Direct Memory Access (DMA) is an important concept in Computer Organization. Questions often test your understanding of different DMA modes.
In this post, we’ll:
- Understand DMA basics
- Learn all DMA transfer modes
- Solve an exam-level MCQ
🔵 What is DMA (Direct Memory Access)?
DMA allows devices to transfer data directly to/from memory without continuous CPU involvement.
👉 This improves system performance by reducing CPU workload.
🔴 Types of DMA Transfer Modes
✅ 1. Burst Mode (Block Transfer Mode)
🧠 Key Idea:
- Entire block of data is transferred at once
- CPU is stopped (put on hold)
✔ Features:
- Fastest transfer
- CPU remains idle during transfer
- DMA gets full control of system bus
👉 Used when large data blocks need to be transferred quickly
✅ 2. Cycle Stealing Mode
🧠 Key Idea:
- DMA transfers one word (or byte) at a time
- CPU is paused briefly
✔ Features:
- CPU and DMA share system bus
- After each transfer, CPU gets control back
👉 “Stealing” cycles from CPU → hence the name
✅ 3. Transparent Mode
🧠 Key Idea:
- DMA transfers data only when CPU is idle
✔ Features:
- CPU is never stopped
- Slowest data transfer
- Most efficient for overall system performance
🔥 Given MCQ
Statements:
(i) In burst mode DMA, CPU can be put on hold and full block is transferred
(ii) In transparent mode, DMA transfers only when CPU is not using system bus
(iii) In cycle stealing mode, only one word is transferred per request
🧠 Step-by-Step Evaluation
✔ Statement (i): Burst Mode
✅ TRUE
- CPU is completely halted
- Full block transfer happens
✔ Statement (ii): Transparent Mode
✅ TRUE
- DMA uses bus only when CPU is idle
✔ Statement (iii): Cycle Stealing
✅ TRUE
- One word/byte per request
- Then CPU regains control
🎯 Final Answer
👉 All are TRUE
⚡ Exam Tricks to Remember
- Burst Mode → CPU stopped, fastest
- Cycle Stealing → One word at a time
- Transparent Mode → CPU never disturbed
🧠 Quick Comparison Table
| Mode | CPU Status | Speed | Data Transfer |
|---|---|---|---|
| Burst Mode | Stopped | Fastest | Block transfer |
| Cycle Stealing | Partially stopped | Medium | One word at a time |
| Transparent Mode | Not stopped | Slowest | Only when CPU idle |
🎯 Conclusion
DMA modes are frequently asked in exams. Understanding how CPU and DMA share the system bus is key to solving such questions quickly.
