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[ The PC Guide | Systems and Components Reference Guide | The Processor | Processor Families | Fifth Generation Processors ]

Intel Pentium OverDrive

In order to allow the easy processor upgrades of earlier motherboards, Intel has made available Pentium-class OverDrive chips. These are internally Pentium chips, but have subtle changes made to them to accommodate the unusual sockets and motherboards they are designed to work in. This allows them to be used where a regular Pentium would not work, which is the whole idea behind the OverDrive product--their interfaces appear to the motherboard like an older chip, so you don't need to worry about whether or not the motherboard supports the higher speed of what would be a native chip, in most cases.

Note: In general, if your motherboard can support a higher-speed regular (non-OverDrive) Pentium, that is the better way to go as opposed to an upgrade, because the regular chips are cheaper than the OverDrives. Your motherboard manual will tell you what your system's limitations are.

OverDrive processors have several changes made so that they can be used in older machines. First, when necessary they include integrated voltage regulators (sandwiched between the chip and the heat sink) so they will work with the voltage of the socket they are intended to go into. Second, they of course have the right pin configuration for the socket. Third, they are hard-wired to a specific clock multiplier; they do not set their multiplier based on motherboard jumpers like regular Pentiums do. This saves the user from worrying about what jumpers to change when replacing the processor.

The Pentium OverDrive comes in three basic flavors:

  • OverDrive for 486 Systems: This Pentium OverDrive is specially modified to fit the 32-bit data bus of a 486 system. As you know a Pentium normally uses a 64-bit data bus, but this is not consistent with a 486 motherboard's design. Therefore, this OverDrive is modified to use an external 32 bit bus. This reduces performance compared to a real Pentium, which is one reason why these chips score below what would be expected for a real Pentium of the same speed. Interestingly, this chip has a 32 KB primary cache, double the size of regular Pentiums. Presumably this was done to help mitigate the effects of this chip running on a 32-bit motherboard. (It's still slow, and in fact, slower than the top-end 486 and 5x86 chips that run in the same motherboards and cost much less).
    The Pentium OverDrive for 486s is available in two speeds: 63 MHz for 25 MHz bus systems and 83 MHz for 33 MHz systems. They obviously use a clock multiplier of 2.5. These chips are designed to work in a 5 volt system, and go into a Socket 2 or Socket 3 (the socket 3 must be set on 5 volts).
  • OverDrive for Pentium 60/66: The original Pentium chips were different than later versions, in terms of voltage, socket size and power consumption. Intel created a clock-doubling OverDrive for these chips, which is sold as one "120/133" chip: when replacing a Pentium 60 it runs at 120 MHz, and when replacing a 66 it runs at 133. This is a true Pentium chip since it is on a Pentium motherboard, although it still benchmarks below the real Pentium 120/133, most likely due to the older design of the Pentium 60/66 motherboards.
  • OverDrives for the Pentium 75, 90 and 100: These three chips run with a multiplier of 1.5 on system buses of 50, 60 and 66 MHz respectively. Intel has made for these OverDrives running at 125, 150 and 166 MHz (clock multiplier of 2.5). The 125 is an oddity because Intel never made a Pentium 125 as a stand-alone processor. Note that these three OverDrives have been replaced by versions of the Pentium with MMX OverDrive running at the same speed.

Except where noted above, the Pentium OverDrive has the same design and internal functioning as the regular Pentium. Refer to the section on the Pentium for more description of the Pentium's functions and features, and improvements over 486-class chips.

For many systems, OverDrive processors are the simplest way to get a performance increase at a reasonable price. Since those who need to use OverDrives are somewhat of a "captive market", Intel has traditionally priced OverDrives significantly higher than equivalent regular Pentiums. In some cases, the price difference can be exorbitant, with OverDrives selling for double what the street price is for the regular chip. The best time to buy OverDrives is after they have been on the market for a while and the prices have begun to decrease. Still, an upgrade of the motherboard and processor is often less expensive than an OverDrive processor, and yields better performance (at the cost of more work and risk, of course).

It's important to remember that increasing the performance of the processor is only part of the solution to increasing overall system performance. Many other factors impact on system performance, and increasing the processor clock while leaving the system bus speed the same is an exercise in diminishing returns, because the processor is increasingly stuck waiting for the rest of the system.

Note: Many older 486 motherboards do not support write-back primary cache. They will run the Pentium OverDrive slower than those that do.

Look here for an explanation of the categories in the processor summary table below, including links to more detailed explanations.

General Information

Manufacturer

Intel

Family Name

Pentium OverDrive

Code name

P24T

!?

Processor Generation

Fifth

Motherboard Generation

Fourth

Fifth

Version

Pentium Over
Drive 63 for 486

Pentium Over
Drive 83 for 486

Pentium Over
Drive 120/133

Pentium Over
Drive 125

Pentium Over
Drive 150

Pentium Over
Drive 166

Introduced

!?

Variants and Licensed Equivalents

--

Speed Specifications

Memory Bus Speed (MHz)

25

33

60 / 66

50

60

66

Processor Clock Multiplier

2.5

2.0

2.5

Processor Speed (MHz)

63

83

120 / 133

125

150

166

"P" Rating

~50

~66

~90 / ~100

125

150

166

Benchmarks

iCOMP Rating

443

581

877 / 970

1070

1176

1308

iCOMP 2.0 Rating

~42

~57

75 / 84

~105

114

127

Norton SI

~180

~240

~340 / ~370

~410

476

529

Norton SI32

~15

~19

~27 / ~30

~32

35

40

CPUmark32

~110

~140

~210 / ~235

~260

308

343

Physical Characteristics

Process Technology

Bipolar CMOS

Circuit Size (microns)

0.6

0.35

Die Size (mm^2)

!?

90

Transistors (millions)

!?

3.3

Voltage, Power and Cooling

External or I/O Voltage (V)

5

3.3 (STD) / 3.52 (VRE)

Internal or Core Voltage (V)

3.3 (STD) / 3.52 (VRE)

Power Management

SMM

Cooling Requirements

Active heat sink (included)

Packaging

Packaging Style

168-Pin PGA

273-Pin PGA

296-Pin SPGA

Motherboard Interface

Socket 2, Socket 3

Socket 4

Socket 5, Socket 7

External Architecture

Data Bus Width (bits)

32

64

Maximum Data Bus Bandwidth (Mbytes/sec)

95.4

127.2

457.8 / 508.6

381.5

457.8

508.6

Address Bus Width (bits)

32

Maximum Addressable Memory

4 GB

Level 2 Cache Type

Motherboard

Level 2 Cache Size

Varies

Usually 256 KB - 512 KB

Level 2 Cache Bus Speed

Same as Memory Bus

Multiprocessing

No

Internal Architecture

Instruction Set

x86 plus Pentium Extensions

MMX Support

No

Processor Modes

Real, Protected, Virtual Real

x86 Execution Method

Native

Internal Components

Register Size (bits)

32

Pipeline Depth (stages)

5

Level 1 Cache Size

16 KB Data, 16 KB Instruction

8 KB Data, 8 KB Instruction

Level 1 Cache Mapping

2-Way Set Associative

Level 1 Cache Write Policy

Write-Through (Data and Instruction), Write-Back (Data Only)

Integer Units

2

Floating Point Unit / Math Coprocessor

Integrated

Instruction Decoders

1

Branch Prediction Buffer Size / Accuracy

256 entries / 80%

Write Buffers

2

Performance Enhancing Features

--

Next: Intel Pentium with MMX Technology ("P55C")


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