1. The Von Neumann Legacy 🏛️

Virtually every computer today is built on the Von Neumann Architecture (1945). Its defining characteristic? The Stored-Program Concept. Both instructions (code) and data reside in the SAME memory space, addressable by location.

The Bottleneck

Because code and data share a bus, the CPU is much faster than the memory. The CPU spends a lot of time waiting for data. This is the Von Neumann Bottleneck.

2. Instruction Set Architecture (ISA) 📜

The interface between hardware and software. The "vocabulary" of the processor.

RISC (Reduced Instruction Set Computer)

Philosophy: Keep hardware simple, let software (compiler) do the work.
Instructions: Simple, fixed length, 1 cycle execution.
Registers: Heavy use of registers (Load/Store architecture).
Examples: ARM (your phone), MIPS, RISC-V.

CISC (Complex Instruction Set Computer)

Philosophy: Hardware should support complex operations to reduce code size.
Instructions: Complex, variable length, multi-cycle.
Memory: Instructions can operate directly on memory.
Examples: x86 (Intel/AMD desktops).

3. Pipelining: The Assembly Line 🚀

Why execute one instruction at a time when you can overlap them? Pipelining divides execution into stages (Fetch, Decode, Execute, Memory, Writeback).

Performance Formula:

Speedup = (Time without pipeline) / (Time with pipeline)

Ideal CPI (Cycles Per Instruction) = 1. But Hazards ruin the party:

Structural: Hardware resource conflict.
Data: Instruction depends on result of previous one (RAW).
Control: Branching makes us fetch wrong instructions.

4. Memory Hierarchy: Need for Speed 🏎️

We want memory to be fast, large, and cheap. We can't have all three. So we build a hierarchy.

Registers: Top speed, tiny capacity. Inside CPU.
L1 Cache: SRAM. Integrated. Split (Instruction/Data).
L2/L3: SRAM. Shared. Bridges CPU and RAM.
Main RAM: DRAM. Large capacity. Volatile.
Disk/SSD: Storage. Massive. Non-volatile. Slow.

Computer Architecture