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Complete GCSE OCR Computer Science Notes
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Complete Notes for GCSE Computer Science Ideal for OCR Exam Board. 8000+ Words Each Unit (1-8) clearly labelled.
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COMPUTER SCIENCEGCSE OCR Computer Science Condensed Notes
,Table of Contents
1 Unit 1 – Computer Systems
4 Unit 2 – Networks
10 Unit 3 – System Software& Security
12 Unit 4 – Ethics
13 Unit 5 – Algorithms
16 Unit 6 – Programming
19 Unit 7 – Logic & Languages
22 Unit 8 – Data Representation
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, GCSE OCR Computer Science Condensed Notes Unit 1 – Computer Systems
Computer Components
Central Processing Unit
Before 1943, early computers stored the data to be worked on in memory, and programs were not stored –
instructions had to be input one at a time using switches or read in from paper tape.
− Computer programs and the data used are stored in the same memory;
- Data then moves between the memory unit and the processor. This is the Fetch-Decode-Execute
Cycle (FDE).
Components of the CPU
− Control Unit
- Coordinates all of the activities taking place within the CPU.
− Arithmetic Logic Unit (ALU)
- Component of the processor where all calculations take place.
− Registers
- Registers are very fast memory locations
- Program Counter (PC)
- Holds the address of the next instruction to be executed
- Memory Address Register (MAR)
- Holds the memory address of the current instruction, and then the data that it uses, so that
it can be fetched from the memory
- Memory Data Register (MDR)
- Holds the actual instruction and then the data that has been fetched from memory
- Accumulator
- Hold the result of an instruction before transferring it to memory.
Memory Address Register & Memory Data Register (MAR & MDR)
− In the FETCH stage of the FDE cycle, the PC copies the address of the instructions to be executed to the MAR.
− The instructions at that address are fetched and copied to the MDR.
− The Control Unit decodes the instruction and decides if data needs to be fetched.
− If data is required, the MAR is used to hold the address of data to be used in the instruction.
− Said data is fetched and copied to the MDR.
Figure 1 Figure 2 Figure 3
Fetch-Decode-Execute Central Processing Unit Input, Output & Components of the CPU
(FDR Cycle) Processes
1
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