Hardware Reference
In-Depth Information
needs is in its own cache, the word is returned. Otherwise, the snooping logic
checks if a copy of that word is available somewhere else on the boardset. If so,
the request is satisfied.
18 address
crossbar switch as described below. The snooping logic can do one snoop per
clock cycle. The system clock runs at 150 MHz, so it is possible to perform 150
million snoops/sec per boardset or 2.7 billion snoops/sec system wide.
Although the snooping logic is logically a bus, as portrayed in Fig. 8-34, phys-
ically it is a device tree, with commands being relayed up and down the tree.
When a CPU or PCI board puts out an address, it goes to an address repeater via a
point-to-point connection, as shown in Fig. 8-35. The two address repeaters con-
verge on the expander board, where the addresses are sent back down the tree for
each device to check for hits. This arrangement is used to avoid having a bus that
involves three boards.
If not, the request is passed on via the 18
×
18 x 18 Address
crossbar switch
18 x 18 Response
crossbar switch
18 x 18 Data
crossbar switch
Level 3
3x3
Data
switch
Level 2
Directory and
snooping management
3 x 3 Data switch
3x3
Data
switch
Addr
rep.
Addr
rep.
Level 1
CPU
C
C
C
C
PCI
cont.
PCI
cont.
5x5
Data
switch
5x5
Data
switch
Level 0
Memory
M
M
M
M
Figure 8-35.
The Sun Fire E25K uses a four-level interconnect. Dashed lines
are address paths. Solid lines are data paths.
Data transfers use a four-level interconnect as depicted in Fig. 8-35. This de-
sign was chosen for high performance. At level 0, pairs of CPU chips and memo-
ries are connected by a small crossbar switch that also has a connection to level 1.
The two groups of CPU-memory pairs are connected by a second crossbar switch
at level 1. The crossbar switches are custom ASICs. For all of them, all the inputs
are available on both the rows and the columns, although not all combinations are
used (or even make sense). All the switching logic on the boards is built from
3
×
3 crossbars.
