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546
March 2002 Release
1-800-255-7778
Virtex-II Pro™ Platform FPGA Documentation
R
Branch-Taken Debug Event
A branch-taken (BT) debug event occurs immediately
before
executing a
resolved (non-speculative) branch instruction. It is enabled by setting
DBCR0[BT]
=
1 and disabled by clearing DBCR0[BT]
=
0. The processor reports
the occurrence of a BT debug event by setting the BT bit in the debug-status
register (DBSR[BT]) to 1. After a BT event is recorded by a debugger, the status
bit should be cleared to prevent ambiguity when recording future debug
events.
The BT debug event
does not
set a DBSR status bit if all of the following are
true:
•
Internal-debug mode is enabled.
•
Debug exceptions are disabled.
•
External-debug mode is disabled.
Branches are a common event and this condition prevents the DBSR from
recording their obvious occurrence when exceptions are disabled.
This debug event is useful for single-stepping through branches to narrow the
search for code sequences of interest. Once identified, debug software can
enable IC debug events and single-step the code sequence instruction-by-
instruction.
If debug interrupts are enabled, the SRR2 register is loaded with the effective
address of the branch instruction that caused the BT event.
Exception-Taken Debug Event
An exception-taken (EDE) debug event occurs immediately
after
an exception
occurs, but before the first instruction in the exception handler is executed. It is
enabled by setting DBCR0[EDE]
=
1 and disabled by clearing DBCR0[EDE]
=
0.
The processor reports the occurrence of an EDE debug event by setting the
EDE bit in the debug-status register (DBSR[EDE]) to 1. After an EDE event is
recorded by a debugger, the status bit should be cleared to prevent ambiguity
when recording future debug events.
Noncritical exceptions always cause an EDE event when EDE is enabled.
Critical exceptions cause an EDE event only when EDE is enabled
and
external-debug mode is enabled.
This debug event is useful for debugging interrupt handlers. Upon entering
an interrupt handler, debug software can enable IC debug events and single-
step the handler instruction-by-instruction.
If debug interrupts are enabled, the SRR2 register is loaded with the 32-bit
exception-vector physical address. This corresponds to the effective address of
the first instruction in the interrupt handler.
Trap-Instruction Debug Event
A trap-instruction (TDE) debug event occurs immediately
before
executing a
trap instruction (
tw
or
twi
), if the conditions are such that a program exception
would normally occur (invoking the system trap-handler). If the trap
conditions are not met, the debug event does not occur and the program
executes normally. The event is enabled by setting DBCR0[TDE]
=
1 and
disabled by clearing DBCR0[TDE]
=
0. The processor reports the occurrence of
a TDE debug event by setting the TDE bit in the debug-status register
(DBSR[TDE]) to 1. After a TDE event is recorded by a debugger, the status bit
should be cleared to prevent ambiguity when recording future debug events.
Содержание Virtex-II Pro PPC405
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