External interrupts —External interrupts are
received from external peripheral devices. They are
used to synchronize the execution of computer
programs to the readiness of the peripheral device to
transmit or receive data. They are also used to identify
peripheral equipment problems/errors to the computer.
Now let’s look at how interrupts work in each major
type of computer.
MICROCOMPUTER INTERRUPT TYPES. —
The microcomputer receives both internal and external
interrupts. Internal interrupts are received from the
real-time clock, system clock, and other conditions that
effect the operation of the microprocessor. External
interrupts are received from disk drives, CD-ROM
drives, sound boards, etc. These are classified as
external interrupts, even though the devices are
physically installed in the microcomputer case.
Microcomputer interrupts fall into two basic categories:
maskable and non-maskable. The CPU of the
microcomputer has two interrupt signal lines, one for
each category of interrupt.
External hardware interrupts are maskable inter-
rupts. The interrupt request signal indicates the pres-
ence of one or more of these interrupts. The specific
interrupt type is defined by accompanying interrupt code
words. The interrupt code and a ROM or program-
mable ROM (PROM) lookup table are used to direct the
processor to the address of the interrupt processor pro-
gram for the particular interrupt type. Maskable inter-
rupts can be masked out or locked out for short periods
of time by the software to allow the CPU to perform
critical operations. The programmer is responsible for
ensuring that interrupts are managed in a timely manner.
Nonmaskable interrupts cannot be masked out.
They are used for conditions that require immediate
attention by the microcomputer. Examples include
interrupts from the internal hard disks, modems, fax
cards, and sometimes a power out-of-tolerance
condition. If this feature is available, a power out-of-
tolerance condition will force the microcomputer to
execute its save data program.
The interrupt request (IRQ) line provides the
input signal path for all interrupts. If the interrupt
enable bit in the status indicating register is set, the
interrupt is processed at the end of the current
instruction cycle. If the interrupt enable bit is clear, the
interrupt signal is ignored by the microcomputer and
the next sequential instruction is executed.
Each hardware interrupt has a unique IRQ channel
assigned. Some of these channels are preassigned and
cannot be changed, while several are available for the
user to install additional hardware into the
microcomputer. Table 5-1 lists the hardware interrupt
channels used by most microcomputers. Note that in
Table 5-1.—Common IRQ Assignments for Microcomputers
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