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5- 36
MC68349 USER’S MANUAL
MOTOROLA
5.6.2.5 SOFTWARE BREAKPOINTS. To support hardware emulation, the CPU32+ must
provide a means of inserting breakpoints into target code and of announcing when a
breakpoint is reached.
The MC68000 and MC68008 can detect an illegal instruction inserted at a breakpoint
when the processor fetches from the illegal instruction exception vector location. Since the
VBR on the CPU32+ allows relocation of exception vectors, the exception vector address
is not a reliable indication of a breakpoint. CPU32+ breakpoint support is provided by
extending the function of a set of illegal instructions ($4848–$484F).
When a breakpoint instruction is executed, the CPU32+ performs a read from CPU space
$0, at a location corresponding to the breakpoint number. If this bus cycle is terminated by
BERR , the processor performs illegal instruction exception processing. If the bus cycle is
terminated by DSACK≈, the processor uses the data returned to replace the breakpoint in
the instruction pipeline and begins execution of that instruction. See Section 3 Bus
Operation for a description of CPU space operations.
5.6.2.6 HARDWARE BREAKPOINTS. The CPU32+ recognizes hardware breakpoint
requests. Hardware breakpoint requests do not force immediate exception processing, but
are left pending. An instruction breakpoint is not made pending until the instruction
corresponding to the request is executed.
A pending breakpoint can be acknowledged between instructions or at the end of
exception processing. To acknowledge a breakpoint, the CPU performs a read from CPU
space $0 at location $1E (see Section 3 Bus Operation).
If the bus cycle terminates normally, instruction execution continues with the next
instruction as if no breakpoint request occurred. If the bus cycle is terminated by BERR ,
the CPU begins exception processing. Data returned during this bus cycle is ignored.
Exception processing follows the regular sequence. Vector number 12 (offset $30) is
internally generated. The PC of the executing instruction, the PC of the next instruction to
be executed, and a copy of the SR are saved on the supervisor stack.
5.6.2.7 FORMAT ERROR. The processor checks certain data values for control
operations. The validity of the stack format code and, in the case of a bus cycle fault
format, the version number of the processor that generated the frame are checked during
execution of the RTE instruction. This check ensures that the program does not make
erroneous assumptions about information in the stack frame.
If the format of the control data is improper, the processor generates a format error
exception. This exception saves a four-word format exception frame and then vectors
through vector table entry number 14. The stacked PC is the address of the RTE
instruction that discovered the format error.