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Table Of Contents  CertiGuide to A+ (A+ 4 Real)
 9  Chapter 8: Motherboards

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Memory Mapping
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 The Future of Motherboards
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Logical Names

The concept of memory locations expressed in hex is important in order to understand what comes next. Each device using resources in the system is assigned a logical, hexadecimal name by the BIOS. If you recall your trip into the Device Manager to review the resources used by a modem, you doubtless noticed that the resources were expressed in hex. These are the memory locations that the resources use and where they can be accessed.

The BIOS assigns Logical Names to the devices using an IRQ and memory address. Now IRQ 3 and I/O address 2F8-2FFh becomes Com 2.

This does not mean that you cannot change the standardized IRQ and I/O address. Be strongly cautioned however against doing this, "just because you can". Moreover, whenever you are building or working on a new system, be strongly advised to create a map of the system before you start making any changes.

Hard-Coded Resource Settings

Remember some programs will not ask the Operating System for details such as IRQ and I/O. Some programs require this information in their own setup. And there are a few older programs, such as games, that require the popular default settings for the ISA Sound Blaster. That is:

IRQ 5, I/O 220h, DMA 1


Table 19 shows a listing of the typical assignments for IRQs and I/O addresses in a PC.


Table 19: Typical IRQ and I/O Address Assignments

IRQ

Used By

I/O Address

0

System Clock

40h

1

Keyboard

60h

2

Cascade

 

3

Com 2/4

2F8h / 2E8h

4

Com 1/3

3F8h / 3E8h

5

LPT2 / Sound

278h / 220h

6

Floppy

3F0h

7

LPT1

378h

8

Real Time Clock

70h

9

Open

 

10

Open / NIC

 

11

Open / VGA

A000

12

PS/2 Mouse

 

13

Math Coprocessor

 

14

Primary IDE

170h

15

Secondary IDE

1F0h

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Memory Mapping
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 The Future of Motherboards
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