A+ Certification: Part 2 – Hardware: System Architecture

Section Objectives

After you complete this section you will:

Understand what an IRQ is and what common IRQs are.

Understand what a DMA Channel is.

Memorize the common I/O addresses.


IRQs (Interrupt Request) are a very important portion of this course as there could be several questions on the exam in regards to this material. In addition, understanding IRQs and what effect they have on your computer can help you when troubleshooting problems in your career as an IT technician.

All CPUs perform tasks one at a time. When a device needs the attention of the processor, it sends an interrupt request. Since the processor needs to know where the request came from, we have the IRQ settings to minimize conflicts and allow multiple devices to interrupt the CPU when needed.

Here is a chart of common IRQ settings; you should memorize these for the exam:

IRQ Device
0 Timer
1 Keyboard
2 Wired to IRQ 9
3 COM 2 (COM 4)
4 COM 1 (COM 3)
5 Available (often LPT2, sound cards, or network cards)
6 Floppy Disk Controller
7 LPT1
8 Clock
9 Wired to IRQ 2
10 Unused
11 Unused
12 Mouse Port
13 Coprocessor
14 Hard Disk Controller
15 Unused


DMA, or Direct Memory Access, is also a very important topic to study and understand. DMA channels allow devices to communicate directly with system memory without involving the CPU allowing for more efficient (faster) transfer of data. Newer PCs no longer require DMA, but older PCs may.

Hard drive controllers, NICs, and floppy disk controllers are three examples of devices that may use DMA in older systems.

In most PCs, there are 8 DMA channels (number 0 – 7). Channels 4-7 are generally available, while channel 0 is used to refresh DRAM, channel 1 is used by the hard disk controller or sound card, and channel 2 is used by the floppy disk controller.

Common I/O Addresses

I/O Addresses are memory locations which provide an interface between the operating system and an I/O device. This is an important area of study for the exam.

Address (Hex) Device
00-0F DMA Controller
20-21 Interrupt Controller
40-43 Timer
1F0-1F8 Hard Disk Controller
200-20F Joystick Controller
238-23B Bus Mouse
278-27F LPT2
2E8-2EF COM4 Serial Port
2F8-2FF COM2 Serial Port
300-30F Ethernet Card
330-33F MIDI Port
378-37F LPT1 Port
3E8-3EF COM3 Serial Port
3F0-3F7 Floppy Disk Controller
3F8-3FF COM1 Serial Port

It’s important you study and memorize the memory addresses, especially the COM ports.

After you have completed this section you should:

Understand what an IRQ is and what common IRQs are.

An IRQ is an interrupt request – a request from a device to gain the attention of the processor.

Understand what a DMA Channel is.

DMA, or Direct Memory Access, is a method by which certain devices can communicate directly with RAM, bypassing the processor.

Memorize the common I/O addresses.

You should memorize the common I/O addresses, especially the COM port I/O addresses. This information will be asked on the exam.

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