Unit 7 - Fundamentals of computer organisation and architecture
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Summary Computer Systems & Architecture
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Unit 7 - Fundamentals of computer organisation and architecture
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AQA
Notes for the computer systems and architecture module of AQA A Level Computer Science. Information is summarised into bullet points and can be easily turned into flashcards. Condense notes with diagrams
Unit 7 - Fundamentals of computer organisation and architecture
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Computer systems and
architecture
Internal hardware components of a computer
There are two types of main memory: includes RAM and ROM.
o ROM stores program instructions and frequently used data.
o Main memory is much faster than secondary storage; so
storing frequently used data in main memory helps the
processor execute instructions faster.
Bus: A series of parallel wires that connects internal components of
a computer system, allowing signals to be passed between them.
o The number of parallel wires in a bus is called the width and it
has a direct relationship to the number of bits that can be
transferred simultaneously by the bus.
o There are 3 buses (that we need to know about):
Address bus
Data bus
Control bus
Address bus: Used to transport memory addresses – specifying
where data is to be sent or retrieved.
o Increasing the width of the bus increases the range of
addresses that it can specify – which therefore increases the
computer’s store of addressable memory. Adding a single
wire will double the number of addressable memory locations.
o Addressable memory location: a portion of memory that
can be accessed by its address.
Number of parallel wires in Number of addressable
address bus memory locationW
1 21 = 2
2 22 = 4
3 23 = 8
Data bus: Sends data and instructions to and from the different
components of the computer system.
o Increasing the width of the data bus increases the volume of
data that can be transferred over the bus at any one time.
Control bus: used to carry control signals that regulate the
operation of the system.
o Carries the computer’s clock signal.
, I/O controllers: (Input/Output controllers) pieces of hardware that
control the communication of data between the processor and
external hardware devices.
Von Neumann and Harvard architectures
Two different ways of setting up a processor’s access to main
memory.
Harvard architecture
The processor will use separate main memory locations for
instruction and data.
This allows the architecture to take advantage of giving each piece
of main memory different characteristics e.g. memory used for
instructions could be read-only.
Harvard architecture is extensively used in embedded systems,
such as digital signal processing.
Von Neumann architecture
Both instructions and data are stored in the same memory.
Systems that are based on von Neumann architecture usually
perform worse; the same buses have to be shared for fetching
both instructions and data.
Von Neumann is used in everyday general-purpose computers – like
laptops.
The stored program concept
A computer that uses the stored program concept is defined as:
“serially fetching and executing machine code instructions
stored in main memory by a processor that performs
arithmetic and logical operations”.
o Serially: Instructions are fetched and executed in order – first
instruction is fetched and executed before the second
instruction is fetched.
o Fetching: Retrieving an instruction from main memory.
o Executing: Carrying out the specified instruction that was
set.
o Machine code instructions: Instructions formed from binary
that the computer can execute directly without translation.
o Main memory: Where the computer stores instructions and
frequently used data.
o Arithmetic: Mathematical operations.
o Logical: Operations that involve the use of logic gates.
Some early computers were designed to execute just one specific
instruction; the stored program concept allows one set of
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