Intel 80C186XL, User Manual

Page: 199 / 405

INTERRUPT CONTROL UNIT
8-4
The priority of each source is programmable. The Interrupt Control register enables the
programmer to assign each source a priority that differs from the default. The priority must still
be between zero (highest) and seven (lowest). Interrupt sources can be programmed to share a
priority. The Interrupt Control Unit uses the default priorities (see Table 8-1) within the shared
priority level to determine which interrupt to service first. For example, assume that INT0 and
INT1 are both programmed to priority seven. Because INT0 has the higher default priority, it is
serviced first.
Interrupt sources can be masked on the basis of their priority. The Priority Mask register masks
all interrupts with priorities lower than its programmed value. After reset, the Priority Mask reg-
ister contains priority seven, which effectively enables all interrupts. The programmer can then
program the register with any valid priority level.
8.2.1.3 Interrupt Nesting
When entering an interrupt handler, the CPU pushes the Processor Status Word onto the stack
and clears the Interrupt Enable bit. The processor enters all interrupt handlers with maskable in-
terrupts disabled. Maskable interrupts remain disabled until either the IRET instruction restores
the Interrupt Enable bit or the programmer explicitly enables interrupts. Enabling maskable in-
terrupts within an interrupt handler allows interrupts to be nested. Otherwise, interrupts are pro-
cessed sequentially; one interrupt handler must finish before another executes.
The simplest way to use the Interrupt Control Unit is without nesting. The operation and servicing
of all sources of maskable interrupts is straightforward. However, the application tradeoff is that
an interrupt handler will finish executing even if a higher priority interrupt occurs. This can add
considerable latency to the higher priority interrupt.
In the simplest terms, the Interrupt Control Unit asserts the maskable interrupt request to the CPU,
waits for the interrupt acknowledge, then presents the interrupt type of the highest priority un-
masked interrupt to the CPU. The CPU then executes the interrupt handler for that interrupt. Be-
cause the interrupt handler never sets the Interrupt Enable bit, it can never be interrupted.
The function of the Interrupt Control Unit is more complicated with interrupt nesting. In this case,
an interrupt can occur during execution of an interrupt handler. That is, one interrupt can preempt
another. Two rules apply for interrupt nesting:
•
An interrupt source cannot preempt interrupts of higher priority.
•
An interrupt source cannot preempt itself. The interrupt handler must finish executing
before the interrupt is serviced again. (Special Fully Nested Mode is an exception. See
“Special Fully Nested Mode” on page 8-8.)