HP Vectra XU 6/XXX HP Vectra XU 6/xxx, Guide to Optimizing performance - Page 63

2 Technical Reference The Intel Pentium Pro Processor Transactional Bus 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. 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