Intel 80C188XL, 80C186XL user manual Chapter Interrupt Control Unit, 8.1FUNCTIONAL OVERVIEW

Models: 80C186XL 80C188XL

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CHAPTER 8

CHAPTER 8

INTERRUPT CONTROL UNIT

The 80C186 Modular Core has a single maskable interrupt input. (See “Interrupts and Exception Handling” on page 2-39.) The Interrupt Control Unit (ICU) expands the interrupt capabilities be- yond a single input. To fulfill this function, the Interrupt Control Unit operates in either of two modes: Master or Slave.

In Master mode, the ICU controls the maskable interrupt input to the CPU. Interrupts can origi- nate from the on-chip peripherals and from four external interrupt pins. The ICU synchronizes and prioritizes all interrupt sources and presents the correct interrupt type vector to the CPU. (See Figure 8-1.) Most systems use master mode.

In Slave mode, an external 8259A module controls the maskable interrupt input to the CPU and acts as the master interrupt controller. The ICU processes only those interrupts from the on-chip peripherals and acts as an interrupt input to the 8259A. (See Figure 8-15 on page 8-24.) This mode can be useful in larger system designs.

The Interrupt Control Unit has the following features:

Programmable priority of each interrupt source

Individual masking of each interrupt source

Nesting of interrupt sources

Support for polled operation

Support for cascading external 8259A modules to expand external interrupt sources

8.1FUNCTIONAL OVERVIEW

All microcomputer systems must communicate in some way with the external world. A typical system might have a keyboard, a disk drive and a communications port, all requiring CPU atten- tion at different times. There are two distinct ways to process peripheral I/O requests: polling and interrupts.

Polling requires that the CPU check each peripheral device in the system periodically to see- whether it requires servicing. It would not be unusual to poll a low-speed peripheral (a serial port, for instance) thousands of times before it required servicing. In most cases, the use of polling has a detrimental effect on system throughput. Any time used to check the peripherals is time spent away from the main processing tasks.

8-1

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Intel 80C188XL, 80C186XL user manual Chapter Interrupt Control Unit, 8.1FUNCTIONAL OVERVIEW