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MPC7400 RISC Microprocessor Users Manual
Memory Segment Model
5.4.5.2 AltiVec Line Fetch Skipping
As described in Chapter 7, òThe AltiVec Technology Implementation,ó there are many
conditions (exceptions, etc.) that cause the stream fetch performed by a VTQ stream engine
to abort. In the case of a VTQ-initiated table search operation, when an exception or
interrupt condition occurs, the stream engine pauses, the line-fetch that caused the table
search operation is effectively dropped, and no MMU exceptions are reported for this
line-fetch. When the stream engine resumes operation, the next line fetch is attempted,
causing a skip of one line fetch in the stream engine.
Also, when a
tlbsync
instruction is executed while a VTQ-initiated table search operation
is in progress, that table search operation is aborted, potentially causing a line fetch skip.
5.4.5.3 Page Table Search Operation Flow
The following is a summary of the page table search process performed by the MPC7400:
1. The 32-bit physical address of the primary PTEG is generated as described in òPage
Table Addressesó in Chapter 7, òMemory Management,ó of
The
Programming
Environments Manual
.
2. The Trst PTE (PTE0) in the primary PTEG is read from memory. PTE reads occur
with an implied WIM memory/cache mode control bit setting of 0b001. Therefore,
they are considered cacheable and read (burst) from memory and placed in the
cache. Because the table search operation is never speculative and is cacheable, the
G-bit has no effect
3. The PTE in the selected PTEG is tested for a match with the virtual page number
(VPN) of the access. The VPN is the VSID concatenated with the page index Teld
of the virtual address. For a match to occur, the following must be true:
PTE[H] = 0
PTE[V] = 1
PTE[VSID] = VA[0:23]
PTE[API] = VA[24:29]
4. If a match is not found, step 3 is repeated for each of the other seven PTEs in the
primary PTEG. If a match is found, the table search process continues as described
in step 8. If a match is not found within the 8 PTEs of the primary PTEG, the address
of the secondary PTEG is generated.
5. The Trst PTE (PTE0) in the secondary PTEG is read from memory. Again, because
PTE reads have a WIM bit combination of 0b001, an entire cache line is read into
the on-chip cache.