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Exception Processing
MOTOROLA
M68060 USER’S MANUAL
8-13
with the MOVE to SR or RTE instruction). The level 6 interrupt request and mask level
example in Figure 8-3 is the same as for all interrupt levels except 7.
Note that a mask value of 6 and a mask value of 7 both inhibit request levels of 1–6 from
being recognized. In addition, neither masks an interrupt request level of 7. The only differ-
ence between mask values of 6 and 7 occurs when the interrupt request level is 7 and the
mask value is 7. If the mask value is lowered to 6, a second level 7 interrupt is recognized.
External circuitry can chain or otherwise merge signals from devices at each level, allowing
an unlimited number of devices to interrupt the processor. When several devices are con-
nected to the same interrupt level, each device should hold its interrupt priority level constant
until its corresponding interrupt acknowledge bus cycle ensures that all requests are pro-
cessed. Refer to
Section 7 Bus Operation
for details on the interrupt acknowledge cycle.
Figure 8-4 illustrates a flowchart for interrupt exception processing. When processing an
interrupt exception, the processor first makes an internal copy of the SR, sets the mode to
supervisor, suppresses tracing, and sets the processor interrupt mask level to the level of
the interrupt being serviced. The processor attempts to obtain a vector number from the
Figure 8-3. Interrupt Recognition Examples
EXTERNAL
IPL2–IPL0
INTERRUPT PRIORITY
MASK (I2–I0) ACTION
LEVEL 6 EXAMPLE
(INITIAL CONDITIONS)
100 ($3) 101 ($5)
(LEVEL COMPARISON)IF 001 ($6) AND 101 ($5) THEN LEVEL 6 INTERRUPT
IF 100 ($3) AND STILL 110 ($6) THEN NO ACTION
IF 001 ($6) AND STILL 110 ($6) THEN NO ACTION
IF STILL 001 ($6) AND RTE SO THAT 101 ($5) THEN LEVEL 6 INTERRUPT (LEVEL COMPARISON)
(TRANSITION)
(INITIAL CONDITIONS)
100 ($3) 101 ($5)
IF 000 ($7) AND 101 ($5) THEN LEVEL 7 INTERRUPT
IF 000 ($7) AND 111 ($7) THEN NO ACTION
IF 100 ($3) AND STILL 111 ($7) THEN NO ACTION
IF 000 ($7) AND STILL 111 ($7) THEN LEVEL 7 INTERRUPT (TRANSITION)
LEVEL 7 EXAMPLE
IF STILL 000 ($7) AND RTE SO THAT 101 ($5) THEN LEVEL 7 INTERRUPT (LEVEL COMPARISON)
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