A computer is an electronic device that performs calculations on data, presenting the results to humans or other computers in a variety of (hopefully useful) ways. The computer system includes not only the hardware, but also software that are necessary to make the computer function.
Computer hardware is the physical part of a computer, including the digital circuitry, as distinguished from the computer software that executes within the hardware.
Computer software is a general term used to describe a collection of computer programs, procedures and documentation that perform some task on a computer system
Computer Organization
General Model of a Computer
A computer performs basically five major operations or functions irrespective of their size and make.
1. Input: This is the process of entering data and programs in to the computer system. You should know that computer is an electronic machine like any other machine which takes as inputs raw data and performs some processing giving out processed data. Therefore, the input unit takes data from us to the computer in an organized manner for processing.
2. Storage: The process of saving data and instructions permanently is known as storage. Data has to be fed into the system before the actual processing starts. It is because the processing speed of Central Processing Unit (CPU) is so fast that the data has to be provided to CPU with the same speed. Therefore the data is first stored in the storage unit for faster access and processing. This storage unit or the primary storage of the computer system is designed to do the above functionality. It provides space for storing data and instructions.The storage unit performs the following major functions: - All data and instructions are stored here before and after processing.- Intermediate results of processing are also stored here.
3. Processing: The task of performing operations like arithmetic and logical operations is called processing. The Central Processing Unit (CPU) takes data and instructions from the storage unit and makes all sorts of calculations based on the instructions given and the type of data provided. It is then sent back to the storage unit.
4. Output: This is the process of producing results from the data for getting useful information. Similarly the output produced by the computer after processing must also be kept somewhere inside the computer before being given to you in human readable form. Again the output is also stored inside the computer for further processing.
5. Control: The manner how instructions are executed and the above operations are performed. Controlling of all operations like input, processing and output are performed by control unit. It takes care of step by step processing of all operations in side the computer. In order to carry out the operations mentioned above, the computer allocates the task between its various functional units. The computer system is divided into several units for its operation.
CPU (central processing unit) : The place where decisions are made, computations are performed, and input/output requests are delegated
Memory: stores information being processed by the CPU
Input devices : allows people to supply information to computers
Buses : a bus is a subsystem that transfers data or power between computer components inside a computer
The basic function of a computer is program execution. When a program is running the executable binary file is copied from the disk drive into memory. The process of program execution is the retrieval of instructions and data from memory, and the execution of the various operations.The cycles with complex instruction sets typically utilize the following stages :
Fetch the instruction from main memory
The CPU presents the value of the program counter (PC) on the address bus. The CPU then fetches the instruction from main memory via the data bus into the Memory Data Register (MDR). The value from the MDR is then placed into the Current Instruction Register (CIR), a circuit that holds the instruction so that it can be decoded and executed.
Decode the instruction
The instruction decoder interprets and implements the instruction.
Fetch data from main memory
Read the effective address from main memory if the instruction has an indirect address. Fetch required data from main memory to be processed and placed into registers.
Execute the instruction
From the instruction register, the data forming the instruction is decoded by the control unit. It then passes the decoded information as a sequence of control signals to the relevant function units of the CPU to perform the actions required by the instruction such as reading values from registers, passing them to the Arithmetic logic unit (ALU) to calculate the result and writing the result back to a register. A condition signal is sent back to the control unit by the ALU if it is involved.
Store results
The result generated by the operation is stored in the main memory, or sent to an output device. Based on the condition feedback from the ALU, the PC is either incremented to address the next instruction or updated to a different address where the next instruction will be fetched. The cycle is then repeated.
The Central Processing Unit (CPU)
You may call CPU as the brain of any computer system. It is the brain that takes all major decisions, makes all sorts of calculations and directs different parts of the computer functions by activating and controlling the operations.CPU has four key parts
- Control Unit
- Arithmetic & Logic Unit
- Registers
- Clock
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Arithmetic Logic Units (ALU)
The ALU, as its name implies, is that portion of the CPU hardware which performs the arithmetic and logical operations on the binary data .The ALU contains an Adder which is capable of combining the contents of two registers in accordance with the logic of binary arithmetic.
Control Unit
The control unit, which extracts instructions from memory and decodes and executes them, calling on the ALU when necessary.
Registers
Registers are temporary storage units within the CPU. Some registers, such as the program counter and instruction register, have dedicated uses. Other registers, such as the accumulator, are for more general purpose use.
Clock
A circuit in a processor that generates a regular sequence of electronic pulses used to synchronize operations of the processor's components. The time between pulses is the cycle time and the number of pulses per second is the clock rate (or frequency). The execution times of instructions on a computer are usually measured by a number of clock cycles rather than seconds. The higher clock rate, the quicker speed of instruction processing. The clock rate for a Pentium 4 processor is about 2.0, 2.2 GHz or higher
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