording head, inducing a pattern of magnetization similar to the signal. A playback head can then pick up the changes in magnetic field from the tape and convert it into an electrical signal.
(i) DVD-R DVD-R is called as Digital Versatile Disc-Recordable. DVD-R has features similar to CD- R, where the drive is capable or recording once to a disc and then read many times after it has been created. DVD-R is an approved standard by DVD Forum and the drives are capable of recording to DVD-R discs. These discs are also known as DVD-5 and DVD -10. DVD-R discs are compatible with most standalone DVD players and computer DVD-ROM drives.
(ii) DVD-RW (DVD-R/W): These are called as Digital Versatile Disc-Read/Write. DVD-RW is an approved standard by DVD Forum. It is based on a technology that enables a user to read and write to a DVD-RW or DVD-R disc several times. DVD-RW discs are compatible with most stand-alone DVD players and computer DVD-ROM drives.
A flash drive is used by inserting it onto the front or back of USB port or hub. After inserting the flash drive, open the My Computer, you can see as "Remove able Disk", "Flash drive", or as the manufacturer's name. When the drive has been identified, data can be copied onto the flash drive by a simple copy paste or just dragging and dropping onto the flash drive icon.
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. Tapes are not much in use these days for faster and more reliable solution like disk drives, hard drives, and flash drives.
DVDs and can hold up to 25GB on a single-layer disc and 50GB on a dual-layer disc. This extra capacity combined with the use of advanced video and audio codecs will offer consumers an unprecedented HD experience.
Secondary Memory Device and their storage Method and capacity.
Secondary memory device |
Storage Method |
Capacity |
Floppy disk (5.25 inch) |
Magnetic |
1.2MB |
Hard Disk |
Magnetic |
up to 1 T B |
CD-ROM |
optical |
640 MB to 680 MB |
DVD-ROM |
optical |
4.7GB to 17GB |
Pen-Drive |
solid state |
1 GB to 512GB |
Magnetic Tape |
Magnetic |
up to 1 TB |
IMPORTANT MEASUREMENT:
1 Bit = Binary digit
The term ‘bit’ is short for Binary digit.
1 Nibble = 4 Bits
8 Bit = 1 Byte = 2 Nibble
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