Default IRQ Numbers And I/O Addresses IRQ I/O Address Device, Adding New Peripherals

An Example Of An IRQ

Consider this example of a device using an IRQ to get service from the CPU, described in non-technical terms:

The CPU is printing a word processing document. As the document is printing, the user presses a key on the keyboard. The keyboard sends an IRQ to the CPU requesting service.

The CPU receives the IRQ and stops the printing. Before the CPU turns its attention to the keyboard, it records all the information that it needs to resume the print job where it left off. This print job information is stored in a stack, or place in memory where information about suspended jobs is kept.

The CPU handles the keyboard IRQ by accessing the I/O addresses assigned to that IRQ. In this address is stored the memory address of the keyboard BIOS. The CPU reads the address, then turns to that address, and executes the program located there. The program (interrupt handler) processes the keystroke and then returns an interrupt completed message to the CPU.

The CPU returns to the stack, retrieves the information about the print job, and continues printing.

Default IRQ Numbers And I/O Addresses IRQ I/O Address Device

0 040-05F System timer

1 060 Keyboard controller

2 0A0 Access to IRQs above 7

3 2F8-2FF COM2 and COM4

4 3F8-3FF COM1 and COM3

5 1F0-1F8 Sound/parallel port 2 (LPT2)

6 3F0-3F7 Floppy drive controller

7 278-27F Printer parallel port (LPT1:)

8 070-07F System clock

9-10 Available

11 SCSI/available

12 Systemboard mouse

13 0F8-0FF Math coprocessor

14 IDE hard drive

15 Secondary IDE hard drive/available

Adding New Peripherals

When adding new peripherals to a computer, consider these fundamental principles: The peripheral is a hardware device that is controlled by software. You must install both the hardware and the software.

The software might exist at different levels. For example, a device could require driver software that interfaces directly with the hardware device and an applications software package that interfaces with the driver. You must install all levels of software.

More than one peripheral device might attempt to use the same computer resources. This conflict could disable a device or cause it to hang up. Possible conflicts arise when more than one device attempts to use:

The same IRQ

The same DMA channel

The same I/O addresses

The same upper memory addresses

Peripheral devices need (in addition to the standard computer resources you already expect to be present) the hardware device itself, the firmware, the device driver, and the applications software.