Identifying System Resources
Throughout this book, you are going to see many references to system resources and configuring devices to use them. Essentially, the computer system is a collection of interconnected and interrelated components all needing to communicate with each other. Each computer component needs to be assigned system resources to allow this communication to take place. There are three types of system resources that components use: input/output (I/O) addresses, interrupt requests (IRQs), and direct memory access (DMA) channels.
I/O Addresses
All computers have a number of I/O devices attached to them such as printers, scanners, floppy drives, hard disks, and just about every other component you can think of. Each of these devices requires a unique I/O address. An I/O address is the hexadecimal address of an area of memory that is used by the peripheral device to exchange information with other devices in the system. You can think of it as a kind of mail slot for the device.
NOTE
I/O addresses are expressed in a numbering system known as hexadecimal. The lower case h is appended to the number to indicate this. Hexadecimal is a numbering system based on 16 characters—the numbers 0 through 9 and the letters A through F. Although you will not be required to understand the hexadecimal numbering system for the A+ exam, it is a useful thing to understand when working in the real world.
Some core components, such as the hard disk controller or video graphics card, have I/O addresses reserved for them, while other devices such as printers need to have I/O addresses assigned. I/O addresses are assigned to add-on peripheral devices through Plug and Play, or you can manually assign I/O addresses. Plug and Play is far and away the preferred method for resource assignment. You can view I/O address assignments on a Microsoft Windows PC using Windows Device Manager, as shown in Figure 1-3.
SEE ALSO
For more information on using Windows Device Manager, refer to Chapter 7.
Figure 1-3. You can view the I/O addresses assigned to devices in Windows Device Manager.
Not only does each component require an I/O address, but each component must use a unique I/O address. This means that when a new device such as a network card or modem is installed in the system, it cannot be assigned an I/O address already being used by another device. If two devices are assigned the same I/O address, one or both devices will fail to function. Table 1-1 identifies the default I/O addresses that are assigned to various devices within the system.
TEST SMART
Each component attached to the computer requires a unique I/O address to communicate with other devices in the system.
|
Table 1-1. Standard I/O Address Assignments |
|
|
I/O Address |
Device |
|
1F0–1F8 |
Hard disk controller |
|
200–207 |
Game I/O |
|
278–27F |
Parallel port (LPT2) |
|
2F8–2FF |
Serial port (COM2) |
|
320–32F |
Hard drive controller, 8-bit ISA |
|
378–37F |
Parallel port (LPT1) |
|
3B0–3BF |
Monochrome graphics adapter |
|
3D0–3DF |
Color graphics adapter |
|
3F0–3F7 |
Floppy drive controller |
|
3F8–3FF |
Serial port (COM1) |
TEST SMART
When studying for the A+ exam, ensure that you know the I/O addresses for key system components such as the COM ports, floppy drive, and hard drive.
Standard IRQ Settings
Once a computer component has been assigned an I/O address, it is able to communicate with other devices and the CPU within the system. However, there needs to be some mechanism by which the devices can initiate communication with the CPU; for that the system uses interrupt request (IRQ) lines. IRQ lines are used in the system to notify the CPU that a peripheral device has started or completed a task. When the CPU receives an interrupt request from a peripheral device, the CPU can identify the device by its unique IRQ.
When working with IRQs, there are a few things to keep in mind. Most components need to have a unique IRQ, whether it is located on the motherboard or it is an add-on device. Secondly, devices should not share an IRQ address; if they do, one or both devices will fail to function. There is one caveat here, because Plug and Play has the ability to share IRQs if necessary; just more of that Plug and Play magic. In modern systems, Plug and Play manages the assignment of IRQ addresses, which greatly reduces the chances of assigning the same resources to two devices. As technicians, we do not have the liberty to trust Plug and Play to manage resources because there will be those times when we will need to troubleshoot IRQ assignments. To do this, we need to have a clear idea of which IRQ is used for which device. Table 1-2 shows system IRQs and the components that use them.
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Table 1-2. IRQ Assignments |
|
|
IRQ Address |
Device |
|
IRQ 0 |
System timer |
|
IRQ 1 |
Keyboard |
|
IRQ 2/9 |
Open (cascade to IRQ 9) |
|
IRQ 3 |
Default IRQ for COM2 and COM4 |
|
IRQ 4 |
Default for IRQ 1 and COM3 |
|
IRQ 5 |
LPT2 (often used with audio cards) |
|
IRQ 6 |
Floppy drive |
|
IRQ 7 |
LPT1 |
|
IRQ 8 |
Real-time clock |
|
IRQ 9 |
Redirect to 2 |
|
IRQ 10 |
Open |
|
IRQ 11 |
Open |
|
IRQ 12 |
Open (often used with PS/2 mouse) |
|
IRQ 13 |
Math coprocessor |
|
IRQ14 |
Primary hard drive controller |
|
IRQ 15 |
Secondary hard drive controller |
TEST SMART
Be sure to study Table 1-2 closely before taking the A+ exam.
Notice in Table 1-2 that IRQ 2 and 9 are connected; there is a good reason for this. Early PCs only had a small number of devices, so the original IRQ controller that supplied eight IRQs was more than sufficient. As PCs developed, more IRQs were needed, which meant that another IRQ controller was needed. The addition of another interrupt controller was made simple by the fact that the second controller was accessed through an interrupt on the first controller. IRQ 2 was set aside for this purpose. In essence, the two interrupt controllers are linked—or, to give it its proper term, cascaded—through IRQ 2. This cascading function provides 16 IRQs (0–15) without requiring a fundamental redesign of PC architecture.
DMA Addresses
Direct memory access (DMA) is a method used by peripheral devices to access the system’s main memory and store data without having to use the CPU. This frees up the CPU to focus on other tasks. DMA data transfers from peripheral devices are held in a special area of memory known as a buffer.
Devices using DMA use channel assignments to bypass the CPU and access the main memory directly. DMA access is not used by every device on the system but might be used by disk drives, tape drives, and even sound cards. Table 1-3 shows the DMA channels and devices that use them.
|
Table 1-3. Default DMA Channels |
|
|
DMA Channel |
Device |
|
0 |
DRAM refresh |
|
1 |
Sound card |
|
2 |
Floppy disk drive |
|
3 |
ECP parallel port |
|
4 |
DMA controller |
|
5 |
Open (sound card) |
|
6 |
Open |
|
7 |
ISA hard disk controller |
Like the other resources mentioned in this section, two devices cannot share a DMA channel. Operating systems today assign DMA channels through Plug and Play; however, some legacy devices require that DMA channels be set using jumper settings. You can view DMA channel assignments in Windows using Device Manager. Figure 1-4 shows the DMA channels used.
Figure 1-4. You can view the DMA channels in use through Windows Device Manager.
