37-8
MPC860 PowerQUICC UserÕs Manual
MOTOROLA
Part VI. Debug and Test
37.2 Watchpoints and Breakpoints Support
Watchpoints, when detected, are reported to the external world (on dedicated pins), but do
not change the timing and ßow of the machine. Breakpoints, when detected, force the
machine to branch to the appropriate exception handler. The core supports watchpoints
generated inside the core and breakpoints generated inside and outside the core.
Internal watchpoints are generated when a user-programmable set of conditions are met.
Internal breakpoints can be programmed to be generated either as an immediate result of
the assertion of one of the internal watchpoints or after an internal watchpoint is asserted
for user-programmable times. Programming a certain internal watchpoint to generate an
internal breakpoint can be done either in software, by setting the corresponding software
trap enable bit or on-the-ßy using the serial interface implemented in the development port
to set the corresponding trap enable bit. External breakpoints can be generated by
peripherals of the system outside of the MPC860 such as an external development system.
Peripherals on the external bus use the serial interface of the development port to assert the
external breakpoint.
In the core, as in other RISC processors, software saves and restores machine state as part
of exception handling. As software saves/restores the machine state, MSR[RI] is cleared.
Exceptions that occur are handled by the core when MSR[RI] is clear and they result in a
nonrestartable machine state. See Section 7.1.5, ÒRecoverability after an Exception.Ó
In general, the core recognizes breakpoints only if MSR[RI] = 1, which guarantees machine
restartability after a breakpoint. In this working mode, breakpoints are said to be masked.
Sometimes it is preferable to enable breakpoints when MSR[RI] is clear, despite the risk of
a nonrestartable machine state. Internal breakpoints also have a programmable nonmasked
mode, and an external development system can choose to assert a nonmaskable external
breakpoint. Watchpoints are not masked and are always reported on external pins,
regardless of the value of MSR[RI]. Although they count watchpoints, counters are part of
the internal breakpoint logic and are not decremented when the core operates in masked
mode and MSR[RI] = 0. Figure 37-1 shows the coreÕs watchpoint and breakpoint support.
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