Von Neumann · ALU · CU · Registers · Buses · Fetch-Decode-Execute
CSZoneEdexcel GCSE Computer Science 1CP2
Von Neumann Architecture
The Stored Program Concept
The Von Neumann architecture is the basis of modern computers. It stores both data and instructions in the same memory. The CPU fetches, decodes, and executes each instruction in turn.
ALU (Arithmetic Logic Unit): performs calculations and logical comparisons
CU (Control Unit): directs operations — fetches instructions and controls data flow
Registers: tiny, ultra-fast storage inside the CPU for current data and instructions
Key Registers
CPU Temporary Storage
PC (Program Counter): holds the address of the NEXT instruction to fetch
MAR (Memory Address Register): holds the address of memory being read/written
MDR (Memory Data Register): holds data that has been fetched or is to be written to memory
ACC (Accumulator): stores the result of ALU calculations
CIR (Current Instruction Register): holds the instruction currently being decoded/executed
Fetch–Decode–Execute Cycle
How the CPU Runs a Program
Fetch: PC → MAR; instruction fetched from RAM via data bus into MDR; MDR → CIR; PC incremented
Decode: CU decodes the instruction in the CIR — identifies the operation and operands
Execute: ALU or CU carries out the instruction; result stored in ACC or memory
Exam Practice
Have a go at this question
Edexcel-style question
Describe the role of the Program Counter (PC) in the Fetch–Decode–Execute cycle.
2 marks
The Program Counter stores the memory address of the next instruction to be fetched [1]. After each fetch, the PC is incremented to point to the following instruction [1].
Key Takeaways
What to Remember
Von Neumann: data and instructions stored in same memory
ALU: calculations; CU: controls; Registers: fast temporary storage
PC → MAR → MDR → CIR: the fetch path; PC increments each cycle
Fetch → Decode → Execute repeats continuously while the program runs