Busses, Interrupts and PC Systems - Computer Busses - page 65

Data received from serial ports interrupts the processor with IRQ3 and IRQ4 and the paral-
lel ports use IRQ5 and IRQ7 . If one of the parallel, or serial, ports does not exist then the
IRQ line normally assigned to it can be used by another device. It is typical for interrupt-
driven I/O cards, such as a sound card, to have a programmable IRQ line which is mapped to
an IRQ line that is not being used.
Note that several devices can use the same interrupt line. A typical example is COM1:
and COM3: sharing IRQ4 and COM2: and COM4: sharing IRQ3 . If they do share then the
ISR must be able to poll the shared devices to determine which of them caused the interrupt.
If two different types of device (such as a sound card and a serial port) use the same IRQ line
then there may be a contention problem as the ISR may not be able to communicate with
different types of interfaces.
Figure 2.11 shows a sample window displaying interrupt usage. In this case it can be seen
that the system timer uses IRQ0 , the keyboard uses IRQ1 , the PIC uses IRQ2 , and so on.
Notice that a sound blaster is using IRQ5 . This interrupt is normally reserved for the secon-
dary printer port. If there is no printer connected then IRQ5 can be used by another device.
Some devices can have their I/O address and interrupt line changed. An example is given in
Figure 2.12. In this case, the IRQ line is set to IRQ7 and the base address is 378h.

Typical uses of interrupts are:

IRQ0 : System timer The system timer uses IRQ0 to interrupt the proc-

essor 18.2 times per second and is used to keep the
time-of-day clock updated.

IRQ1 : Keyboard data ready The keyboard uses IRQ1 to signal to the processor

that data is ready to be received from the key-
board. This data is normally a scan code.

IRQ2 : Redirection of IRQ9 The BIOS redirects the interrupt for IRQ9 back

here.

Figure 2.11 Standard usage of IRQ lines