Nexus3/ Module
e200z3 Power Architecture Core Reference Manual, Rev. 2
Freescale Semiconductor
10-33
switch, the program trace message resulting from that branch will indicate the previous execution state.
The new state will not be signaled until the next program trace message.
In some cases, a program correlation message is generated to indicate execution mode status. Refer to
Section 10.7.2.6, “Program Correlation Messages
,” for more information on these cases.
10.7.3.4
Branch/Predicate Instruction History (HIST)
If DC1[PTM] is set, BTM messaging uses the branch history format. The branch history (HIST) packet in
these messages provides a history of direct branch execution used for reconstructing program flow. This
packet is implemented as a left-shifting shift register. The register is always pre-loaded with a value of one.
This bit acts as a stop bit so that the development tools can determine which bit is the end of the history
information. The pre-loaded bit itself is not part of the history but is transmitted with the packet.
A value of one is shifted into the history buffer on a taken branch, conditional or unconditional, and on any
instruction whose predicate condition executed as true. A value of zero is shifted into the history buffer on
any instruction whose predicate condition executed as false, as well as on branches not taken. This includes
indirect as well as direct branches not taken. For the evsel instruction, two bits are shifted in, corresponding
to the low element shifted in first, and the high element shifted in second.
10.7.3.5
Sequential Instruction Count (I-CNT)
The I-CNT packet is present in all BTM messages. For traditional branch messages, I-CNT represents the
number of sequential instructions, or non-taken branches, in between direct/indirect branch messages.
For branch history messages, I-CNT represents the number of instructions executed since the last
taken/non-taken direct branch, predicate instruction, last taken/not-taken indirect branch, or exception.
Branch instructions that trigger message generation are included in the I-CNT. Instructions that generate
history bits are not included in the I-CNT.
The sequential instruction counter overflows when its value reaches 255 and is reset to 0. The next BTM
message (corresponding to the 256th or later instruction) is converted to a synchronization type message.
10.7.3.6
Program Trace Queueing
Nexus3/ implements a programmable depth queue (a minimum of 32 entries is recommended)
for queuing all messages. Messages that enter the queue are transmitted through the auxiliary pins in the
order in which they are queued.
NOTE
If multiple trace messages need to be queued at the same time, watchpoint
messages have the highest priority:
(WPM
→
OTM
→
BTM
→
DTM).
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