Download Chapter 2: Constants, variables and data types

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Chapter 2: Systems architecture
Von Neumann architecture describes
a computer architecture in which:
• the data and the program are both stored
in the computer’s memory in the same
place
• all instructions and data will be stored in
the same place as binary numbers.
This means data and instructions are
indistinguishable from each other in memory.
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The central processing unit (CPU)
• The CPU carries out all the processing for
the computer system by following all the
instructions given to it.
• A CPU is made from billions of transistors
(electronic switches).
• The transistors combine to build logic
gates to process the data and instructions.
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Processors work at incredible speeds.
• A typical processor might work at 4GHz.
• That’s four billion instructions per
second.
• The speed is determined by a vibrating
crystal.
• The crystal maintains a constant speed.
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The processor is continuously fetching,
decoding and executing instructions.
The fetch–decode–execute cycle:
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• When a computer is switched on, the CPU
looks at a specific place in ROM.
• ROM contains the first program to load
and execute: the boot program.
• This is always stored in ROM in a specific
place so that the CPU knows where to find
it when the computer starts.
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• The boot program gets the computer up
and running and the operating system
started.
• After the operating system is started the
boot program hands control over to the
operating system.
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A simplified processor:
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• Control unit: decodes instructions and
sends signals to control how data moves
around the CPU and memory
• ALU: carries out calculations and logical
decisions, e.g. addition, subtraction and
comparison
• Registers: memory locations within the
CPU; they can be accessed very quickly
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• PC (program counter): holds the address
of the next instruction to be carried out
• MDR (memory data register): holds data
being moved in or out of the CPU
• MAR (memory address register): holds
the address of data to be fetched or where
data is to stored
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• CIR (current instruction register): holds
the most recently fetched instruction
waiting to be decoded and executed
• ACC (accumulator): stores the results of
any calculations made by the ALU
• Address bus: carries memory addresses
for locations to be read from or written to
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• Control bus: carries control signals
around the CPU and memory
• Data bus: carries data between the CPU
and memory
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Watch how the instruction to load the data in
memory location 6 to the accumulator works
in the Chapter 2 animation: CPU, buses and
memory.