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Bus Operation
7-76
MC68030 USER’S MANUAL
MOTOROLA
7.4.1.3 SPURIOUS INTERRUPT CYCLE.
When a device does not respond to an interrupt
acknowledge cycle with AVEC, STERM, or DSACKx, the external logic typically returns
BERR. The MC68030 automatically generates the spurious interrupt vector number, 24,
instead of the interrupt vector number in this case. If HALT is also asserted, the processor
retries the cycle.
7.4.2 Breakpoint Acknowledge Cycle
The breakpoint acknowledge cycle is generated by the execution of a breakpoint instruction
(BKPT). The breakpoint acknowledge cycle allows the external hardware to provide an
instruction word directly into the instruction pipeline as the program executes. This cycle
accesses the CPU space with a type field of zero and provides the breakpoint number
specified by the instruction on address lines A2–A4. If the external hardware terminates the
cycle with DSACKx or STERM, the data on the bus (an instruction word) is inserted into the
instruction pipe, replacing the breakpoint opcode, and is executed after the breakpoint
acknowledge cycle completes. The breakpoint instruction requires a word to be transferred
so that if the first bus cycle accesses an 8-bit port, a second cycle is required. If the external
logic terminates the breakpoint acknowledge cycle with BERR (i.e., no instruction word
available), the processor takes an illegal instruction exception. Figure 7-46 is a flowchart of
the breakpoint acknowledge cycle. Figure 7-47 shows the timing for a breakpoint
acknowledge cycle that returns an instruction word. Figure 7-48 shows the timing for a
breakpoint acknowledge cycle that signals an exception.
7.4.3 Coprocessor Communication Cycles
The MC68030 coprocessor interface provides instruction-oriented communication between
the processor and as many as seven coprocessors. The bus communication required to
support coprocessor operations uses the MC68030 CPU space with a type field of $2.
Coprocessor accesses use the MC68030 bus protocol except that the address bus supplies
access information rather than a 32-bit address. The CPU space type field (A16-A19) for a
coprocessor operation is $2. A13-A15 contain the coprocessor identification number (CpID),
and A0–A4 specify the coprocessor interface register to be accessed. Coprocessor
accesses to a CpID of zero correspond to MMU instructions and are not generated by the
MC68030 as a result of the coprocessor interface. These cycles can only be generated by
the MOVES instruction. Refer to
Section 10 Coprocessor Interface Description
for
further information.