MICROCOMPUTER INTERRUPT TYPES

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 5-11