1.1 · CPU Architecture

Goal: name the major parts of a CPU, explain what each does, and reason about CPU performance.

What the CPU does

The Central Processing Unit (CPU) is the brain of the computer. Its single job is to execute instructions — billions of them per second.

Major parts

            ┌──────────────── CPU ──────────────────┐
            │                                       │
 ┌────┐     │   ┌─────────────┐   ┌──────────┐     │
 │ ⏤ │ ──▶ │   │ Control Unit │ ─▶│ Registers│     │
 └────┘     │   └─────┬────────┘   └────▲─────┘     │
            │         │                 │           │
            │   ┌─────▼────────┐  ┌─────┴──────┐    │
            │   │     ALU      │  │ Cache (L1) │    │
            │   └──────────────┘  └────────────┘    │
            └───────────▲──────────────────▲────────┘
                        │  System buses    │
                        ▼                  ▼
                  ┌──────────┐      ┌──────────┐
                  │  RAM     │      │ I/O dev  │
                  └──────────┘      └──────────┘

Part	Role
Control Unit (CU)	Fetches and decodes instructions; sends control signals to other parts
Arithmetic Logic Unit (ALU)	Performs arithmetic (+ − × ÷) and logic (AND OR NOT, comparisons)
Registers	Tiny, ultra-fast storage inside the CPU for the current instruction and its operands
Cache	Small, fast memory close to the CPU for recently-used data
Buses	Wires that carry data, addresses and control signals between parts

Key registers you should recognise

Register	Stands for	Holds
PC	Program Counter	Address of the next instruction
IR	Instruction Register	The instruction currently being decoded
MAR	Memory Address Register	Address that the CPU is about to read/write
MDR	Memory Data Register	Data being transferred to/from memory
ACC	Accumulator	Working value during arithmetic

In the exam

You may be given a labelled CPU diagram and asked which register stores what. Memorise PC, IR, MAR, MDR, ACC at minimum.

How CPU speed is measured

• Clock frequency in GHz (10⁹ cycles per second). 3 GHz = 3 billion ticks per second.
• One clock cycle ≈ 0.33 ns at 3 GHz.
• Modern CPUs do multiple operations per cycle (pipelining, superscalar).

Time units to know:

Unit	Time
1 ms (millisecond)	10⁻³ s
1 μs (microsecond)	10⁻⁶ s
1 ns (nanosecond)	10⁻⁹ s
1 ps (picosecond)	10⁻¹² s

CPU vs GPU

The C&A Guide names two kinds of processing unit explicitly:

Unit	Strength	Used for
CPU	Fast at sequential, complex logic	OS, browsers, anything general purpose
GPU	Fast at parallel simple maths	Graphics rendering, AI training, scientific compute

A modern laptop has both: the CPU runs your OS while the GPU draws the screen.

What affects performance

• Clock speed — more ticks per second.
• Number of cores — multiple CPUs on one chip can do parallel work.
• Cache size — more on-chip cache reduces slow RAM access.
• Word length — wider buses (32-bit, 64-bit) move more data per cycle.
• Pipelining / branch prediction — advanced tricks not in syllabus.

Exam-style question

Q (4 marks): Describe the function of the Control Unit, ALU, registers, and explain how cache memory improves CPU performance.

Sample answer:

• Control Unit (CU): decodes instructions and sends control signals to direct other components; coordinates the fetch–decode–execute cycle.
• ALU: carries out arithmetic and logical operations on data supplied by the registers.
• Registers: very small, very fast storage inside the CPU used to hold the instruction being decoded, the operands, and intermediate results.
• Cache: sits between CPU and main memory; stores recently used data and instructions so the CPU can fetch them in nanoseconds instead of waiting for slower RAM. This reduces average memory access time and increases overall throughput.

Key takeaways

• CPU = CU + ALU + Registers (+ cache on modern chips).
• Common registers: PC, IR, MAR, MDR, ACC.
• Performance grows with clock speed, cores, cache, word length.
• CPU is general purpose; GPU specialises in parallel maths.

➡️ Next: 1.2 Fetch–Decode–Execute