Service manual

Overview 1-3
What do I need to know about the system clock, CPU variants,
and the system box QBBs?
Early GS160/320 systems used the B4125 CPU module, which supported a
4-Mbyte backup cache. The QBBs in the early system boxes (BA51A-AA)
supported the 4-Mbyte B-cache. With the introduction of the B4166 CPU
modules which support an 8-Mbyte B-cache comes a system box (BA51A-BA)
with QBBs that support the larger B-cache. Furthermore, the CPU derives its
operating clock speed from the system clock, so you need to know the setting of
the system clock. Most of these components can coexist in the same system.
But for optimum performance the B4166 CPUs should be in a system box with
QBBs that support the 8-Mbyte B-cache and in a system with the 9.0 ns clock
setting.
Table 1–3 compares the B4125 and B4166 CPUs, and Table 1–4 compares the
system boxes.
Table 1–3 Comparison of CPUs
CPU Part
Number
B-cache
Size
Speed
in MHz Comments
B4125 4 Mbytes 731
The B4125 must operate at this speed and requires
the system clock to run at 9.6 ns.
B4166
4or8
Mbytes
1001
The B4166 runs at this speed and uses all its avail-
able B-cache when all CPUs in the system are B4166,
the clock runs at 9.0 ns, and the CPUs are in a system
box with duplicate tag support of the 8-MB cache. If
the system box duplicate tag supports only 4-MB
cache, then only 4 of the 8 Mbytes of B-cache on the
CPU are used.
4or8
Mbytes
940
The B4166 runs at this speed and uses all its avail-
able B-cache when all CPUs in the QBB are B4166,
and the clock runs at 9.6 ns. This occurs because the
system clock is run at 9.6 ns. If the system box du-
plicate tag supports only 4-MB cache, then only 4 of
the 8 Mbytes of B-cache on the CPU are used.
Uses 4 of
its 8 Mbyte
B-cache
731
The B4166 runs at this speed and uses 4 Mbytes of its
B-cache when both CPU variants (B4125 and B4166)
are placed in the same QBB and the clock runs at 9.6
ns.