The Stored Program Concept and Von Neumann Architecture

The stored program concept, developed by John von Neumann in the 1940s, states that both program instructions and data are stored together in the same memory (RAM) in binary form. The CPU fetches each instruction from memory, decodes it, and executes it sequentially. This is the foundation of virtually all modern computers.

The key insight of Von Neumann architecture is that because instructions are stored as binary data, the same memory can hold both a program and the data it operates on. This allows programs to be loaded and changed without altering the hardware — unlike earlier machines where programs were hardwired.

Components of the CPU

Control Unit (CU)

The Control Unit is the component that coordinates all CPU operations. It does not perform calculations itself, but instead:

• Fetches instructions from memory
• Decodes (interprets) each instruction to determine what must be done
• Sends control signals to other components to carry out the instruction
• Controls the timing and sequencing of all operations via the control bus

Arithmetic Logic Unit (ALU)

The ALU performs all computational operations on data. At A Level, you need to know the ALU handles three types of operation:

• Arithmetic operations: addition, subtraction, multiplication, division
• Logical operations: AND, OR, NOT, XOR (bitwise)
• Shift operations: logical left/right shift and arithmetic shift of binary values

Results of ALU operations are stored temporarily in the Accumulator register.

Registers

Registers are extremely fast, small storage locations inside the CPU. They hold values that are currently being processed. Accessing a register is far faster than accessing main memory (RAM), which is why registers are used for active data during processing.

The Status Register (A Level specific)

The status register contains individual bits called flags that record the outcome of the last arithmetic or logical operation:

• Carry flag (C): set if the result produced a carry out of the most significant bit
• Zero flag (Z): set if the result is zero
• Negative flag (N): set if the result is negative (MSB = 1)
• Overflow flag (V): set if signed arithmetic produced an overflow

The status register is used by conditional branch instructions — the CPU checks flags to decide whether to jump to a different part of the program.

General Purpose Registers (A Level specific)

Modern processors include multiple general purpose registers (e.g., R0–R15 in ARM). These hold operands and intermediate results during computation, avoiding the need to constantly read from and write to slower RAM. More general purpose registers generally mean fewer memory accesses and faster execution.

The Three Buses

Buses are sets of parallel wires that transfer data between CPU components and main memory:

• Address bus: carries memory addresses from CPU to RAM (one-directional, CPU → memory)
• Data bus: carries data or instructions between CPU and memory (bidirectional)
• Control bus: carries control signals (e.g., read/write signals) from the Control Unit (bidirectional)

The width of the address bus determines the maximum addressable memory (2ⁿ locations for an n-bit bus). The width of the data bus determines how many bits are transferred per clock cycle.