
Sun Microelectronics
8
UltraSPARC User’s Manual
1.3.6 Memory Management Unit (MMU)
The MMU provides mapping between a 44-bit virtual address and a 41-bit phys-
ical address. This is accomplished through a 64-entry iTLB for instructions and a
64-entry dTLB for data; both TLBs are fully associative. UltraSPARC provides
hardware support for a software-based TLB miss strategy. A separate set of glo-
bal registers is available to process MMU traps. Page sizes of 8Kb (13-bit offset),
64Kb (16-bit offset), 512Kb (19-bit offset), and 4Mb (22-bit offset) are supported.
1.3.7 Load/Store Unit (LSU)
The LSU is responsible for generating the virtual address of all loads and stores
(including atomics and ASI loads), for accessing the D-Cache, for decoupling
load misses from the pipeline through the Load Buffer, and for decoupling stores
through the Store Buffer. One load or one store can be issued per cycle.
1.3.8 Data Cache (D-Cache)
The D-Cache is a write-through, non-allocating, 16Kb direct-mapped cache with
two 16-byte sub-blocks per line. It is virtually indexed and physically tagged
(VIPT). The tag array is dual ported, so tag updates due to line fills do not collide
with tag reads for incoming loads. Snoops to the D-Cache use the second tag
port, so they do not delay incoming loads.
1.3.9 External Cache Unit (ECU)
The main role of the ECU is to handle I-Cache and D-Cache misses efficiently.
The ECU can handle one access per cycle to the External Cache (E-Cache). Ac-
cesses to the E-Cache are pipelined, which effectively makes the E-Cache part of
the instruction pipeline. Programs with large data sets can keep data in the
E-Cache and can schedule instructions with load latencies based on E-Cache la-
tency. Floating-point code can use this feature to effectively hide D-Cache misses.
Table 1-5 on page 10 shows the E-Cache sizes that each UltraSPARC model sup-
ports. Regardless of model, however, the E-Cache line size is always 64 bytes.
UltraSPARC uses a MOESI (Modified, Own, Exclusive, Shared, Invalid) protocol
to maintain coherence across the system.
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