Computer Science Paper 1 GCSE OCR - All Notes
1.1.1 - Architecture of the CPU
A computer is an electronic device that takes data, process it and then outputs it. A
computer system contains hardware and software that work together to process data
and complete tasks.
Hardware – This is the physical things that make up a computer
Examples: Motherboard, Printer, CPU and Monitor
Software- These are the programs and applications that a computer system
runs
Examples: Video game or Operating system.
The CPU
The CPU is the Central Processing Unit and processes all of the data and
instructions that make a system work.
The CPU contains 3 main parts:
The
control unit (CU) sends signals to control the flow of data inside the CPU and
outside the CPU. Its main job is to manage the fetching, decoding and execution of
instructions.
The arithmetic logic unit (ALU) is responsible for carrying out all arithmetic
calculations such as addition, subtraction and multiplication. It also performs logical
operations such as ‘AND’, ‘OR’ AND ‘NOT’ and binary shifts
The cache is a small amount of very fast memory situated in the CPU. It stores
frequently used instructions and data so that the CPU can access them quickly if
they are needed. Cache has a very low capacity and is usually more expensive than
RAM and secondary storage.
It also contains various registers that temporarily hold data that is needed by the
CPU. They are super-fast to read and write and all have specific functions.
Computer Science Paper 1 GCSE OCR - All Notes 1
, Accumulator- This register is stored in the ALU and stores all the results of
calculations from the ALU.
Memory Address Register (MAR) – Holds the address of where data is to be
fetched or stored.
Memory Data Register (MDR) – Holds the actual data or instruction that may
have been fetched or be waiting to be written to memory.
Program Counter (PC) – Holds the memory address of the instruction.
Von-Neumann Architecture
John Von Neumann was the first person to design computers that had stored programs. Prior to this, computers had fixed
problems, they could only be used for one purpose.
The Von Neumann designed revolutionised computing. The Von Neumann architecture describes a system where the CPU
runs programs stored in memory.
The programs consists of instructions and data which are stored in memory addresses.
A Von Neumann architecture will contain:
CPU
Arithmetic logic unit
Control unit
Cache
Registers
The Fetch-Decode-Execute Cycle
The fetch-decode-execute cycle describes how the CPU carries out all the
instructions.
Fetch:
1. The memory address is copied from the Program counter to the MAR.
2. MAR will send a request to RAM to fetch the data from a specific data slot. RAM
receives the signal and sends a signal so that the data is sent to the MDR.
3. The Program counter is increased to point to the address of the next instruction so
that is ready for the next cycle.
Decode:
The MDR sends the data to a register called the Current Instruction register (CIR).
The CIR looks up the sequence in the CPU's instruction set to see what it should do.
Execute:
The instruction has been decoded so the CPU knows what to do with it. The
instruction is performed.
If the instruction contained a number it would be sent to the ALU.
When the decoded instruction has been executed the CIR sends a signal to the
Control Unit so that the next FDE cycle can begin
Computer Science Paper 1 GCSE OCR - All Notes 2
, 1.1.2 - CPU Performance
A variety of things can affect the speed and performance of the CPU. The main ones are:
Clock Speed
Cache size
Number of cores
RAM
GPU
Clock Speed
Clock speed is the number of instructions a single processor can carry out per second. It is measured in hertz (hz).
If the clock speed is higher, a higher number of instructions can be carried out per second as the CPU is executing more
FDE cycles per second. This will lead to a faster and smoother response from the computer and led to programs being
executed at a faster rate.
Cache size
Cache is a small amount of very fast memory situated in the CPU. It stores frequently used instructions and data so that
the CPU can access them quickly if they are needed.
A larger cache size means that a higher number of frequently used instructions can be stored in the cache. This means the
processor does not have to keep performing the FDE cycle for instructions that are used regularly. Therefore, the CPU has
faster access to more data which will lead to a faster, smoother and more efficient performance.
However, too much cache can be detrimental as it could take too long to find instructions in the cache therefore defeating
the purpose of it.
Number of Cores
Each core in a CPU processes data independently.
A higher number of cores means that FDE cycles can be performed simultaneously so instruction sets can be performed
faster. This leads to a faster and smoother performance overall.
Random Access Memory - RAM
More RAM means that more applications can smoothly run making the overall performance faster.
If a computer has too little RAM it could run down slowly due to the use of virtual memory.
However, if a computer already has enough RAM to run everything that the user wants, increasing the RAM will not affect
the speed of the computer.
Computer Science Paper 1 GCSE OCR - All Notes 3
, Graphics Processing Units - GPUs
GPUs are specialised to handle graphics and image processing. They relieve the processing load on the CPU enabling it to
do other things.
Computers normally have basic GPUs integrated into their motherboard or CPU but for better graphics performance these
can be replaced with a dedicated graphics card.
Using a dedicated GPU can greatly improve performance in applications that require graphics. Examples: PC gaming, Photo
editing
1.1.3 - Embedded Systems
Embedded systems are small computers built into a larger system, device or machine.
They are often used as control systems, they monitor and control machinery in order to reach a desired result.
Example: In a dishwasher, the embedded system could control the water pumps and water release mechanisms managing
the dishwasher cycles and making sure that the thermostat keeps the water at a suitable temperature.
They are usually dedicated to a single task so are easier to produce, cheaper and more efficient at doing their specified
task than a general purpose computer. They also tend to require less power and can be built using cheaper, less powerful
processors.
Examples:
Dishwasher
Washing machine
Microwaves
Central heating systems
Digital watches
Electronic calculators
GPS systems
Fitness trackers
1.2.1 - Primary Storage
Computer Science Paper 1 GCSE OCR - All Notes 4
1.1.1 - Architecture of the CPU
A computer is an electronic device that takes data, process it and then outputs it. A
computer system contains hardware and software that work together to process data
and complete tasks.
