
MOTOROLA
8-4
DEVELOPMENT SUPPORT
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RCPU
REFERENCE MANUAL
branch instructions, the processor marks these instructions as indirect branch in-
structions (VF = 101, see
Table 8-3
) and marks the subsequent instruction address
with the indirect change-of-flow attribute, as if it were an indirect branch target.
Therefore, when the processor detects one of these instructions, the address of the
following instruction is visible externally. This enables the reconstructing software
to correctly evaluate the effect of these instructions.
8.1.2 Instruction Fetch Show Cycle Control
Instruction fetch show cycles are controlled by the bits in the ICTRL and the state
of VSYNC, as illustrated in
Table 8-2
.
Note that when the value of the ISCTL field is changed (with the
mtspr
instruction),
the new value does not take effect until two instructions after the
mtspr
instruction.
The instruction immediately following
mtspr
is under control of the old ISCTL val-
ue.
In order to keep the pin count of the chip as low as possible, VSYNC is not imple-
mented as an external pin; rather, it is asserted and negated using the develop-
ment port serial interface. For more information on this interface refer to
8.3.5 Trap-
Enable Input Transmissions
.
The assertion and negation of VSYNC forces the machine to synchronize and the
first fetch after this synchronization to be marked as an indirect change-of-flow cy-
cle and to be visible on the external bus. This enables the external hardware to syn-
chronize with the internal activity of the processor.
When either VSYNC is asserted or the ISCTL bits in the I-bus control register are
programmed to a value of 0b10, cycles resulting from an indirect change-of-flow
are shown on the external bus. By programming the ISCTL bits to show all indirect
flow changes, the user can thus ensure that the processor maintains exactly the
same behavior when VSYNC is asserted as when it is negated. The loss of perfor-
mance the user can expect from the additional external bus cycles is minimal.
For additional information on the ISCTL bits and the ICTRL register, refer to
8.8 De-
velopment Support Registers
. For more information on the use of VSYNC during
program trace, refer to
8.1.4 External Hardware During Program Trace
.
Table 8-2 Fetch Show Cycles Control
VSYNC
ISCTL (Instruction Fetch
Show Cycle Control Bits)
Show Cycles Generated
X
00
All fetch cycles
X
01
All change-of-flow (direct & indirect)
X
10
All indirect change-of-flow
0
11
No show cycles are performed
1
11
All indirect change-of-flow
F
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n
.