
Sun Microelectronics
44
UltraSPARC User’s Manual
Note:
The E-bit does not force an uncacheable access. It is expected, but not
required, that the CP and CV bits will be set to zero when the E-bit is set.
P
:
Privileged. If the P bit is set, only the supervisor can access the page
mapped by the TTE. If the P bit is set and an access to the page is
attempted when PSTATE.PRIV=0, the MMU will signal an
instruction_access_exception
or
data_access_exception
trap (FT=1
16
).
W
:
Writable. If the W bit is set, the page mapped by this TTE has write
permission granted. Otherwise, write permission is not granted and the
MMU will cause a
data_access_protection
trap if a write is attempted. The
W-bit in the I-MMU is read as zero and ignored when written.
G
:
Global. This bit must be identical to the Global bit in the TTE tag. Similar
to the case of the Valid bit, the Global bit in the TTE tag is necessary for
the TSB hit comparison, while the Global bit in the TTE data facilitates
the loading of a TLB entry.
Compatibility Note:
Referenced and Modified bits are maintained by software. The Global, Privileged,
and Writable fields replace the 3-bit ACC field of the SPARC-V8 Reference MMU
Page Translation Entry.
6.3 Translation Storage Buffer (TSB)
The TSB is an array of TTEs managed entirely by software. It serves as a cache of
the Software Translation Table, used to quickly reload the TLB in the event of a
TLB miss. The discussion in this section assumes the use of the hardware support
for TSB access described in Section 6.3.1, “Hardware Support for TSB Access,” on
page 45, although the operating system is not required to make use of this sup-
port hardware.
Inclusion of the TLB entries in the TSB is not required; that is, translation infor-
mation may exist in the TLB that is not present in the TSB.
The TSB is arranged as a direct-mapped cache of TTEs. The UltraSPARC MMU
provides precomputed pointers into the TSB for the 8 Kb and 64 Kb page TTEs.
In each case, N least significant bits of the respective virtual page number are
used as the offset from the TSB base address, with N equal to log base 2 of the
number of TTEs in the TSB.
A bit in the TSB register allows the TSB 64 Kb pointer to be computed for the case
of common or split 8 Kb/64 Kb TSB(s).
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