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IDT EJTAG System
Hardware Breakpoints
79RC32438 User Reference Manual
20 - 41
November 4, 2002
Notes
Thus, in the case a data breakpoint with data value compare is set up on a load instruction, the load
does occur from the external memory, since the data value is required to evaluate the match condition, but
the destination register is not updated, so the loaded value is simply discarded. The rules shown in Table
20.26 describe update of the BS bits when several data breakpoints match the same access and generate
a debug exception
Any BS bit set prior to the match and debug exception is kept set, since only debug software can clear
the BS bits.
The debug handler usually returns to the instruction that caused the Debug Data Break Load/Store
exception, whereby the instruction is re-executed. This re-execution results in a repeated load from system
memory after a data breakpoint with a data value compare on a load, because the load occurred previously
in order to allow evaluation of the breakpoint as described above. Memory-mapped devices with side
Instruction and
Data
Breakpoint
Load/Store
Instruction
Execution
Destination
Register
External
Memory
System
Access
Store with/without
value match
Not completed
Not updated
1
1.
This applies to the Store Conditional Word/Doubleword (SC/SCD instructions.
2.
This includes side effects like Load Linked Word/Doubleword (LL/LLD) instructions.
Store to memory is
not committed
Load without value
match
Not updated
2
Load from memory
does not occur
Loan with value match
Load from memory
does occur
Table 20.25 Behavior on Precise Exceptions from Data Breakpoints
Instruction
Breakpoints that Match
Update of BS Bits Matching
Data Breakpoints
Without
Value
Compare
With Value
Compare
Without Value
Compare
With Value
Compare
Load / Store
One or more
None
BS bits set for all
No matching break-
points
Load
One or more
One or more
BS bits set for all
Unchanged BS bits
since load of data
value does not
occur, so match of
the breakpoint cant
be determined
Load
None
One or more
No matching break-
points
BS bits set for all
Store
One or more
One or more
BS bits set for all
Optional to either set
BS bits for all, or
change none of the
BS bits
Store
None
One or more
No matching break-
points
BS bits set for all
Table 20.26 Behavior on Precise Exceptions from Data Breakpoints