
Development Capabilities and Interface
20-8
MPC823 USER’S MANUAL
MOTOROLA
DEVELOPMENT
20
CAPABILITIES
&
INTERFACE
20.2.1.3 COMPRESSION OF CANCELLED INSTRUCTIONS. To store all the information
generated on the pins during program trace (5 bits per clock + 30 bits per show cycle), a
large memory buffer is required. However, since this information includes events that were
cancelled, compression is possible and can be very beneficial in this situation. External
hardware can be added to eliminate all canceled instructions and reports only on taken or
not taken branches, indirect flow change, and the number of sequential instructions after the
last ow change.
20.2.2 Controlling Instruction Fetch Show Cycles
Instruction fetch show cycles are controlled by the bits in the ICTRL register and the state
of the VSYNC signal. The following table defines the level of fetch show cycles generated
by the core.
Table 20-3 shows the types of fetch show cycles determined by the ISCT_SER
field. A cycle marked with the program trace cycle attribute is generated for any change in
the state of VSYNC.
20.3 GENERATING WATCHPOINTS AND BREAKPOINTS
When detected, watchpoints are reported to the external world on dedicated pins but do not
change the timing and flow of the machine. When breakpoints are detected, they force the
machine to branch to the appropriate exception handler. The core supports watchpoints that
are generated inside the core as well as breakpoints that are generated inside and outside
the core.
In the core, as in other RISC processors, saving and restoring the machine state on the
stack during exception handling is done in the software. When the software is in the middle
of saving and restoring, the MSRRI bit is cleared. Exceptions that occur are handled by the
core when the MSRRI bit is clear and they result in a nonrestartable machine state. For more
Table 20-3. Fetch Show Cycle Types
VSYNC
ISCT_SER INSTRUCTION
FETCH SHOW CYCLE
CONTROL FIELD
SHOW CYCLES
GENERATED
X
000
All Fetch Cycles
X
X01
All Change of Flow (Direct and
Indirect)
X
X10
All Indirect Change of Flow
0
X11
No Show Cycles Are Performed
1
X11
All Indirect Change of Flow
NOTE: Only cycles that access storage assert the TS signal. All cycles that involve “show
cycles” are marked by asserting the STS signal. When you need to sample the
show cycle address and attributes, the STS signal should be enabled by