IBM PowerPC 604 User Manual page 221

-
PA+- PA+ 8
(Fetch Next PTE in PTEG)
Secondary Page
Table Search
Generate PA Using Secondary Hash Function
PA+- Base PA
ot
PTEG
Fetch PTE from PTEG
Fetch PTE
(64
Bits)
from PA
otherwise
-~
PTE [VSID, API, H,
VI=
Segment DeSCf\:>tor [VSID), EA[APg, 1, 1
Secondary Page
Table Search Hit
Last PTE in PTEG
J:··~·
(See Figure S-9)
Instruction Access Data Access
ISi Exception
OSI Exception
Figure s-10. Secondary Page Table Search Flow
H the address in one of the two selected TLB entries is valid and matches the virtual
address, that TLB entry contains the physical address. If no match is found, a TLB miss
occurs and, if this is an in-order access, a hardware table search operation begins. Once the
matching PTE is found in memory, it is loaded into the appropriate TLB entry depending
on the LRU bit setting and translation continues.
The LSU initiates out-of-order accesses without knowledge of whether it is legal to do so.
Therefore, the MMU does not perform hardware table search due to TLB misses until the
request is nonspeculative. In these out-of-order cases, the MMU does detect protection
violations and whether a dcbz instruction specifies a page marked as write-through or
cache-inhibited. The MMU also detects alignment exceptions caused by the dcbz
instruction, which prevents the changed bit in the PTE from being updated erroneously.
Note that when a TLB miss occurs, the MMU does not begin the table search operation if
the access is out of order.
5-32 PowerPC 604 RISC Microprocessor User's Manual