EXCEPTION PROCESSING STATE (INTERRUPT PROCESSING)
MOTOROLA PROCESSING STATES 7 - 21
There are two cases in which the stacked address will not point to the illegal instruction:
1. If the illegal instruction is one of the two instructions at an interrupt vector loca-
tion and is fetched during a regular interrupt service, the processor will stack
the address of the next sequential instruction in the normal instruction flow (the
regular return address of the interrupt routine that had the illegal opcode in its
vector).
2. If the illegal instruction follows an REP instruction (see Figure 7-6), the proces-
sor will effectively execute the illegal instruction as a repeated NOP and the
interrupt vector will then be inserted in the pipeline. The next instruction will be
fetched but will not be decoded or executed. The processor will stack the
address of the next sequential instruction, which is two instructions after the
illegal instruction.
In DO loops, if the illegal instruction is in the loop address (LA) location and the instruc-
tion preceding it (i.e., at LA-1) is being interrupted, the loop counter (LC) will be decre-
mented as if the loop had reached the LA instruction. When the interrupt service ends
and the instruction flow returns to the loop, the illegal instruction will be refetched (since
it is the next sequential instruction in the flow). The loop state machine will again decre-
ment LC because the LA instruction is being executed. At this point, the illegal instruction
will trigger the III. The result is that the loop state machine decrements LC twice in one
loop due to the presence of the illegal opcode at the LA location.
INTERRUPT CONTROL CYCLE 1 i
INTERRUPT CONTROL CYCLE 2 i
FETCH n1 n2 n3 n4 n5 n6 n7 — — — ii1 ii2 n8
DECODE n1 n2 n3 n4 REP II — — — — ii1 ii2 n8
EXECUTE n1 n2 n3 n4 REP REP NOP — — — ii1 ii2 n8
INSTRUCTION CYCLE COUNT 1 2 3 4 5 6 7 8 9 10111213141516
i = INTERRUPT
ii = INTERRUPT INSTRUCTION WORD
II = ILLEGAL INSTRUCTION
n = NORMAL INSTRUCTION WORD
ILLEGAL INSTRUCTION INTERRUPT
RECOGNIZED AS PENDING
Figure 7-6 Repeated Illegal Instruction