Processor Types and Specifications - Upgrading and Repairing PCs - page 91

The Pentium is fully compatible with previous Intel processors, but it differs from them in many
ways. At least one of these differences is revolutionary: The Pentium features twin data pipelines,
which enable it to execute two instructions at the same time. The 486 and all preceding chips can
perform only a single instruction at a time. Intel calls the capability to execute two instructions at the
same time superscalar technology . This technology provides additional performance compared to
the 486.
With superscalar technology, the Pentium can execute many instructions at a rate of two instructions
per cycle. Superscalar architecture usually is associated with high-output RISC chips. The Pentium is
one of the first CISC chips to be considered superscalar. The Pentium is almost like having two 486
chips under the hood. Table 3.12 shows the Pentium processor specifications.
Table 3.12. Pentium Processor Specifications
The two instruction pipelines within the chip are called the u- and v-pipes. The u-pipe , which is the
primary pipe, can execute all integer and floating-point instructions. The v-pipe is a secondary pipe
that can execute only simple integer instructions and certain floating-point instructions. The process of
operating on two instructions simultaneously in the different pipes is called pairing . Not all
sequentially executing instructions can be paired, and when pairing is not possible, only the u-pipe is
used. To optimize the Pentium's efficiency, you can recompile software to enable more instructions to
be paired.
The Pentium processor has a branch target buffer (BTB), which employs a technique called branch
prediction . It minimizes stalls in one or more of the pipes caused by delays in fetching instructions
that branch to nonlinear memory locations. The BTB attempts to predict whether a program branch
will be taken and then fetches the appropriate instructions. The use of branch prediction enables the
Pentium to keep both pipelines operating at full speed. Figure 3.25 shows the internal architecture of
the Pentium processor.