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MOTOROLA M68040 USER’S MANUAL 7- 31
The M68040 takes an interrupt exception for a pending interrupt within one instruction
boundary after processing any other pending exception with a higher priority. Thus, the
M68040 executes at least one instruction in an interrupt exception handler before
recognizing another interrupt request. The following paragraphs describe the various kinds
of interrupt acknowledge bus cycles that can be executed as part of interrupt exception
processing. Table 7-4 provides a summary of the possible interrupt acknowledge
terminations and the exception processing results.
Table 7-4. Interrupt Acknowledge Termination Summary
Termination Condition
High High Don’t Care Insert Waits
High Low Don’t Care Take Spurious Interrupt Exception
Low High High Latch Vector Number on D7–D0 and Take Interrupt
Exception
Low High Low Take Autovectored Interrupt Exception
Low Low Don’t Care Retry Interrupt Acknowledge Cycle
7.5.1.1 INTERRUPT ACKNOWLEDGE BUS CYCLE (TERMINATED NORMALLY).
When the M68040 processes an interrupt exception, it performs an interrupt acknowledge
bus cycle to obtain the vector number that contains the starting location of the interrupt
exception handler. Some interrupting devices have programmable vector registers that
contain the interrupt vectors for the exception handlers they use. Other interrupting
conditions or devices cannot supply a vector number and use the autovector bus cycle
described in 7.5.1.2 Autovector Interrupt Acknowledge Bus Cycle.
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