Sun Microelectronics
62
UltraSPARC User’s Manual
Split
:
When Split=1, the TSB 64 Kb Pointer address is calculated assuming
separate (but abutting and equally-sized) TSB regions for the 8 Kb and
the 64 Kb TTEs. In this case, TSB_Size refers to the size of each TSB, and
therefore the TSB 8Kb Pointer address calculation is not affected by the
value of the Split bit. When Split=0, the TSB 64 Kb Pointer address is
calculated assuming that the same lines in the TSB are shared by 8 Kb
and 64 Kb TTEs, called a “common TSB” configuration.
Warning –
In the “common TSB” configuration (TSB.Split=0), 8 Kb and 64 Kb
page TTEs can conflict, unless the TLB miss handler explicitly checks the TTE for
page size. Therefore, do not use the common TSB mode in an optimized handler.
For example, suppose an 8K page at VA=2000
16
and a 64K page at VA=10000
16
both exist, which is a legal situation. These both want to exist at the second TSB
line (line 1), and have the same VA tag of 0. Therefore, there is no way for the
miss handler to distinguish these TTEs based on the TTE tag alone, and unless it
reads the TTE data, it may load an incorrect TTE.
I/D TSB_Size
: The Size field provides the size of the TSB according to the
following:
•
Number of entries in the TSB (or each TSB if split)=512
×
2
TSB_Size
.
•
Number of entries in the TSB ranges from 512 entries at TSB_Size=0
(8 Kb common TSB, 16 Kb split TSB), to 64 Kb entries at TSB_Size=7
(1 Mb common TSB, 2 Mb split TSB).
Note:
Any update to the TSB register immediately affects the data that is
returned from later reads of the Tag Target and TSB Pointer registers.
6.9.7 I-/D-TLB Tag Access Registers
In each MMU the Tag Access register is used as a temporary buffer for writing
the TLB Entry tag information. The Tag Access register may be updated during
either of the following operations:
1.
When the MMU signals a trap due to a miss, exception, or protection. The
MMU hardware automatically writes the missing VA and the appropriate
Context into the Tag Access register to facilitate formation of the TSB Tag
Target register. See Table 6-4 on page 51 for the SFSR and Tag Access
register update policy.
2.
An ASI write to the Tag Access register. Before an ASI store to the TLB
Data Access registers, the operating system must set the Tag Access
register to the values desired in the TLB Entry. Note that an ASI store to the
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