Freescale Semiconductor PowerPC e500 Core, Reference Manual

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PowerPC e500 Core Family Reference Manual, Rev. 1
3-44 Freescale Semiconductor
Instruction Model
3.3.6 Context Synchronization
Context synchronization is achieved by post- and presynchronizing instructions. An instruction is
presynchronized by completing all instructions before dispatching the presynchronized
instruction. Post-synchronizing is implemented by not dispatching any later instructions until the
post-synchronized instruction is completely finished.
3.4 Memory Access Alignment Support
The e500 core complex provides hardware support for misaligned memory accesses, but at the cost
of performance degradation. For loads that hit in the cache, the LSU’s throughput degrades to one
misaligned load every 3 cycles. Similarly, stores can be translated at a rate of one misaligned store
every 3 cycles. Additionally, after translation, each misaligned store is treated as two distinct
entries in the store queue, each requiring a cache access.
A word or half-word memory access requires multiple accesses if it crosses a double-word
boundary but not if it crosses a natural boundary. Vector loads and stores cause alignment
interrupts if they cross natural alignment boundaries (as shown in
Table 3-33 ).
Frequent use of misaligned memory accesses can greatly degrade performance.
Any load word or load half word that crosses a double-word boundary is interruptible, and
therefore can restart. If the first access has been performed when the interrupt occurs, it is
performed again when the instruction is restarted, even if it is to a page marked as guarded. Any
load word or load half word that crosses a translation boundary may take a translation exception
on the second access. In this case, the first access may have already occurred.
Table 3-33. Natural Alignment Boundaries for Extended Vector Instructions
Instruction Boundary
evld{d,w,h}
evld{d,w,h}x
evstd{d,w,h}
evstd{d,w,h}x
Double word
evlwwsplat{x}
evlwhe{x}
evlwhou{x}
evlwhos{x}
evlwhsplat{x}
evstwwe{x}
evstwwo{x}
evstwhe{x}
evstwho{x}
Word
evlhhesplat{x}
evlhhousplat{x}
evlhhossplat{x}
Half word