Which component carries out calculations and logical comparisons?

the arithmetic logic unit. ALU: arithmetic and logic.

What does the program counter hold?

the address of the next instruction to be fetched. It is increased by one each time an instruction is fetched.

Which register holds the result of a calculation?

ACC. The accumulator stores the ALU's results.

Which bus carries data in both directions?

the data bus. Data is read from memory and written to it.

What is meant by Von Neumann architecture?

a design in which instructions and data are stored in the same memory. This is the stored program idea.

What is an instruction set?

the list of all the commands a CPU can process. The commands are in machine code.

The CPU and Von Neumann architecture · Cambridge IGCSE Computer Science

In plain words

The central processing unit is the part of a computer that actually does things. It takes in instructions and data, processes them, and produces results.

In the Von Neumann design, used by nearly every computer, the instructions and the data live together in the same memory. The CPU fetches them one at a time using a set of small, very fast stores called registers, and three sets of wires called buses.

7 things to know

The CPU processes the instructions and data that are input into the computer, so that the result can be output. A microprocessor is a type of integrated circuit: a CPU on a single chip.
The arithmetic logic unit (ALU) carries out calculations and logical comparisons. The control unit (CU) decodes instructions and sends control signals to manage the other components.
The program counter (PC) holds the address of the next instruction to be fetched.
The memory address register (MAR) holds the address in memory that is about to be read from or written to. The memory data register (MDR) holds the data or instruction that has just been fetched, or is about to be written.
The current instruction register (CIR) holds the instruction that is being decoded and executed. The accumulator (ACC) holds the result of a calculation in the ALU.
The address bus carries addresses from the CPU to memory. The data bus carries data and instructions between the CPU and memory, in both directions. The control bus carries control signals.
The instruction set is the list of all the commands, in machine code, that a CPU can process.

Worked example

Which register would hold each of these: the address of the next instruction, the result of an addition, and the instruction being carried out now?

The address of the next instruction is in the program counter (PC).
The result of an addition done by the ALU is put in the accumulator (ACC).
The instruction being carried out is in the current instruction register (CIR).

Tips and tricks

"Address" in the name means it holds an address: the MAR. "Data" means it holds what was found there: the MDR. Do not swap them.
The PC holds the address of the next instruction, not the instruction itself.

6 questions, about 2 minutes.

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The CPU and Von Neumann architecture: 6 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

Which component carries out calculations and logical comparisons?
- the control unit
- the arithmetic logic unit (the answer)
- the program counter
- the address bus
ALU: arithmetic and logic.
What does the program counter hold?
- the result of the last calculation
- the instruction being executed
- the address of the next instruction to be fetched (the answer)
- the number of programs running
It is increased by one each time an instruction is fetched.
Which register holds the result of a calculation?
- MAR
- CIR
- PC
- ACC (the answer)
The accumulator stores the ALU's results.
Which bus carries data in both directions?
- the address bus
- the data bus (the answer)
- the control bus
- none of them
Data is read from memory and written to it.
What is meant by Von Neumann architecture?
- a computer with no memory
- a design in which instructions and data are stored in the same memory (the answer)
- a computer with two processors
- a design that needs no registers
This is the stored program idea.
What is an instruction set?
- the programs installed on a computer
- the list of all the commands a CPU can process (the answer)
- a set of user manuals
- the keys on a keyboard
The commands are in machine code.

6 questions

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Worksheet

2 questions, 5 marks. Write your answers on paper, then check them.

The CPU and Von Neumann architecture

Cambridge IGCSE Computer Science 0478 · 5 marks · papermunch.org

Name ______________________________ Date ______________

Describe the role of the control unit.[2]
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It decodes instructions and sends control signals to the other components to manage what they do and when.
I got this one
State the purpose of the address bus, the data bus and the control bus.[3]
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The address bus carries the address of a memory location from the CPU to memory. The data bus carries data and instructions between the CPU and memory. The control bus carries control signals from the control unit.
I got this one

Answers: The CPU and Von Neumann architecture

1. It decodes instructions and sends control signals to the other components to manage what they do and when.
2. The address bus carries the address of a memory location from the CPU to memory. The data bus carries data and instructions between the CPU and memory. The control bus carries control signals from the control unit.