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Rev. 5.00, 09/03, page 66 of 760
31 16 111217 0
Virtual address
Ways 0−3
VPN(31−17) VPN(11−10)
ASID(7−0) V
0
Address array Data array
PPN(28−10) PR(1−0) SZ C D SH
Index
31
Figure 3.7 TLB Indexing (IX = 0)
3.3.3 TLB Address Comparison
The results of address comparison determine whether a specific virtual page number is registered
in the TLB. The virtual page number of the virtual address that accesses external memory is
compared to the virtual page number of the indexed TLB entry. The ASID within the PTEH is
compared to the ASID of the indexed TLB entry. All four ways are searched simultaneously. If the
compared values match, and the indexed TLB entry is valid (V bit = 1), the hit is registered.
It is necessary to have software ensure that TLB hits do not occur simultaneously in more than one
way, as hardware operation is not guaranteed if this occurs. For example, if there are two identical
TLB entries with the same VPN and a setting is made such that a TLB hit is made only by a
process with ASID = H'FF when one is in the shared state (SH = 1) and the other in the non-shared
state (SH = 0), then if the ASID in PTEH is set to H'FF, there is a possibility of simultaneous TLB
hits in both these ways. It is therefore necessary to ensure that this kind of setting is not made by
software.
The object compared varies depending on the page management information (SZ, SH) in the TLB
entry. It also varies depending on whether the system supports multiple virtual memory or single
virtual memory.
The page-size information determines whether VPN (11–10) is compared. VPN (11–10) is
compared for 1-kbyte pages (SZ = 0) but not for 4-kbyte pages (SZ = 1).