Sun Microelectronics
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6. MMU Internal Architecture
6.11 TLB Hardware
6.11.1 TLB Operations
The TLB supports exactly one of the following operations per clock cycle:
•
Normal translation. The TLB receives a virtual address and a context identifier
as input and produces a physical address and page attributes as output.
•
Bypass. The TLB receives a virtual address as input and produces a physical
address equal to the truncated virtual address page attributes as output.
•
Demap operation. The TLB receives a virtual address and a context identifier
as input and sets the Valid bit to zero for any entry matching the demap page
or demap context criteria. This operation produces no output.
•
Read operation. The TLB reads either the CAM or RAM portion of the
specified entry. (Since the TLB entry is greater than 64 bits, the CAM and
RAM portions must be returned in separate reads. See Section 6.9.9, “I-/D-
TLB Data-In/Data-Access/Tag-Read Registers,” on page 64.)
•
Write operation. The TLB simultaneously writes the CAM and RAM portion
of the specified entry, or the entry given by the replacement policy described
in Section 6.11.2 .
•
No operation. The TLB performs no operation.
6.11.2 TLB Replacement Policy
UltraSPARC uses a 1-bit LRU scheme, very similar to that used in SuperSPARC.
Each TLB entry has an associated “valid,” “used,” and “lock” bit. On an automat-
ic write to the TLB initiated through an ASI store to register TLB Data In, the TLB
picks the entry to write based on the following rules:
1.
The first invalid entry will be replaced (measuring from TLB entry 0). If
there is no invalid entry, then:
2.
The first unused entry with its lock bit set to zero will be replaced
(measuring from TLB entry 0). If no unused entry has its lock bit set to
zero, then:
3.
All used bits are reset, and the process is repeated from Step 2 above.
Arbitrary entries may have their lock bit set, however, operation of the TLB is un-
defined if all entries have their lock bit set.
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