80C186EC/80C188EC Microprocessor User's Manual

OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
The CPU can recognize interrupts only on valid instruction boundaries. A valid instruction
boundary usually occurs when the current instruction finishes. The following is a list of exceptions:
1. MOVs and POPs referencing a segment register delay the servicing of interrupts until
after the following instruction. The delay allows a 32-bit load to the SS and SP without an
interrupt occurring between the two loads.
2. The CPU allows interrupts between repeated string instructions. If multiple prefixes
precede a string instruction and the instruction is interrupted, only the one prefix
preceding the string primitive is restored.
3. The CPU can be interrupted during a WAIT instruction. The CPU will return to the WAIT
instruction.
2.3.4 Interrupt Response Time
Interrupt response time is the time from the CPU recognizing an interrupt until the first instruction
in the service routine is executed. Interrupt response time is less for interrupts or exceptions
which supply their own vector type. The maskable interrupt has a longer response time because
the vector type must be supplied by the Interrupt Control Unit (see Chapter 7, “Interrupt Control
Unit”).
Figure 2-27 shows the events that dictate interrupt response time for the interrupts that supply
their type. Note that an on-chip bus master, such as the DRAM Refresh Unit, can make use of
idle bus cycles. This can increase interrupt response time.
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