Hardware – This is the physical things that make up a computer
Examples: Motherboard, Printer, CPU and Monitor
Software- These are the programs and applications that a computer system
runs
Examples: Video game or Operating system.
The CPU
The CPU is the Central Processing Unit and processes all of the data and
instructions that make a system work.
The CPU contains 3 main parts:
The
control unit (CU) sends signals to control the flow of data inside the CPU and
outside the CPU. Its main job is to manage the fetching, decoding and execution of
instructions.
The arithmetic logic unit (ALU) is responsible for carrying out all arithmetic
calculations such as addition, subtraction and multiplication. It also performs logical
operations such as ‘AND’, ‘OR’ AND ‘NOT’ and binary shifts
The cache is a small amount of very fast memory situated in the CPU. It stores
frequently used instructions and data so that the CPU can access them quickly if
they are needed. Cache has a very low capacity and is usually more expensive than
RAM and secondary storage.
It also contains various registers that temporarily hold data that is needed by the
CPU. They are super-fast to read and write and all have specific functions.
Computer Science Paper 1 GCSE OCR - All Notes 1
, Accumulator- This register is stored in the ALU and stores all the results of
calculations from the ALU.
Memory Address Register (MAR) – Holds the address of where data is to be
fetched or stored.
Memory Data Register (MDR) – Holds the actual data or instruction that may
have been fetched or be waiting to be written to memory.
Program Counter (PC) – Holds the memory address of the instruction.
Von-Neumann Architecture
John Von Neumann was the first person to design computers that had stored programs. Prior to this, computers had fixed
problems, they could only be used for one purpose.
The Von Neumann designed revolutionised computing. The Von Neumann architecture describes a system where the CPU
runs programs stored in memory.
The programs consists of instructions and data which are stored in memory addresses.
A Von Neumann architecture will contain:
CPU
Arithmetic logic unit
Control unit
Cache
Registers
The Fetch-Decode-Execute Cycle
The fetch-decode-execute cycle describes how the CPU carries out all the
instructions.
Fetch:
1. The memory address is copied from the Program counter to the MAR.
2. MAR will send a request to RAM to fetch the data from a specific data slot. RAM
receives the signal and sends a signal so that the data is sent to the MDR.
3. The Program counter is increased to point to the address of the next instruction so
that is ready for the next cycle.
Decode:
The MDR sends the data to a register called the Current Instruction register (CIR).
The CIR looks up the sequence in the CPU's instruction set to see what it should do.
Execute:
The instruction has been decoded so the CPU knows what to do with it. The
instruction is performed.
If the instruction contained a number it would be sent to the ALU.
When the decoded instruction has been executed the CIR sends a signal to the
Control Unit so that the next FDE cycle can begin
Computer Science Paper 1 GCSE OCR - All Notes 2
, 1.1.2 - CPU Performance
A variety of things can affect the speed and performance of the CPU. The main ones are:
Clock Speed
Cache size
Number of cores
RAM
GPU
Clock Speed
Clock speed is the number of instructions a single processor can carry out per second. It is measured in hertz (hz).
If the clock speed is higher, a higher number of instructions can be carried out per second as the CPU is executing more
FDE cycles per second. This will lead to a faster and smoother response from the computer and led to programs being
executed at a faster rate.
Cache size
Cache is a small amount of very fast memory situated in the CPU. It stores frequently used instructions and data so that
the CPU can access them quickly if they are needed.
A larger cache size means that a higher number of frequently used instructions can be stored in the cache. This means the
processor does not have to keep performing the FDE cycle for instructions that are used regularly. Therefore, the CPU has
faster access to more data which will lead to a faster, smoother and more efficient performance.
However, too much cache can be detrimental as it could take too long to find instructions in the cache therefore defeating
the purpose of it.
Number of Cores
Each core in a CPU processes data independently.
A higher number of cores means that FDE cycles can be performed simultaneously so instruction sets can be performed
faster. This leads to a faster and smoother performance overall.
Random Access Memory - RAM
More RAM means that more applications can smoothly run making the overall performance faster.
If a computer has too little RAM it could run down slowly due to the use of virtual memory.
However, if a computer already has enough RAM to run everything that the user wants, increasing the RAM will not affect
the speed of the computer.
Computer Science Paper 1 GCSE OCR - All Notes 3
, Graphics Processing Units - GPUs
GPUs are specialised to handle graphics and image processing. They relieve the processing load on the CPU enabling it to
do other things.
Computers normally have basic GPUs integrated into their motherboard or CPU but for better graphics performance these
can be replaced with a dedicated graphics card.
Using a dedicated GPU can greatly improve performance in applications that require graphics. Examples: PC gaming, Photo
editing
1.1.3 - Embedded Systems
Embedded systems are small computers built into a larger system, device or machine.
They are often used as control systems, they monitor and control machinery in order to reach a desired result.
Example: In a dishwasher, the embedded system could control the water pumps and water release mechanisms managing
the dishwasher cycles and making sure that the thermostat keeps the water at a suitable temperature.
They are usually dedicated to a single task so are easier to produce, cheaper and more efficient at doing their specified
task than a general purpose computer. They also tend to require less power and can be built using cheaper, less powerful
processors.
Examples:
Dishwasher
Washing machine
Microwaves
Central heating systems
Digital watches
Electronic calculators
GPS systems
Fitness trackers
1.2.1 - Primary Storage
Computer Science Paper 1 GCSE OCR - All Notes 4
