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 is a heavily modified, 3.3 volt Pentium P54 core manufactured on 0.6 micrometer technology. It is fitted with a 486-compatible bus unit (though with an increased pin-count), an integrated heatsink and fan, and 32 kB of level 1 cache, double the 16 kB offered on regular P54C chips. As the data bus was effectively reduced to 32-bit width, per-clock performance was much lower than that of a 'regular' Pentium, though still substantially faster compared to a similarly-clocked 486 owing to the Pentium's architectural improvements, such as the much improved FPU. It was also equipped with an integrated 3.3 volt power regulator as many 486 motherboards only provided 5 volt power.
The 63 MHz model was launched in February 1995, and supported 25 MHz bus systems. The much faster 83 MHz version, which supported both 25 (63 MHz effective) and 33 MHz bus systems, launched much later the same year on October, and was very expensive at $299 compared to other upgrade alternatives, such as those based on AMD's 5x86 and Cyrix's Cx5x86 chips.
The processor's heatsink is permanently attached, and the removable fan module is powered via spring-like metal prongs that connect to a trio of conductors on the surface of the chip. The clip that releases the fan is visible in the first photo, 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.
Compatibility and performance
During development, Intel had changed the design specification, causing various compatibility and performance problems with some boards that were previously fully compatible. For instance, the Packard Bell 450 motherboard required a specially-designed interposer to be installed between the processor and the motherboard to cope with the changed specification, with the unfortunate consequence of precluding access to the motherboard's level 2 cache, resulting in sub-par performance. In addition, some older chipsets do not support the write-back functionality of the chip's level 1 cache, which could also reduce performance. Fortunately, the majority of Socket 3 motherboards, particularly later (post-1994) VLB and most PCI boards, provide proper support for the Pentium OverDrive including fully operational access to the level 2 cache, and many earlier boards also support the processor with varying levels of compatibility and performance.
Performance-wise, many popular synthetic benchmarks of the time showed the Pentium OverDrive under-performing its much cheaper and higher-clocked rivals, though its real-world performance (given the motherboard cache was being optimally used) could be much different: programs that were floating-point dependent or optimized for the Pentium architecture (as were both becoming increasingly common in the mid to late nineties) derived a more substantial benefit from the Pentium OverDrive, particularly the 83 MHz version, over competing solutions. In addition, it fully supported programs and operating systems specifically coded for the Pentium architecture, such as many emulators, multimedia utilities and even later Windows operating systems and games; however, the benefit of running such programs on a clock- and motherboard bus-constrained system may be questionable.
- 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.
- UKT Support page for the Packard Bell 450 motherboard
- 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