Chapter 7. The AltiVec Technology Implementation
7-9
AltiVec Technology and the Programming Model
Like all other termination conditions not mentioned in Table 7-5, asserting SRESET does
not terminate a
dst
x
stream.
7.1.2.5.3 Line Fetch Skipping
When an exception condition occurs, the MPC7400 terminates any
dst
x-
initiated table
search operations and pauses the stream engine that initiated the table search. In this
situation, the line fetch of the
dst
that caused the table search is effectively dropped and any
translation exception that would have terminated the stream had the table search operation
completed does not occur. Instead, the engine attempts the next line fetch when the stream
resumes. This, in effect, causes a skip of one line fetch in the stream engine.
Also note that the execution of a
tlbsync
instruction cancels any
dst
x
-initiated table search
operation in progress, which can cause a line fetch skip.
7.1.2.5.4 Context Awareness and Stream Pausing
Stream accesses can take place only when data translation is enabled (MSR[DR] = 1), and
when the processor is in the same privilege state as it was when the
dst
x
instruction was
executed.
If the value privilege level changes or if data translation is disabled, the stream engine
suspends generation of new accesses. Any outstanding transactions initiated before the
pause (like cache reTlls and bus activity) Tnish normally. The stream engine resumes when
translation is again enabled and the privilege level again matches the level in place when
the
dst
x
instruction for that stream was executed.
Table 7-5. The dst
x
Stream Termination Conditions
Conditions
Successfully reached end of stream
dst
x
stream is still speculative with respect to program ow and the control unit issues a cancel due to a mispredicted
branch or exception.
Another
dst
x
instruction to this stream tag is executed and this new
dst
x
is non-speculative with respect to branch
prediction.
A
dss
instruction to this stream tag is completed.
Current line fetch caused a table search operation that did not Tnd a matching entry in the page table.
Current line fetch is translated as cache-inhibited.
Current line fetch is translated as write-through and the stream is a touch-for-store.
Current line fetch is translated to direct-store space.
Current line fetch is to a protected page.
L1 data cache is locked or disabled.
The processor has encountered a condition that causes a machine check exception.