OCR A Level Computer Science

1.1.1 Structure and Function of the Processor

The Arithmetic and Logic Unit (ALU), Control Unit, Registers (PC, ACC, MAR, MDR, CIR), buses, and how this relates to assembly language programs.

Overview of the CPU

The Central Processing Unit (CPU) is responsible for executing instructions, processing data, and controlling the operation of the computer.

The CPU follows the Von Neumann architecture, meaning instructions and data share the same memory and a single set of buses is used.

Arithmetic and Logic Unit (ALU)

The ALU performs all arithmetic operations → addition, subtraction, multiplication, division.

The ALU performs all logical operations → AND, OR, NOT, XOR, comparisons.

Results are stored in the Accumulator (ACC).

Internal Components of the ALU

Arithmetic circuit → performs calculations
Logic circuit → performs logical operations
Registers → temporary storage
Zero flag → result = 0
Overflow flag → result too large

Control Unit (CU)

The Control Unit manages the CPU and coordinates all operations.

Key Functions

Controls the Fetch-Decode-Execute (FDE) cycle
Decodes instructions
Sends control signals (e.g. read, write)
Manages the flow of data between components

Registers

Registers are very small, very fast memory locations inside the CPU.

They are used for temporary storage during processing.

They are faster than RAM and secondary storage.

Each register has a specific purpose.

Program Counter (PC)

Holds the address of the next instruction.

After fetching, it is incremented.

Accumulator (ACC)

Stores results of ALU operations.

Stores intermediate data.

Stores recently fetched data.

Memory Address Register (MAR)

Stores the address of data/instruction.

Sent via the address bus.

Memory Data Register (MDR)

Also called MBR.

Stores the actual data or instruction.

Transfers data between CPU and memory.

Current Instruction Register (CIR)

Holds the current instruction.

Instruction is copied from MDR.

Instruction is split into opcode + operand.

Instruction is sent to CU for decoding.

System Buses

Buses are communication pathways that connect CPU, memory, and input/output devices.

Address Bus

Carries memory addresses.

Direction: Unidirectional (CPU → memory).

Data Bus

Carries data and instructions.

Direction: Bidirectional.

Control Bus

Carries control signals.

Examples: Read, Write, Interrupt signals.

Direction: Bidirectional.

Internal Buses

Exist inside the CPU.

Connect registers, ALU, and cache.

Link to Assembly Language Programs

1. Direct Register Manipulation

Assembly instructions work directly with registers.

LDA (Load Accumulator) → Loads data from memory into ACC
STA (Store Accumulator) → Stores data from ACC into memory

This directly uses MAR (address), MDR (data), and ACC (storage).

2. Controlling the Program Counter

Branch instructions modify program flow.

BRA (Branch Always)
BRZ (Branch if Zero)

These instructions change the value in the PC and control which instruction is executed next.

3. Interaction with Buses

Assembly instructions trigger:

Address bus → send memory address (via MAR)
Data bus → transfer data (via MDR)
Control bus → send read/write signals

4. Efficiency of Assembly

Provides low-level control of hardware.

More efficient than high-level languages.

Used in embedded systems and performance-critical applications.

5. Translation

Assembly is translated into machine code by an assembler.

CPU executes binary instructions only.

Bringing It All Together (Big Picture)

PC holds next instruction address
Address sent to MAR
Instruction fetched into MDR
Copied to CIR
CU decodes instruction
ALU executes operation
Result stored in ACC
Data transferred via buses

Exam Focus (Common Errors)

MAR = address
MDR = data
Address bus = one way
Data bus = two way
CU = control + decoding
ALU = calculations

Exam Sentence Starters

“The Control Unit coordinates the Fetch-Decode-Execute cycle by…”
“The MAR stores the memory address which is sent via the address bus…”
“The MDR holds the data being transferred between memory and the CPU…”
“Assembly language allows direct manipulation of registers such as the ACC…”