Alternate Modes Of Operation: The Poll Command - Intel 80C188EC User Manual

INTERRUPT CONTROL UNIT
8.3.9

Alternate Modes of Operation: The Poll Command

Conventional polling requires that the CPU check each peripheral device to determine whether it
needs servicing. Polling can also be accomplished with an 8259A module by using the Poll com-
mand. This method improves polling efficiency because the CPU needs to check only the 8259A
module, not each of the devices connected to it.
The Poll command is useful in various situations. For example, if more than 64 interrupt sources
are required in a system (64 is the limit for cascaded 8259A modules) the interrupt capability can
be expanded using polling. The number of interrupt request sources in a polled 8259A module
system is limited only by the number of 8259A modules that can be addressed.
The Poll command takes the place of a standard interrupt acknowledge sequence. The external
maskable interrupt request of the CPU must be disabled either by disconnecting it from the 8259A
module (when possible) or by clearing the Interrupt Enable Flag in the CPU (with a CLI instruc-
tion). Polling is covered in greater detail in "Special Mask Mode, Poll Mode and Register Read-
ing: OCW3" on page 8-34.
8.4 PROGRAMMING THE 8259A MODULE
This section describes the programming of a single 8259A module. Programming requirements
that are specific to the 80C186EC/C188EC are covered in "Module Integration: The 80C186EC
Interrupt Control Unit" on page 8-36.
8.4.1 Initialization and Operation Command Words
The command register set of the 8259A module is divided into two types of words: Initialization
Command Words (ICWs) and Operation Command Words (OCWs). The Initialization Com-
mand Words are usually written only once during program execution (during system initializa-
tion). The Operation Command Words can be written at any time during program execution (after
initialization is complete).
The Initialization Command Words specify information that does not change during execution.
For example, the base interrupt type for the module does not change and is specified by an Ini-
tialization Command Word. The Operation Command Words specify conditions that may change
during execution. The Interrupt Mask Register, for example, is accessed through an Operation
Command Word.
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