Motorola MC68020 - Interrupt Recognition Examples

6- 14 M68020 USER’S MANUAL MOTOROLA
level 6 interrupt is not processed. However, if the MC68020/EC020 is handling a level 7
interrupt (I2–I0 in the SR set to 111) and the external request is lowered to level 3 and
then raised back to level 7, a second level 7 interrupt is processed. The second level 7
interrupt is processed because the level 7 interrupt is transition sensitive. A level 7
interrupt is also generated by a level comparison if the request level and mask level are at
7 and the priority mask is then set to a lower level (with the MOVE to SR or RTE
instruction, for example). As shown in Figure 6-3 for level 6 interrupt request level and
mask level, this is the case for all interrupt levels.
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 a transition to an interrupt request level of 7.
The only difference 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 IPL2–IPL0
INTERRUPT PRIORITY MASK (I2–I0 IN SR)
ACTION
LEVEL 6 EXAMPLE
INITIAL CONDITIONS
100 ($3)
101 ($5)
(LEVEL COMPARISON)
IF
001 ($6)
THEN
110 ($6)
AND
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)
(TRANSITION)
LEVEL 7 EXAMPLE
INITIAL CONDITIONS
100 ($3)
101 ($5)
IF
000 ($7)
THEN
111 ($7)
AND
LEVEL 7 INTERRUPT
IF
100 ($3)
AND STILL
111 ($7)
THEN
NO ACTION
IF
000 ($7)
AND STILL
111 ($7)
THEN
NO ACTION
IF STILL
000 ($7)
AND RTE SO THAT
101 ($5)
THEN
LEVEL 7 INTERRUPT
(LEVEL COMPARISON)
Figure 6-3. Interrupt Recognition Examples