Computer Architecture

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The processor is the brain in the system. It is
responsible for executing programs and
supervising the functioning of the other
components Processor
The main memory is the simplest part of the computer. In a
The Processor
Von Neumann computer it is used as a store for program A computer system can be considered as
instructions and data. Main memory its available in one or Main Memory Three-Box Model: Von Neumann architecture consisting of three boxes Main Memory
more integrated circuits. Memory chips with the capacity to I/O
store 512kb are often use din PC systems. Each byte goes A computer device that is not part of the CPU is
into a separate area of the chip, known as a memory called a peripheral. The system bus is not
location connected directly to I/O devices, it is connected
to an I/O controller then the I/O controller is
Random Access memory is used for memory that More components
RAM Components Peripherals connected to the I/O device.
is readable and writeable. It is used by the
processor to store data and to read it back. The The processor exchanges data with a peripheral
contents of Ram are volatile. device through a part of an I/O controller called
an I/O port. It is a set of data, command and
Read Only Memory isn't volatile. Provides random status registers. Registers are storage locations
access like RAM but it can't be written to once it ROM
I/O Port that can be written to and read from.
is set up. It is used to hold fixed programs such
as the bootstrap program program. It is a permanent storage memory, not directly
Secondary Storage connected to the processor. Therefore secondary
Electrically erasable programmable read-only EEPROM storage is a type of peripheral. A magnetic hard
memory; its contents may be altered but writing disk is a typical example of it. It is connected to
is about 100 times slower. the processor through an I/O controller. Data is
transferred from the storage device, to the I/O
The boxes of the three-box model are connected controller and then across the system bus to the
by a bus called the system bus or external bus, main memory.
which acts as a connecting tube between the
three components. This connecting tube can System Buses The processor needs to be able to distinguish different parts of the main
transmit a single binary word between the memory that stores individual bytes, so each main memory location has a main
Computing: Computer Architecture
processor and the store or I/O component, and memory address, like a house address. It is a unique numeric code. When the
send an address to the store or I/O component. processor needs to select a main memory location, it puts the unique address
A bi-directional bus, typically consisting of 32
corresponding to this location on the address bus. The processor then asserts,
wires, used to transport data between the three Addressable memory
Data bus over the control bus, whether it wishes to read from this location or write to this
components of the three box model A computer bus is a set of parallel wires that
location. Finally, the processor uses the data bus to transfer a byte or bytes
connect independent components of a computer
A unidirectional bus, typically consisting of 32 between itself and the addressed memory location
system in order to pass signals between them.
wires, used to address memory and I/O locations. Address bus Some signals represent data, some, an address Proposed by John von neumann and Alan turning
and others control information. For this reason, in 1945
A bi-directional bus, typically consisting of eight
the system bus is split into three separate buses;
wires, used to transport control signals between Stored Program Concept A program must be in main memory to be
Control bus Characteristics of a processor
the three components of the three-box model executed
To be useful, the CPU needs to communicate
Clock signal, for timing purposes. with devices known as peripherals. The control Machine code instructions are fetched, one after another,
bus carries signals such as these; from main memory in sequence and are executed, one at
Reset signal, used to initialise components. a time in the processor.
Memory read, used to assert that the memory Von neumann and Harvard In the Von Neumann architecture, instruction
location currently in use is being read from.
fetches and data reference share the same bus,
Memory write, used to assert that the memory Microcontroller: It is a complete computer on a so they compete for resources. In a harvard
location currently in use is being written single chip. (processor, memory and I/O) architecture there is no competition, so
instruction fetched and reading and writing of
I/O: used to indicate that the processor wishes to
I/O data can take place in parallel. This increases the
use an I/O controller and not main memory when
overall processing speed.
I/O and main memory share memory addresses.

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