MPC555
/
MPC556
CENTRAL PROCESSING UNIT
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
3-39
tation-dependent software emulation interrupt. Invalid and preferred instruction forms
treatment by the MPC555 / MPC556 is described under the specific processor compli-
ance sections.
3.13.4 Exceptions
Invocation of the system software for any instruction-caused exception in the MPC555
/ MPC556 is precise, regardless of the type and setting.
3.13.5 The Branch Processor
3.13.6 Instruction Fetching
The core fetches a number of instructions into its internal buffer (the instruction pre-
fetch queue) prior to execution. If a program modifies the instructions it intends to ex-
ecute, it should call a system library program to ensure that the modifications have
been made visible to the instruction fetching mechanism prior to execution of the mod-
ified instructions.
3.13.7 Branch Instructions
The core implements all the instructions defined for the branch processor by the
UISA
in the hardware. For performance of various instructions, refer to
manual.
3.13.7.1 Invalid Branch Instruction Forms
Bits marked with
z
in the BO encoding definition are discarded by the MPC555 /
MPC556 decoding. Thus, these types of invalid form instructions yield result of the de-
fined instructions with the
z
bit zero. If the decrement and test CTR option is specified
for the
bcctr
or
bcctrl
instructions, the target address of the branch is the new value
of the CTR. Condition is evaluated correctly, including the value of the counter after
decrement.
3.13.7.2 Branch Prediction
The core uses the
y
bit to predict path for pre-fetch. Prediction is only done for not-
ready branch conditions. No prediction is done for branches to link or count register if
the target address is not ready. Refer to RCPU Reference Manual
(
Conditional Branch
Control) for more information.
3.13.8 The Fixed-Point Processor
3.13.8.1 Fixed-Point Instructions
The core implements the following instructions:
• Fixed-point arithmetic instructions
• Fixed-point compare instructions
• Fixed-point trap instructions
• Fixed-point logical instructions
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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