Page 16 - Computer Science V2.0 Class 11

Central Processing
Unit

Input Output
Device Device

Memory Unit

Fig 1.1: Components of a computer system (Von Neumann Architecture)
The input unit, Central Processing Unit (CPU), memory unit, and output unit constitute the hardware of a computer.
A sequence of instructions given to a computer is called a program. A computer manufacturer provides several useful
programs that help the end user to use the computer conveniently. A set of programs that performs a useful task
is called software. In contrast to the computer hardware, the software is not visible to the human eye. A computer,
having several input-output devices, is also called a computer system.
Fig 1.2 presents the IPO cycle ("Input > Process > Output" cycle), which represents the sequential stages through which
data or actions flow in a system where initial data (input) is transformed (process) to produce a result (output), which
can then influence subsequent actions (input).

INPUT

OUTPUT PROCESS

Fig 1.2: IPO Cycle

Electronic Numerical Integrator and Computer (ENIAC) was the first electronic programmable computer.

1.1 CPU
The Central Processing Unit (CPU), also called the processor, is the computer's brain and is responsible for carrying
out all the processing in the computer system. A CPU is often fabricated as a semiconductor chip, called Integrated
Circuit(IC). Given a task, the CPU is provided with a sequence of instructions in the form of a program. For the execution
of an instruction in memory, it goes through an instruction cycle which first fetches the instruction and the data
(if required) from memory, decodes the instruction (what action to perform), and performs the action. The result of
the computation may be stored again in the memory. Thus, the CPU controls the entire data flow and instructions
inside the computer. To facilitate its functioning, the CPU comprises of the following components (Fig 1.3):
Registers: While processing an instruction, the CPU needs to store in its local storage the instruction, memory addresses
being referred during the instruction execution, data operands (on which operation is to be carried out), and the

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