Computer Architecture – Overview

This post is a conceptual guide to computer architecture, covering key principles, essential terminology, and core concepts.

In a Nutshell

The Von Neumann architecture is the foundational model for modern computers: the CPU fetches instructions and data from shared memory over a shared bus (a potential bottleneck).

The Von Neumann Architecture (Foundational Model)

Main components:

1. Arithmetic Logic Unit (ALU)
2. Control Unit
3. Memory for both program and data
4. Input/Output (I/O)

All components connect via a shared bus. This creates the Von Neumann bottleneck because instructions and data compete for the same pathway.

The CPU – the “Brain”

Typical execution flow: Fetch → Decode → Execute → Writeback.

Key concepts:

• Registers (fast, small, on the CPU)
• Instruction set (e.g., x86/ARM); CISC vs. RISC
• Clock frequency (GHz) is not the only performance metric
• Multiple cores enable parallel processing

Memory Hierarchy

From fastest/most expensive to slowest/cheapest:

1. Registers
2. Cache (L1/L2/L3)
3. RAM
4. SSD/Hard disk

Cache relies on the principle of locality:

• Spatial locality
• Temporal locality

Peripherals and Bus Systems

• Bus types: data bus, address bus, control bus

I/O methods:

• Programmed I/O (polling)
• Interrupt-driven I/O
• DMA (Direct Memory Access): data transfers directly between a device and RAM without constant CPU intervention

Performance Evaluation

Performance depends on more than just GHz:

• IPC (Instructions per Cycle)
• Number of cores
• Memory latency and bandwidth

Exam Answer (Oral)

A computer is a machine that operates according to the Von Neumann architecture.
A CPU (ALU plus control unit) fetches instructions and data from shared memory
over a bus, performs calculations, and writes results back. To compensate for
slow memory access, the system uses a memory hierarchy with fast caches. I/O is
managed via interrupts or DMA to reduce CPU overhead.