HP Vectra XU 6/XXX HP Vectra XU 6/XXX - Guide to Optimization Performance - Page 35

CHARACTERISTICS The Pentium Pro processor uses a 64-bit external data path and a 36-bit address bus. It integrates two 8 KB level-one cache memories (one for instructions, one for data) and a 256 KB level-two cache. Internally it uses three pipelines and five execution units for instruction execution. The table below summarizes the Pentium Pro's characteristics and compares them with those of the Intel Pentium processor. Data bus Address bus Level-one cache Level-two cache Superscaler design Execution units Branch prediction Speculative execution Out-of-order execution Intel Pentium Pro 64-bit 36-bit 8 KB data, 8 KB instruction 256 KB Three-way Five Advanced dynamic Yes Yes Intel Pentium 64-bit 32-bit 8 KB data, 8 KB instruction None Two-way Three Dynamic No No SUPERPIPELINE ARCHITECTURE Three Pipelines The key to the Pentium Pro processor's improved performance is its superpipelined architecture, which allows it to execute instructions more quickly. Each instruction is executed in a number of steps through a mechanism called a pipeline. In fact the Pentium Pro processor uses three pipelines, which allows it to execute more than one instruction simultaneously. Each Pentium Pro's pipeline consists of up to 14 individual steps. Pipelines have been used by previous generations of PC processors, such as the i486™ and the Pentium, to accelerate instruction execution. The reason for this is that x86 compatible instructions require a number of separate operations to complete. By using a pipeline, these processors are able to queue instructions through the individual operations so that one instruction can be completed with each clock cycle. Micro-ops The Pentium Pro processor takes the pipeline approach one stage further, by breaking the x86 instructions down into simpler operations called micro-ops. These micro-ops are then executed through the Pentium Pro's pipelines. The micro-ops are much simpler to handle than x86 instructions, and allow the Pentium Pro processor more flexibility in the way it executes them. The Pentium Pro can reorder the micro-ops within its pipelines for greater efficiency and can frequently execute multiple micro-ops in parallel. Multiple Execution Units The core step in a processor pipeline is the execution unit; this is where the real work is done. The pipeline steps before the execution units are mostly concerned with preparing the instruction for execution. The steps after the execution units involve the re-assembly of data