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MPC7400 RISC Microprocessor Users Manual
Execution Unit Timings
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Little-endian store operations
Floating-point store operations
If store gathering is enabled and the stores do not fall under the above categories, an
eieio
or
sync
instruction must be used to prevent two stores from being gathered.
6.4.6 System Register Unit Execution Timing
Most instructions executed by the SRU either directly access renamed registers or either
access or modify nonrenamed registers. Instructions generally execute in strict order.
Results from these instructions are not available to subsequent instructions until the
instruction completes and is retired. See Section 6.3.4.2, òInstruction Serialization,ó for
more information on serializing instructions executed by the SRU. Table 6-4 and Table 6-5
show SRU instruction execution timings.
6.4.7 AltiVec Instructions Executed by the LSU
The LSU execute the AltiVec LRU and transient instructions.
6.4.7.1 LRU Instructions
The AltiVec architecture speciTes that the
lvxl
and
stvxl
instructions differ from other
AltiVec load and store instructions in that they leave cache entries in a least-recently-used
(LRU) state instead of a most-recently-used state. This is used to identify data that is known
to have little reuse and poor caching characteristics.
On the MPC7400, these instructions follow the cache allocation and replacement policies
described in Chapter 3, òL1 and L2 Cache Operation,ó but they leave their addressed cache
entries in the LRU state. In addition, all LRU instructions are also interpreted to be transient
and are also treated as described in the next section. Additional discussion on LRU effects
may be found in Chapter 3, òL1 and L2 Cache Operation.ó
6.4.7.2 Transient Instructions
The AltiVec architecture describes a difference between static and transient memory
accesses.
A static memory access should have some reasonable degree of locality and be referenced
several times or reused over some reasonably long period of time. A transient memory
reference has poor locality and is likely to be referenced a very few times or over a very
short period of time.
The MPC7400 supports both static and transient memory access behavior.