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real page number (RPN), access control bits (ZSEL, EX, WR), and storage attributes (W, I, M, G, E,
UO) ..
PID (Process 10)
o
TLBHI (Tag entry)
o
TLBLO (Data entry)
o
EPN
RPN
23 24
31
ID
21 22 242526 27 128
21 22 23 24
2728293031
Figure
6·2.
TLB Entries
The virtual address space is extended by adding an 8-bit translation ID (TID) loaded from the Process
ID (PID) register during a TLB access. The PID identifies one of 255 unique software entities, usually
used as a process or thread ID. TLBHI[TID] is compared to the PID during a TLB look-up.
Tag and data entries are written by copying data from GPRs and the PID, using the tlbwe instruction.
Tag and data entries are read by copying data to GPRs and the PID, using the tlbre instruction.
Software can search for specific entries using the tlbsx instruction.
6.3.2
TLB Fields
Each TLB entry describes a page that is enabled for translation and access controls. Fields in the TLB
entry fall into four categories:
• Information required to identify the page to the hardware translation mechanism
• Control information specifying the translation
• Access control information
• Storage attribute control information
6.3.2.1
Page Identification Fields
When an EA is presented to the MMU for processing, the MMU applies several selection criteria to
each TLB entry to select the appropriate entry. Although it is possible to place multiple entries into the
TLB to match a specific EA and PID, this is considered a programming error, and the result of a TLB
lookup for such an EA is undefined. The following fields in the TLB entry identify the page. Except as
noted, all comparisons must succeed to validate an entry for subsequent use.
EPN (effective page number, 22 bits)
Compared to some number of the EAo:21 bits presented to the MMU. The number of bits corresponds
to the page size.
Preliminary
Memory Management
6-3
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