Memory Organisation

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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 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.

Memory is primarily of three types:

  1. Cache Memory
  2. Primary Memory/Main Memory
  3. Secondary Memory
  • 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 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.
  • 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.

(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)

(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.

  • 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 nonvolatile
    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.

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