1. Chapter 4: Representing instructions
OCR GCSE Computing
Chapter 4: Representing instructions
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2. Chapter 4: Instructions
The instruction set of a CPU
Each processor type has a limited range of instructions that it can carry out.
This is called its instruction set.
Instructions include:
changing data;
moving data;
controlling program flow.
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3. Chapter 4: Instructions
Arithmetic operations:
plus, minus, multiply, divide.
Logical operations:
comparisons;
AND, OR, NOT – these are bitwise operators;
they change the bits in a byte.
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4. Chapter 4: Instructions
When a computer is instructed to run a program it is directed to the start address for these data and instructions.
The CPU fetches the first instruction from this start location and decodes it to find out what to do next.
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5. Chapter 4: Instructions
A program instruction has two parts – the instruction part or operator, and a data part or operand. 1 1 1
Operator
ADD to the accumulator
Operand
The contents of memory location 1101
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6. Chapter 4: Instructions
This is a part of memory in a PC.
It shows some instructions, both in hexadecimal notation and also unassembled, so you can see the assembly language equivalent.
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7. Chapter 4: Instructions
How does a computer tell the difference between program instructions and data?
It all depends on where the data is in memory.
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