HP Vectra XU 6/XXX Optimization Manual page 16

16-bit Applications
Many of today's more popular applications, including any designed for Windows for
Workgroups, are 16-bit applications. These use 16-bit instructions, and handle both data and
memory addresses as 16-bit quantities.
The Pentium Pro Processor
The Pentium Pro processor in your PC is a 32-bit processor. This means that it has been
specifically designed to execute 32-bit instructions, and has 32-bit registers for storing data and
addresses. Your PC will give optimized performance when running 32-bit applications.
If you run 16-bit applications, you may be disappointed with the performance of your PC. This
is because 16-bit applications are unable to use many of the Pentium Pro processor's
performance-enhancing features.
Using Pipelines to Accelerate Instruction Execution
The processor executes instructions in a number of steps through a sequential process called a
pipeline. The Pentium Pro processor has three pipelines, each of which may contain a queue of
instructions.
With 32-bit instructions, the processor will keep the pipeline's queue full; as an instruction
progresses down the pipeline, another will be loaded in behind it. With each pulse of the
processor's internal clock, all instructions will move one stage further down the queue. The
result of using this mechanism is that one instruction can be completed by each pipeline with
each pulse of the clock.
Why 16-bit Applications are Slower
When executing 16-bit instructions, the Pentium Pro processor is limited by two constraints.
First, the Pentium Pro cannot mix 32-bit and 16-bit instructions in a pipeline. Second, at any
moment only one 16-bit instruction can normally be present in a pipeline.
When running a 16-bit application, all instructions are 16-bits and so each instruction is
executed singly, without use of the pipeline's queuing facility. In effect, the pipeline is not used.