Chapter 8 – Physical Memory & DRAM

Core idea

Physical memory (DRAM) is not uniform and has performance constraints.

DRAM basics

• SRAM
  • Very fast, expensive
  • Used for CPU caches
• DRAM
  • Slower, cheaper
  • Needs constant refresh
• Memory access happens in bursts (e.g. 64 bytes)

Modern RAM

• SDRAM – synchronized with CPU clock
• DDR (Double Data Rate) – two transfers per cycle
• DDR4 / DDR5
  • Larger prefetch buffers
  • Higher bandwidth

Exam traps

• RAM is not byte-by-byte accessed
• Cache line size matters for performance