The Pentium OverDrive was a microprocessor marketing brand name used by Intel, to cover a variety of consumer upgrade products sold in the mid-1990s. It was originally released for 486 motherboards, and later some Pentium sockets. Intel dropped the brand, as it failed to appeal to corporate buyers, and discouraged new system sales.
The Pentium OverDrive was claimed to enable owners of 486 type motherboards to upgrade their machines to Pentium performance, without the cost of having to replace the entire system. The chip was a heavily modified Pentium P54 architecture, made with 0.6 micrometre technology and operating on 3.3 volts, but with a half-wide data bus (32-bit) and a larger 32 kB (16 kB + 16 kB) L1 cache, double its P5-platform Pentium peers. Unfortunately the design was plagued with various compatibility problems. Intel changed the specification during development, rendering previously-compatible motherboard designs incompatible. The chip also did not always benefit from the motherboard's cache RAM, resulting in sub-par performance.
When the Pentium OverDrive 83 MHz launched, significantly later than the mere 63 MHz version, it did so at $299, an exorbitant price compared to other upgrade alternatives. The AMD 5x86 and Cyrix Cx5x86 processors were usually faster and were considerably cheaper. Even Intel's own DX4, based on an older chip architecture, was typically faster. Only on some applications, where floating point arithmetic was used, could the Pentium OverDrive outperform its predecessors.
Two interesting parts of the Pentium OverDrive for 486 systems are the integrated fan/heatsink combination and the onboard voltage regulation. The processor cooler is permanently attached and the fan is powered by a trio of conductors on the surface of the chip. They power the fan through spring-loaded metal points in the fan assembly, which is removable to allow replacement of the fan if necessary. The clip that releases the fan is viewable in the photo above, at the top left corner of the CPU. The central plastic "column" that leads from the center of the fan houses the fan wiring and leads down the side of the heatsink at this corner. The small plastic points at each top left of this column are the locking mechanism for the fan and are released by squeezing them. The opposite corner of the CPU has a latch that locks the fan around underneath the heatsink, by swinging into place upon assembly. The processor monitors the fan and will throttle back on clock speed to prevent overheating and damage if the fan is not operating. This is a predecessor to the internal temperature detection and protection in Intel's modern processors.
The onboard power regulation circuitry, partly visible near the bottom of the photo, allows the CPU to operate on boards that provide only 5 volts to the CPU. This is necessary because the processor (die) itself operates at 3.3 V like a regular P54C-core Pentium. Late-model 486 motherboards did provide this voltage, because some late-model 486 CPUs like the AMD 5x86 required it, but many boards only provided 5 V power.
- Introduced February 3, 1995
- 235 pins, P24T pinout
- 5 or 3.3 volts
- L1 Cache 32 kB (16 kB + 16 kB)
- 63 MHz on 25 MHz front side bus (25 × 2.5)
- Introduced October 1995
- 237 pins, P24T pinout
- 5 or 3.3 volts
- L1 Cache 32 kB (16 kB + 16 kB)
- 83 MHz on 33 MHz front side bus (33 × 2.5)
The original Pentium chips ran at higher voltages than later models, with a slower 60 or 66 MHz front side bus speed (Socket 4, 5V). Although little known, Intel did in fact release an OverDrive chip for these sockets, that used an internal clock multiplier of 2, to change them to a "120/133" machine.
- PODP5V120: 120 MHz on 60 MHz bus
- PODP5V133: 133 MHz on 66 MHz bus or 120 MHz on 60 MHz bus
The OverDrive Processors for the Pentium 75, 90 and 100 were also released (Socket 5, 3.3V), 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.
- PODP3V125: 125 MHz on 50 MHz bus
- PODP3V150: 150 MHz on 60 MHz bus
- PODP3V166: 166 MHz on 66 MHz bus
These were replaced by Pentium OverDrive MMX, which also upgraded the Pentium 120 - 200 MHz to the faster with MMX technology.
- PODPMT66X200: up to 200 MHz on 66 MHz bus (clock multiplier of 3.0)
- PODPMT66X166: up to 166 MHz on 66 MHz bus (clock multiplier of 2.5)
- PODPMT60X180: up to 180 MHz on 60 MHz bus (clock multiplier of 3.0)
- PODPMT60X150: up to 150 MHz on 60 MHz bus (clock multiplier of 2.5)
In 1998 the Pentium II OverDrive, part number PODP66X333, was released as an upgrade path for Pentium Pro owners. This upgrade could be used in single and dual processor Socket 8 systems, or in two sockets of quad processor Socket 8 systems with CPU 3 and 4 removed.
Combining the Pentium II Deschutes core in a flip-chip package with a 512 kB full speed L2 cache chip from the Pentium II Xeon into a Socket 8-compatible module resulted in a 300 or 333 MHz processor that could run on a 60 or 66 MHz front side bus. This combination brought together some of the more attractive aspects of the Pentium II and the Pentium II Xeon: MMX support/improved 16-bit performance and full-speed L2 cache, respectively. The later "Dixon" mobile Pentium II core would emulate this combination with its 256 kB of full-speed cache.
In Intel's "Family/Model/Stepping" scheme, the Pentium II OverDrive CPU is family 6, model 3. Though it was based on the Deschutes core, when queried by the CPUID command, it identified as a Klamath Pentium II. As noted in the Pentium II Processor update documentation from Intel, "Please note that although this processor has a CPUID of 163xh, it uses a Pentium II processor CPUID 065xh processor core." 
The major customer for the production of these chips was Sandia National Laboratories' ASCI Red supercomputer, which had all 4,510 CPUs upgraded in 1999. After the upgrade the system was once again the world's fastest on the Top500.
- INTEL ANNOUNCES FIRST PENTIUM(TM) OVERDRIVE(TM) PROCESSOR, Press Release, Google Groups, February, 1995.
- Crothers, Brooke. System upgrades: Intel plans OverDrive for Pentium systems. Infoworld, September 11, 1995, Vol. 17, Issue 37.
- http://web.archive.org/web/19990914131901/http://www.heise.de/ct/english/98/18/020/ , Wayback machine archive of Heise, accessed April 12, 2009
- Specification Update for the Pentium II Processor, page 15, note 3