MPC885 Instruction Set
MPC885 PowerQUICC Family Reference Manual, Rev. 2
5-18
Freescale Semiconductor
The original purpose of the sync instruction was to synchronize coherent memory with other processors
in a multiprocessor system; it makes sure that memory as seen by one processor is the same as memory
seen by the other processors, and broadcasts a special signal to signal that the action is taking place.
However, the MPC885 does not support this enforcement of coherency in a multiprocessor system, and it
broadcasts no special synchronization signal. The MPC885 simply expects other processors not to rely on
coherency of memory that it has cached in copy-back mode.
The only case where a sync instruction would be useful in an MPC8xx system is if software modified the
page table structure associated with the SMMU only and needed to guarantee that data accesses after that
instruction would be executed in the new data context. However, this is an unexpected special case; isync
would work here, but the pipeline need not be flushed in this case, so sync is sufficient.
5.2.5
PowerPC VEA Instructions
The PowerPC VEA describes the semantics of the memory model that can be assumed by software
processes, and includes descriptions of the cache model, cache control instructions, address aliasing, and
other related issues.
5.2.5.1
Processor Control Instructions
In addition to the move to condition register instructions specified by the UISA, the VEA defines the Move
from Time Base (mftb) instruction for reading the contents of the time base register. The mftb is a
user-level instruction and is shown in
Table 5-17
Simplified mnemonics are provided for the mftb instruction so it can be coded with the TBR name as part
of the mnemonic rather than requiring it to be coded as an operand. The mftb instruction serves as both a
basic and simplified mnemonic. Assemblers recognize an mftb mnemonic with two operands as the basic
form, and an mftb mnemonic with one operand as the simplified form. Simplified mnemonics are also
provided for Move from Time Base Upper (mftbu), which is a variant of the mftb instruction rather than
of mfspr. The MPC885 ignores the extended opcode differences between mftb and mfspr by ignoring bit
25 of both instructions and treating them both identically. For more information refer to Appendix F,
“Simplified Mnemonics,” in the Programming Environments Manual.
5.2.5.2
Memory Synchronization Instructions—VEA
Memory synchronization instructions control the order in which memory operations are completed with
respect to asynchronous events, and the order in which memory operations are seen by other processors or
memory access mechanisms. See
Chapter 7, “Instruction and Data Caches,”
for additional information
about these instructions and about related aspects of memory synchronization.
Table 5-18
lists the VEA memory synchronization instructions for the MPC885.
Table 5-17. Move from Time Base Instruction
Name
Mnemonic
Syntax
Move from Time Base
mftb
r
D, TBR
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