Information Technology Reference
In-Depth Information
Interrupt vectors
Each device that requires to be 'interrupt-driven' is assigned an IRQ (interrupt request) line.
Each IRQ is active high. The first eight (
IRQ0
-
IRQ7
) map into interrupts 8 to 15 (08h-0Fh)
and the next eight (
IRQ8
-
IRQ15
) into interrupts 112 to 119 (70h-77h). Table 2.6 outlines
the usage of each of these interrupts. When
IRQ0
is made active, the ISR corresponds to
interrupt vector 8.
IRQ0
normally connects to the system timer, the keyboard to
IRQ1
, and
so on. The standard set-up of these interrupts is illustrated in Figure 2.10. The system timer
interrupts the processor 18.2 times per second and is used to update the system time. When
the keyboard has data, it interrupts the processor with the
IRQ1
line.
System timer
IRQ0
Keyboard
IRQ1
Serial port
(COM2:)
Processor
Programmable
interrupt
controller (PIC)
IRQ3
INT
Serial port
(COM1:)
IRQ4
Parallel port
(LPT2:)
IRQ5
Interrupt
service
routine
Floppy disk
controller
IRQ6
Parallel port
(LPT1:)
IRQ7
Figure 2.10
Standard usage of IRQ lines
Table 2.6
Interrupt handling
Interrupt
Name
Generated by
08 (08h)
System timer
IRQ0
09 (09h)
Keyboard
IRQ1
10 (0Ah)
Reserved
IRQ2
11 (0Bh)
Serial communications (COM2:)
IRQ3
12 (0Ch)
Serial communications (COM1:)
IRQ4
13 (0Dh)
Parallel port (LPT2:)
IRQ5
14 (0Eh)
Floppy disk controller
IRQ6
15 (0Fh)
Parallel printer (LPT1:)
IRQ7
112 (70h)
Real-time clock
IRQ8
113 (71h)
Redirection of IRQ2
IRQ9
114 (72h)
Reserved
IRQ10
115 (73h)
Reserved
IRQ11
116 (74h)
Reserved
IRQ12
117 (75h)
Math co-processor
IRQ13
118 (76h)
Hard disk controller
IRQ14
119 (77h)
Reserved
IRQ15
