UM0434 datasheet(357/390 Pages) STMICROELECTRONICS

357 / 390 page

Nexus3 module

UM0434

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Execution mode indication

In order for a development tool to properly interpret instruction count and history

information, it must be aware of the execution mode context of that information. VLE

instructions will be interpreted differently from non-VLE instructions.

Program trace messages provide the execution mode status in the least significant bit of the

reconstructed address field. A value of zero indicates that preceding instruction count and

history information should be interpreted in a non-VLE context. A value of one indicates that

the preceding instruction count and history information should be interpreted in a VLE

context. Note that when a branch results in an execution mode 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

Chapter : Program correlation messages on page 354,” for more information

on these cases.

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

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.

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.

Table 257.

Relative address generation and re-creation example

Message generation

0000 0000 0000 0011 1111 1100 0000 0001

0000 0000 0000 0011 1111 0011 0110 0101

⊕ A2

0000 0000 0000 0000 0000 1111 0110 0100

(Address Message)

1111 0110 0100

Address Re-creation

0000 0000 0000 0011 1111 1100 0000 0001

0000 0000 0000 0000 0000 1111 0110 0100

⊕ M1 (A2)

0000 0000 0000 0011 1111 0011 0110 0101

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