Central Processing Unit
The CPU
Information :
Is responsible for executing the instructions given to it in a program and to process data
and instructions . It controls the rest of the computer system .
It follows the instructions in order to do something useful .
The microprocessor relies on other devices :
- To allow users to input the instructions
- To store the instructions
- To transfer the instructions to it so that it can carry them out
- To carry out the commands it issues : e.g. to print an essay or display an image
Purpose :
The CPU is made up of three main components : The Control Unit ; controls the flow of data
within a system through fetching, decoding and executing . It organises the execution of
instructions and coordinates all the CPU’s actions . The Immediate Access Store ; where the
CPU holds all the data and programs it is currently using and The Arithmetic and Logic Unit ;
where the CPU performs operations involving arithmetic ( 1 + 2 = 3 ) and logic including AND
OR and NOT ( 2 > 1 or == ) as well as acting as a getaway between primary memory and
secondary storage - non volatile memory external to the CPU and used for long term
storage of programs and data.
The CPU also contains Registers ; small amounts of high speed memory - 32 or 64 bits -
used to store small amounts of data such as memory addresses, instructions and data.
There are general purpose registers and special - purpose registers for specific purposes ,
Cache ; small amounts of random access memory ( RAM ) built directly within the processor
used to temporarily hold data. When running programs, a computer system constantly
swaps data in and out of cache memory and RAM. When an instruction ( or set of ) are
frequently used, it is inefficient for the processor to keep fetching these from RAM. To
overcome this, cache memory is used . There are three levels of cache, the fastest and
smallest is level one, the slowest and biggest is level three, see image below . All three cache
, levels are faster than RAM . In a multi-core process each core has its own L1 and L2 caches
while the last level cache is usually shared by all the cores.
These usually include a program counter, memory address register which holds current data
( MAR ) , memory data register and temporarily holds data needed next ( MDR ) , current
instruction register holding the current instruction itself ( CIR ) and an accumulator which
stores the ALU’s results ( ACC ) . The program counter ( PC ) keeps track of where we are .
Buses ; high speed internal connection used to send control signals through address buses -
carries from the processor to other components, data buses and control buses. A Clock is
used to issue a control signal that continually switches between a low (0) and a high (1)
state to coordinate all of the computer's components and synchronise them like a conductor
of an orchestra ( the speed of a clock is measured in giga-hertz ) .
Embedded System :
A computer system that uses programmed processors embedded onto a single circuit
board. Often used as part of a larger device. Designed to carry out a small number of
specific tasks very efficiently. Embedded systems are used to control the function of
electronic devices such as those commonly found in the home. They often don’t need a full
operating system because they perform limited and very specific tasks. Their input is often
controlled by a button or switch. Must be reliable because they cannot be modified once
manufactured. The program that controls them is held in Read Only Memory ( ROM ) in
primary memory . Examples of embedded systems include washing machines, pacemakers and
car systems. Characteristics include a CPU, fixed amounts of ROM, and a microcontroller.
The CPU
Information :
Is responsible for executing the instructions given to it in a program and to process data
and instructions . It controls the rest of the computer system .
It follows the instructions in order to do something useful .
The microprocessor relies on other devices :
- To allow users to input the instructions
- To store the instructions
- To transfer the instructions to it so that it can carry them out
- To carry out the commands it issues : e.g. to print an essay or display an image
Purpose :
The CPU is made up of three main components : The Control Unit ; controls the flow of data
within a system through fetching, decoding and executing . It organises the execution of
instructions and coordinates all the CPU’s actions . The Immediate Access Store ; where the
CPU holds all the data and programs it is currently using and The Arithmetic and Logic Unit ;
where the CPU performs operations involving arithmetic ( 1 + 2 = 3 ) and logic including AND
OR and NOT ( 2 > 1 or == ) as well as acting as a getaway between primary memory and
secondary storage - non volatile memory external to the CPU and used for long term
storage of programs and data.
The CPU also contains Registers ; small amounts of high speed memory - 32 or 64 bits -
used to store small amounts of data such as memory addresses, instructions and data.
There are general purpose registers and special - purpose registers for specific purposes ,
Cache ; small amounts of random access memory ( RAM ) built directly within the processor
used to temporarily hold data. When running programs, a computer system constantly
swaps data in and out of cache memory and RAM. When an instruction ( or set of ) are
frequently used, it is inefficient for the processor to keep fetching these from RAM. To
overcome this, cache memory is used . There are three levels of cache, the fastest and
smallest is level one, the slowest and biggest is level three, see image below . All three cache
, levels are faster than RAM . In a multi-core process each core has its own L1 and L2 caches
while the last level cache is usually shared by all the cores.
These usually include a program counter, memory address register which holds current data
( MAR ) , memory data register and temporarily holds data needed next ( MDR ) , current
instruction register holding the current instruction itself ( CIR ) and an accumulator which
stores the ALU’s results ( ACC ) . The program counter ( PC ) keeps track of where we are .
Buses ; high speed internal connection used to send control signals through address buses -
carries from the processor to other components, data buses and control buses. A Clock is
used to issue a control signal that continually switches between a low (0) and a high (1)
state to coordinate all of the computer's components and synchronise them like a conductor
of an orchestra ( the speed of a clock is measured in giga-hertz ) .
Embedded System :
A computer system that uses programmed processors embedded onto a single circuit
board. Often used as part of a larger device. Designed to carry out a small number of
specific tasks very efficiently. Embedded systems are used to control the function of
electronic devices such as those commonly found in the home. They often don’t need a full
operating system because they perform limited and very specific tasks. Their input is often
controlled by a button or switch. Must be reliable because they cannot be modified once
manufactured. The program that controls them is held in Read Only Memory ( ROM ) in
primary memory . Examples of embedded systems include washing machines, pacemakers and
car systems. Characteristics include a CPU, fixed amounts of ROM, and a microcontroller.
