Direct Memory Access (DMA)

 

1. Direct Memory Access (DMA)

🔷 The Problem with Programmed I/O (PIO)

In Programmed I/O (PIO):

• The CPU transfers data between memory and device

• Data is copied word by word

❌ Issues:

• CPU spends too much time on simple data movement

• Reduces time available for executing processes

• Becomes a bottleneck for large data transfers

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💡 The Solution: DMA

🔷 What is DMA?

Direct Memory Access (DMA) is a technique where a special hardware unit (DMA controller) transfers data:

• Directly between memory and device

• Without continuous CPU involvement

👉 Key idea:

• Offload data movement from CPU → DMA controller

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2. How DMA Works (Step-by-Step)

1. OS Programs the DMA Controller

The OS provides:

• 📍 Memory location of data

• 📦 Size of data

• 🎯 Target device

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2. DMA Takes Over

• DMA controller handles the entire data transfer

• CPU is not involved in copying

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3. CPU Continues Other Work

• OS schedules another process (e.g., Process 2)

• CPU utilization improves

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4. DMA Completion

• DMA controller sends an interrupt

• OS is notified that transfer is done

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3. Execution Comparison

🔴 With PIO

CPU:   Process1 → Copying → Copying → Process2
Disk:            Idle → Working

• CPU stuck copying data (c c c)

• Inefficient

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🟢 With DMA

CPU:   Process1 → Process2 → Process2 → Process1
DMA:            Copying data
Disk:            Working

• DMA handles copying

• CPU runs other processes

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 4. Advantages of DMA

✅ 1. Better CPU Utilization

• CPU is free during data transfer

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✅ 2. Faster Data Movement

• Dedicated hardware is more efficient

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✅ 3. Overlap of Work

• CPU + DMA + Device operate in parallel

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✅ 4. Ideal for Large Transfers

• Especially useful for:

  • Disk I/O

  • Network transfers

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5. Key Insight

👉 DMA + Interrupts work together:

• DMA handles data transfer

• Interrupt notifies completion

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6. Simple Analogy

• PIO → Teacher copies notes to board line by line

• DMA → Assign a helper (DMA) to copy notes while teacher continues teaching

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7. Key Takeaways 

• DMA removes CPU involvement in data transfer

• OS only initiates transfer

• DMA controller performs copying

• Completion is signaled via interrupt

• Improves performance and efficiency