1. System Architecture 1
Chapter 2

2. OCR GCSE Computer Science
Chapter 2: Systems architecture
Von Neumann architecture describes a computer architecture in which:
the data and the program are both stored in the computer’s memory in the same place
all instructions and data will be stored in the same place as binary numbers.
This means data and instructions are indistinguishable from each other in memory.

3. OCR GCSE Computer Science
Chapter 2: Systems architecture
The central processing unit (CPU)
The CPU carries out all the processing for the computer system by following all the instructions given to it.
A CPU is made from billions of transistors (electronic switches). 
The transistors combine to build logic gates to process the data and instructions.

4. OCR GCSE Computer Science
Chapter 2: Systems architecture
Von Neumann architecture describes a computer architecture in which:
the data and the program are both stored in the computer’s memory in the same place
all instructions and data will be stored in the same place as binary numbers.
This means data and instructions are indistinguishable from each other in memory.

5. OCR GCSE Computer Science
Chapter 2: Systems architecture
The central processing unit (CPU)
The CPU carries out all the processing for the computer system by following all the instructions given to it.
A CPU is made from billions of transistors (electronic switches). 
The transistors combine to build logic gates to process the data and instructions.

6. OCR GCSE Computer Science
Chapter 2: Systems architecture
Processors work at incredible speeds.
A typical processor might work at 4GHz.
That’s four billion instructions per second.
The speed is determined by a vibrating crystal.
The crystal maintains a constant speed.

7. OCR GCSE Computer Science
Chapter 2: Systems architecture
The processor is continuously fetching, decoding and executing instructions. The fetch–decode–execute cycle:

8. OCR GCSE Computer Science
Chapter 2: Systems architecture
When a computer is switched on, the CPU looks at a specific place in ROM.
ROM contains the first program to load and execute: the boot program.
This is always stored in ROM in a specific place so that the CPU knows where to find it when the computer starts.

9. OCR GCSE Computer Science
Chapter 2: Systems architecture
The boot program gets the computer up and running and the operating system started.
After the operating system is started the boot program hands control over to the operating system.

10. OCR GCSE Computer Science
Chapter 2: Systems architecture
A simplified processor:

11. OCR GCSE Computer Science
Chapter 2: Systems architecture
Control unit: decodes instructions and sends signals to control how data moves around the CPU and memory
ALU: carries out calculations and logical decisions, e.g. addition, subtraction and comparison
Registers: memory locations within the CPU; they can be accessed very quickly

12. OCR GCSE Computer Science
Chapter 2: Systems architecture
PC (program counter): holds the address of the next instruction to be carried out
MDR (memory data register): holds data being moved in or out of the CPU
MAR (memory address register): holds the address of data to be fetched or where data is to stored

13. OCR GCSE Computer Science
Chapter 2: Systems architecture
CIR (current instruction register): holds the most recently fetched instruction waiting to be decoded and executed
ACC (accumulator): stores the results of any calculations made by the ALU
Address bus: carries memory addresses for locations to be read from or written to

14. OCR GCSE Computer Science
Chapter 2: Systems architecture
Control bus: carries control signals around the CPU and memory
Data bus: carries data between the CPU and memory

15. OCR GCSE Computer Science
Chapter 2: Systems architecture
Watch how the instruction to load the data in memory location 6 to the accumulator works in the Chapter 2 animation: CPU, buses and memory.