Matilda Baines GCSE Computer Science Year11
GCSE COMPUTER
SCIENCE REVISION
Paper 1 - Theory
,Matilda Baines GCSE Computer Science Year 11
System Architecture
This is called Von
Neumann architecture
where the program
instructions and the data
the programs are using
are both stored in the
same memory.
Central Processing Unit (CPU)
The Central Processing Unit or CPU is arguably the most important component of a
computer. Its main process is to do the FDE cycle (Fetch, Decode, Execute) and this runs the
instructions given to the computer. It also co-ordinates and manages hardware resources.
The Central Processing Unit has three main components to it in which allow the cycle to
occur properly. These include the:
o Control Unit (CU) o Arithmetic Logic o Registers (+buses)
Unit (ALU)
Control Unit (CU)
The control unit coordinates and controls all the activities taking place within the CPU:
It decodes instruction and executes them.
It ensures the execution of instructions in the correct sequence.
It receives signals from the system clock.
It sends and receives control signals to and from other components.
Makes sure that data goes to the correct place at the correct time.
It directs the timing and control of other parts of the CPU.
Within the control unit, it contains the system clock.
The clock signal is used to synchronise the operations
of the processor components. It generates regular
clock pulses by emitting a signal that continuously
switches between a low (0) and a high (1) state.
The time taken between two sequential rising edges is
called a clock cycle or period. The clock speed is measured
by the number of clock cycles in a second.
1 clock cycle per second = 1 Hertz.
, Matilda Baines GCSE Computer Science Year 11
Arithmetic Logic Unit (ALU)
The Arithmetic Logic Unit is where the actual arithmetic and logical operations are done. For
the arithmetic this includes addition, subtraction, multiplication and division. For the logical,
whether something is bigger or smaller etc. But the ALU contains the accumulator which is
one of the registers.
Registers
There are 5 main registers within the CPU:
Program Counter (PC) – Holds the location of the next instruction/data address in
main memory.
Memory Address Register (MAR) –
Holds the memory address of the
current instruction about to be
executed.
Memory Data Register (MDR) –
Stores the data to be sent/received
from RAM.
Accumulator (ACC) – Holds the
result of an instruction before it is
transferred to memory.
Current Instruction Register (CIR) –
Holds the current instruction that the processor is executing.
Fetch – Decode – Execute Cycle (FDE)
The CPU operates by repeating three operations: fetch, decode and
execute.
1) The Program Counter (PC) contains the address of the memory
location that has the next instruction which must be fetched.
2) This address is then copied to the Memory Address Register
(MAR). The Program Counter increments by 1 so it can do the
next instruction.
3) The contents at the MAR are copied to the RAM so that it can
find the value in this address which is then copied to the Memory Data Register
(MDR).
4) The contents in the MDR are then copied to the Current Instruction Register (CIR)
5) The Control Unit (CU) decodes and executes the instruction in the CIR and is sent to
the ALU if a calculation is being done.
GCSE COMPUTER
SCIENCE REVISION
Paper 1 - Theory
,Matilda Baines GCSE Computer Science Year 11
System Architecture
This is called Von
Neumann architecture
where the program
instructions and the data
the programs are using
are both stored in the
same memory.
Central Processing Unit (CPU)
The Central Processing Unit or CPU is arguably the most important component of a
computer. Its main process is to do the FDE cycle (Fetch, Decode, Execute) and this runs the
instructions given to the computer. It also co-ordinates and manages hardware resources.
The Central Processing Unit has three main components to it in which allow the cycle to
occur properly. These include the:
o Control Unit (CU) o Arithmetic Logic o Registers (+buses)
Unit (ALU)
Control Unit (CU)
The control unit coordinates and controls all the activities taking place within the CPU:
It decodes instruction and executes them.
It ensures the execution of instructions in the correct sequence.
It receives signals from the system clock.
It sends and receives control signals to and from other components.
Makes sure that data goes to the correct place at the correct time.
It directs the timing and control of other parts of the CPU.
Within the control unit, it contains the system clock.
The clock signal is used to synchronise the operations
of the processor components. It generates regular
clock pulses by emitting a signal that continuously
switches between a low (0) and a high (1) state.
The time taken between two sequential rising edges is
called a clock cycle or period. The clock speed is measured
by the number of clock cycles in a second.
1 clock cycle per second = 1 Hertz.
, Matilda Baines GCSE Computer Science Year 11
Arithmetic Logic Unit (ALU)
The Arithmetic Logic Unit is where the actual arithmetic and logical operations are done. For
the arithmetic this includes addition, subtraction, multiplication and division. For the logical,
whether something is bigger or smaller etc. But the ALU contains the accumulator which is
one of the registers.
Registers
There are 5 main registers within the CPU:
Program Counter (PC) – Holds the location of the next instruction/data address in
main memory.
Memory Address Register (MAR) –
Holds the memory address of the
current instruction about to be
executed.
Memory Data Register (MDR) –
Stores the data to be sent/received
from RAM.
Accumulator (ACC) – Holds the
result of an instruction before it is
transferred to memory.
Current Instruction Register (CIR) –
Holds the current instruction that the processor is executing.
Fetch – Decode – Execute Cycle (FDE)
The CPU operates by repeating three operations: fetch, decode and
execute.
1) The Program Counter (PC) contains the address of the memory
location that has the next instruction which must be fetched.
2) This address is then copied to the Memory Address Register
(MAR). The Program Counter increments by 1 so it can do the
next instruction.
3) The contents at the MAR are copied to the RAM so that it can
find the value in this address which is then copied to the Memory Data Register
(MDR).
4) The contents in the MDR are then copied to the Current Instruction Register (CIR)
5) The Control Unit (CU) decodes and executes the instruction in the CIR and is sent to
the ALU if a calculation is being done.
