---
title: "Configurations of DMA"
subject: "Computer Organization and Architecture"
chapter: 7
key: "semester_5/computer_organization_and_architecture:dma-configurations"
source: https://www.bctengineer.com/q/semester_5/computer_organization_and_architecture/dma-configurations
site: BCT Engineering
---

# Configurations of DMA

Computer Organization and Architecture · Chapter 7: Input-Output Organization

- Asked in: 2067 Asar (old course), 8 marks
- Video: [5 DMA CONFIGURATION](https://www.youtube.com/watch?v=2nMajoVQkwo) — Mohanraj Ramasamy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/computer_organization_and_architecture/dma-configurations
- Whole chapter: https://www.bctengineer.com/md/semester_5/computer_organization_and_architecture/chapter-7

## Question

Mention the three possible configurations of DMA and compare them.

## Short answer

Three configurations: detached DMA on a single bus (two bus uses per transfer), integrated DMA in I/O modules (one bus use), and a separate I/O bus (one system bus use). The detached is simplest but least efficient; integrated and I/O bus offer higher efficiency and lower bus traffic.

## Answer

Three common ways of arranging the DMA controller in a system are:

#### 1. Single bus, detached DMA controller
```text
  CPU    DMA    I/O    I/O    Memory
   |      |      |      |       |
  ======================================  system bus
```
All modules share one bus, and the DMA controller acts as a substitute CPU for I/O. A transfer from a device to memory uses the bus **twice**: device to DMA controller, then DMA controller to memory.

#### 2. Single bus, integrated DMA-I/O
```text
  CPU    DMA+I/O    DMA+I/O    Memory
   |        |          |         |
  ======================================  system bus
```
The DMA logic is part of one or more I/O modules (or the DMA controller controls several I/O modules through an I/O bus). A transfer needs the system bus **once**.

#### 3. I/O bus
```text
  CPU     DMA      Memory
   |       |          |
  ====================================  system bus
           |
  =========================  I/O bus
     |        |        |
    I/O      I/O      I/O
```
The I/O modules are connected to the DMA controller by a separate I/O bus. The system bus is used once per transfer, and the number of interfaces on the DMA controller is reduced to one.

#### Comparison
| Point | Detached | Integrated | I/O bus |
|---|---|---|---|
| System bus uses per word | Two | One | One |
| Cost | Low, simple | Higher, DMA in each module | Moderate |
| Efficiency | Lowest | Highest | High |
| Flexibility | Easy to add devices | Less flexible | Easy to add I/O modules |
| Bus traffic | Highest | Low | Low (I/O traffic on separate bus) |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How many times is the system bus used per word in the single-bus detached DMA configuration?
   A. Never
   B. Once
   C. Three times
   D. Twice
   Answer: D. Data goes device to DMA controller, then DMA controller to memory. (Key point: Detached DMA bus use)

2. Why is the integrated DMA-I/O configuration more efficient than detached?
   A. DMA logic is in the I/O module, so the transfer needs the system bus only once
   B. It needs no I/O
   C. It has no memory
   D. It uses two buses per word
   Answer: A. A transfer goes straight between the module and memory. (Key point: Integrated DMA-I/O)

3. What does the I/O bus configuration do?
   A. Connects I/O modules to the DMA controller by a separate I/O bus, reducing the DMA interfaces to one
   B. Removes the system bus
   C. Puts DMA in every module
   D. Connects memory to I/O directly
   Answer: A. A separate I/O bus carries I/O traffic and the system bus is used once per transfer. (Key point: I/O bus configuration)

4. Which configuration has the lowest efficiency and highest bus traffic?
   A. Integrated
   B. I/O bus
   C. Detached
   D. All equal
   Answer: C. Detached uses the system bus twice per word. (Key point: Comparison)

## Related in Computer Organization and Architecture

- [Programmed vs interrupt-driven vs DMA](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Aio-techniques-compare) — Computer Organization and Architecture, chapter 7, asked 5×

## Related in other subjects

- [Compare status check I/O, interrupt I/O and DMA](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Acompare-status-interrupt-dma) — Instrumentation II, chapter 1
- [Principle and configurations of grounding](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Agrounding-principle-configs) — Instrumentation II, chapter 6
- [Fibre configurations and single mode advantages](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Afiber-configurations-single-mode) — Propagation and Antenna, chapter 6
- [Compare Gouraud and Phong shading](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Acompare-gouraud-phong) — Computer Graphics, chapter 8, asked 7×