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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