Higher Clock Speeds; The Pentium Pro Local Bus - HP Vectra XU 6/XXX Optimization Manual

2 Technical Reference
The Intel Pentium Pro Processor

Higher Clock Speeds

The Pentium Pro processor operates at higher internal clock
frequencies than any previous x86 family processor. Typically, the
Pentium Pro uses an internal clock that is up to three times faster than
the clock used for the local bus.
This increase in clock speeds has been made possible by superpipeline
architecture and the additional functions that the Pentium Pro
integrates. With a 256 KB level-two cache memory, the Pentium Pro is
able to perform much of its work internally, without needing to access
its local bus.

The Pentium Pro Local Bus

The new features implemented by the Pentium Pro processor are not
limited to its internal operation; the Pentium Pro also uses a new
protocol for memory accesses performed over its local bus.
Previous generation processors in the x86 family used a simple
protocol for memory accesses on the local bus; once a bus cycle had
been initiated it had to be completed before another access could be
begun. As memory accesses require a number of clock cycles to
complete, this meant that the local bus would be stalled until the
access was complete.
Transactional Bus
The Pentium Pro supports a transactional local bus, which allows up to
eight memory accesses to be outstanding at any one time. Once a
memory access has been issued on the local bus, the Pentium Pro can
issue other memory accesses while waiting for the first access to
complete. By using this transactional protocol, the Pentium Pro
processor is able to make better use of its local bus.
To use the transactional protocol, the Pentium Pro processor requires
support from other devices on the local bus, such as the memory
controller. If this support is not available, the Pentium Pro will use the
simple protocol, used by previous generation x86 family processors, for
memory accesses.
English 57