Chapter 11 – DMA (Direct Memory Access)

Core idea

DMA allows devices to transfer data directly to RAM, bypassing the CPU.

Why DMA exists

• CPU is too slow for large I/O copies
• Example:
  • Disk → RAM
  • Network → RAM

How it works

• DMA controller handles transfer

• CPU sets it up, then continues working

• Interrupt signals completion

Limitations

• Uses physical addresses
• Doesn’t understand virtual memory
• Requires pinned (non-swappable) memory
• Security risks → IOMMU

Exam angles

• DMA improves throughput
• CPU still involved in setup and completion