THE INTEL PENTIUM MICROPROCESSOR
The Pentium is a 32-bit architecture processor on a 64-bit bus, and is 100% software
compatible with Intel’s family of x86 processors. All application software that has been written
for Intel 80386 and Intel 80486 processors can run on the Pentium without modification. The
Pentium processor contains all the features of the Intel 80486 processor, with the following
added features which enhance performance:
Superscalar Architecture
The Pentium processor’s
static superscalar architecture has two instruction pipelines and a
floating-point unit, each capable of independent operation. The two pipelines allow the Pentium
to execute two integer instructions in parallel, in a single clock cycle. This is called instruction
pairing. Each instruction must be simple. One pipeline will always receive the next sequential
instruction of the one issued to the other pipeline. Using the pipelines in this way halves the
instruction execution time and almost doubles the performance of the processor, compared
with an Intel 80486 microprocessor of the same frequency.
FPU
The
floating point unit (FPU) incorporates optimized algorithms and dedicated hardware for
multiply, divide, and add functions. This increases the processing speed of common
operations.
Dynamic Branch Prediction
To implement the Pentium’s 4-state
dynamic branch prediction, the processor uses two
prefetch buffers. One buffer is used to prefetch instruction code in a linear way, and one to
prefetch instruction code depending on the contents of the
branch target buffer (BTB). The BTB
is a small cache which keeps a record of the way that the instruction branched the last time it
was used. When this information leads to a correct prediction on the subsequent branch, the
branch is executed without delay, thereby enhancing performance.
Bus Frequencies
Like the 80486 DX2 processor, the Pentium uses internal clock multiplication. For example,
the Pentium 150 MHz processor multiplies the 60 MHz system clock by 2.5. Switches 1 and 2,
on the system board switch bank, set the frequency of the Processor-Local bus. Switches 3, 4
and 7 set the clock multiplier ratio. The relationship of the switch settings to Processor-Local
bus and processor frequencies is summarized in the following table:
Switch
Processor-
Local Bus
Frequency
Switch
Frequency
Ratio
Processor :
Local Bus
Processor
Frequency
1
2
3
4
7
Closed
Closed
50 MHz
Open
Open
Open
1.5 : 1
75 MHz
Closed
Open
60 MHz
Open
Open
Closed
1.5 : 1
90 MHz*
Open
Closed
66 MHz
Open
Open
Closed
1.5 : 1
100 MHz
Closed
Open
60 MHz
Closed
Open
Closed
2 : 1
120 MHz
Open
Closed
66 MHz
Closed
Open
Closed
2 : 1
133 MHz
Closed
Open
60 MHz
Closed
Closed
Closed
2.5 : 1
150 MHz
Open
Closed
66 MHz
Closed
Closed
Closed
2.5 : 1
166 MHz
*The 90 MHz model is not available for the HP Vectra XM 5/xx series 4 PCs at the time of printing.
This information is provided for completeness only.
