Sun Microelectronics
46
UltraSPARC User’s Manual
A typical TLB miss and refill sequence is as follows:
1.
A TLB miss causes either an
instruction_access_MMU_miss
or a
data_access_MMU_miss
exception.
2.
The appropriate TLB miss handler loads the TSB Pointers and the TTE Tag
Target with loads from the MMU alternate space
3.
Using this information, the TLB miss handler checks to see if the desired
TTE exists in the TSB. If so, the TTE Data is loaded into the TLB Data In
register to initiate an atomic write of the TLB entry chosen by the
replacement algorithm.
4.
If the TTE does not exist in the TSB, the TLB miss handler jumps to a more
sophisticated (and slower) TSB miss handler.
The virtual address used in the formation of the pointer addresses comes from
the Tag Access register, which holds the virtual address and context of the load or
store responsible for the MMU exception. See Section 6.9, “MMU Internal Regis-
ters and ASI Operations,” on page 55. (Note that there are no separate physical
registers in UltraSPARC hardware for the Pointer registers, but rather they are
implemented through a dynamic re-ordering of the data stored in the Tag Access
and the TSB registers.)
Pointers are provided by hardware for the most common cases of 8 Kb and 64 Kb
page miss processing. These pointers give the virtual addresses where the 8 Kb
and 64 Kb TTEs would be stored if either is present in the TSB.
N is defined to be the TSB_Size field of the TSB register; it ranges from 0 to 7.
Note that TSB_Size refers to the size of each TSB when the TSB is split.
For a shared TSB (TSB register split field=0):
8K_POINTER = TSB_Base<63:13+N>
VA<21+N:13>
0000
64K_POINTER = TSB_Base<63:13+N>
VA<24+N:16>
0000
For a split TSB (TSB register split field=1):
8K_POINTER = TSB_Base<63:14+N>
0
VA<21+N:13>
0000
64K_POINTER = TSB_Base<63:14+N>
1
VA<24+N:16>
0000
For a more detailed description of the pointer logic with pseudo-code and hard-
ware implementation, see Section 6.11.3, “TSB Pointer Logic Hardware Descrip-
tion,” on page 70.
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