Sun Microelectronics
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4. Overview of the MMU
Figure 4-2
UltraSPARC’s 44-bit Virtual Address Space, with Hole (Same as Figure 14-2)
Note:
Throughout this document, when virtual address fields are specified as
64-bit quantities, they are assumed to be sign-extended based on VA<43>.
The operating system maintains translation information in a data structure called
the Software Translation Table. The I- and D-MMU each contain a hardware
Translation Lookaside Buffer (iTLB and dTLB); these act as independent caches of
the Software Translation Table, providing one-cycle translation for the more fre-
quently accessed virtual pages.
Figure 4-3 on page 24 shows a general software view of the UltraSPARC MMU.
The TLBs, which are part of the MMU hardware, are small and fast. The Software
Translation Table, which is kept in memory, is likely to be large and complex. The
Translation Storage Buffer (TSB), which acts like a direct-mapped cache, is the in-
terface between the two. The TSB can be shared by all processes running on a
processor, or it can be process specific. The hardware does not require any partic-
ular scheme.
The term “TLB hit” means that the desired translation is present in the MMU’s
on-chip TLB. The term “TLB miss” means that the desired translation is not
present in the MMU’s on-chip TLB. On a TLB miss the MMU immediately traps
to software for TLB miss processing. The TLB miss handler has the option of fill-
ing the TLB by any means available, but it is likely to take advantage of the TLB
miss support features provided by the MMU, since the TLB miss handler is time
critical code. Hardware support is described in Section 6.3.1, “Hardware Support
for TSB Access,” on page 45.
FFFF FFFF FFFF FFFF
FFFF F800 0000 0000
0000 0000 0000 0000
0000 07FF FFFF FFFF
Out of Range VA
(VA “Hole”)
FFFF F7FF FFFF FFFF
0000 0800 0000 0000
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