The
architecture of Pentium Microprocessor:
Contributed**
by Rajesh Kothandapani
A Brief History
of the Pentium Processor Family
The Pentium family of processors,
which has its roots in the Intel486(TM)
processor, uses the Intel486
instruction set (with a few
additional instructions). The
term ''Pentium processor'' refers
to a family of microprocessors
that share a common architecture
and instruction set. The first
Pentium processors (the P5 variety)
were introduced in 1993. This
5.0-V processor was fabricated
in 0.8-micron bipolar complementary
metal oxide semiconductor (BiCMOS)
technology. The P5 processor
runs at a clock frequency of
either 60 or 66 MHz and has
3.1 million transistors.
The next version of the Pentium
processor family, the P54C processor,
was introduced in 1994. The
P54C processors are fabricated
in 3.3-V, 0.6-micron BiCMOS
technology. The P54C processor
also has System Management Mode
(SMM) for advanced power management
The Intel Pentium processor,
like its predecessor the Intel486
microprocessor, is fully software
compatible with the installed
base of over 100 million compatible
Intel architecture systems.
In addition, the Intel Pentium
processor provides new levels
of performance to new and existing
software through a reimplementation
of the Intel 32-bit instruction
set architecture using the latest,
most advanced, design techniques.
Optimized, dual execution units
provide one-clock execution
for "core" instructions,
while advanced technology, such
as superscalar architecture,
branch prediction, and execution
pipelining, enables multiple
instructions to execute in parallel
with high efficiency. Separate
code and data caches combined
with wide 128-bit and 256-bit
internal data paths and a 64-bit,
burstable, external bus allow
these performance levels to
be sustained in cost-effective
systems. The application of
this advanced technology in
the Intel Pentium processor
brings "state of the art"
performance and capability to
existing Intel architecture
software as well as new and
advanced applications.
The Pentium processor has two
primary operating modes and
a "system management mode."
The operating mode determines
which instructions and architectural
features are accessible.
These modes are:
· Protected Mode
This is the native state of
the microprocessor. In this
mode all instructions and architectural
features are available, providing
the highest performance and
capability. This is the recommended
mode that all new applications
and operating systems should
target. Among the capabilities
of protected mode is the ability
to directly execute "real-address
mode" 8086 software in
a protected, multi-tasking environment.
This feature is known as Virtual-8086
"mode" (or "V86
mode"). Virtual-8086 "mode"
however, is not actually a processor
"mode," it is in fact
an attribute which can be enabled
for any task (with
appropriate software) while
in protected mode.
· Real-Address Mode
(also called "real mode")
This mode provides the programming
environment of the Intel 8086
processor, with a few extensions
(such as the ability to break
out of this mode). Reset initialization
places the processor in real
mode where, with a single instruction,
it can switch to protected mode.
· System Management
Mode
The Pentium microprocessor
also provides support for System
Management Mode (SMM). SMM is
a standard architectural feature
unique to all new Intel microprocessors,
beginning with the Intel386
SL processor, which provides
an operating-system and application
independent and transparent
mechanism to implement system
power management and OEM differentiation
features. SMM is entered through
activation of an external interrupt
pin (SMI#), which switches the
CPU to a separate address space
while saving the entire context
of the CPU. SMM-specific code
may then be executed transparently.
The operation is reversed upon
returning.
Advanced Features
The Pentium P54C processor
is the product of a marriage
between the Pentium processor's
architecture and Intel's 0.6-micron,
3.3-V BiCMOS process The Pentium
processor achieves higher performance
than the fastest Intel486 processor
by making use of the following
advanced technologies.
· Superscalar Execution:
The Intel486 processor can execute
only one instruction at a time.
With superscalar execution,
the Pentium processor can sometimes
execute two instructions simultaneously.
· Pipeline Architecture:
Like the Intel486 processor,
the Pentium processor executes
instructions in five stages.
This staging, or pipelining,
allows the processor to overlap
multiple instructions so that
it takes less time to execute
two instructions in a row. Because
of its superscalar architecture,
the Pentium processor has two
independent processor pipelines.
· Branch Target Buffer:
The Pentium processor fetches
the branch target instruction
before it executes the branch
instruction.
· Dual 8-KB On-Chip
Caches: The Pentium processor
has two separate 8-kilobyte
(KB) caches on chip--one for
instructions and one for data--which
allows the Pentium processor
to fetch data and instructions
from the cache simultaneously.
· Write-Back Cache:
When data is modified; only
the data in the cache is changed.
Memory data is changed only
when the Pentium processor replaces
the modified data in the cache
with a different set of data
· 64-Bit Bus: With its
64-bit-wide external data bus
(in contrast to the Intel486
processor's 32-bit- wide external
bus) the Pentium processor can
handle up to twice the data
load of the Intel486 processor
at the same clock frequency.
· Instruction Optimization:
The Pentium processor has been
optimized to run critical instructions
in fewer clock cycles than the
Intel486 processor.
· Floating-Point Optimization:
The Pentium processor executes
individual instructions faster
through execution pipelining,
which allows multiple floating-point
instructions to be executed
at the same time.
· Pentium Extensions:
The Pentium processor has fewer
instruction set extensions than
the Intel486 processors. The
Pentium processor also has a
set of extensions for multiprocessor
(MP) operation. This makes a
computer with multiple Pentium
processors possible.
A Pentium system, with its
wide, fast buses, advanced write-back
cache/memory subsystem, and
powerful processor, will deliver
more power for today's software
applications, and also optimize
the performance of advanced
32-bit operating systems (such
as Windows 95) and 32-bit software
applications.
Block
diagram of pentium
