Determining if you have a 3-volt regulator: First check your motherboard User's Manual for references to voltage jumper settings and/or settings for DX4 CPUs. If you are now using a 486DX4 or 5x86 CPU, the motherboard supports 3-volt processors. Otherwise, open your case and look at the motherboard. If the manual refers to jumper settings for voltage and/or DX4 operation, verify that these jumpers are actually on the motherboard. If not, the motherboard probably omitted the voltage regulator. As a final check, look for the regulator itself _ a black rectangular transistor about index fingernail size with three leads soldered to the motherboard; this may be mounted on a small finned metal heatsink, and is usually near the CPU socket. If you find the regulator, the board should support 3-volt CPUs and you should also be able to find the jumpers for voltage settings; otherwise it probably does not have an on-board 3-volt source. If your motherboard doesn't support 3-volt CPUs, check out Tips for Upgrading Older 486 Computers.

IBM 486BLX (Blue Lightning) motherboards can be upgraded to 100 MHz with the IBM 486BLX3-100 CPU. Cyrix FasCache motherboards can be upgraded to 100 MHz with the Cyrix Cx486DX4-100 CPU. These clock tripled CPUs were the fastest made by their manufacturers, so that any higher performance would require motherboard and CPU replacement.

Clock-tripled DX4 processors are made by Intel ("P24C" at 75/100 MHz) and AMD (100/120 MHz). Initial versions used write-through on-board (L1) cache (16 kbytes in the Intel CPUs/8 kbytes in the AMD). Later versions (Intel i80486DX4WB/SL and AMD Am486DX4 Enhanced/SV8B) have enhancements including write-back cache. Selection of X2 or X3 bus clock operation is by motherboard jumper settings controlling the CLKMUL (B-13) pin state. Enhanced versions from both manufacturers are pin compatible with one another. These DX4 processors have generally been surpassed by the following 5x86 CPUs from AMD and Cyrix.

The AMD Am5x86-133 ("P75") CPU is a clock quadrupled version of the DX4. It has a 16 kbyte write-back cache and other enhancements, and the CLKMUL pin state determines either X3 or X4 operation (the X4 condition corresponds to the X2 condition for the Intel DX4). This CPU is popular with upgraders because it is now inexpensive and can usually be overclocked successfully to 150 or 160 MHz. Note that 150 MHz operation is obtained with the 3X setting and a 50 MHz CLK on the motherboard, and 160 MHz from 4X with a 40 MHz CLK, and so the motherboard must permit selection of these clock speeds for such overclocking to be possible. Disregarding floating point unit (FPU) performance, when operated at 133 MHz benchmarks are comparable to a Pentium 75 (hence the P75 designation); at 160 MHz, benchmarks are between the Pentium 90 and Pentium 100. FPU performance, however, is somewhat lower than the comparable Pentium's. Manufacturer data.

The Cyrix 5x86-100/120 MHz CPU uses 5th generation architectural features in a 486DX compatible processor, including a 16-kbyte, write-back, 4-way associative cache. In benchmarks, it outperforms the AMD 5x86 at the same clockspeed but, unlike the AMC chip, cannot normally be overclocked (although this is subject to correction as more experience becomes available). The 100 and 120 MHz versions can be operated with X2 (2 x 50 = 100) and X3 (3 x 33 = 100 MHz / 3 x 40 = 120 MHz) multiplication. Performance at 100 MHz is at about Pentium 75 level. A 133 MHz version was also produced in limited quantities which used a 4X multiplier with a 33 MHz CLK for 133 MHz internal rate, but this version is no longer available. These CPUs may have integrated heatsinks (without fans) mounted on the CPU. Manufacturer data.

The "enhanced" versions of the DX4 and the 5x86s from both Cyrix and AMD will usually run in older motherboards not designed for them, but with lower performance since features such as the SL enhanced (SMM) mode, Stop Clock and write-back L1 cache are automatically disabled; in other cases, however, they do not run satisfactorily and either a BIOS upgrade or new motherboard may be necessary. Since you can get close to 100 MHz Pentium performance with either the Cyrix or AMD 5x86s and their prices are lower than the comparable Pentium CPUs, buying a new motherboard optimized to use them is a practical upgrade option. If you buy a 5x86 CPU from ComputerNerd USA and it doesn't work on your motherboard, you may be able to trade in your old motherboard against the purchase price of a new motherboard optimized for maximum performance with these advanced processors.

By "Newer", we are referring to computers having motherboards with on-board 3-volt regulators, which will generally have been made after about 1993/1994.

Determining if you have a 3-volt regulator: First check your motherboard User's Manual for references to voltage jumper settings and/or settings for DX4 CPUs. If you are now using a 486DX4 or 5x86 CPU, the motherboard supports 3-volt processors. Otherwise, open your case and look at the motherboard. If the manual refers to jumper settings for voltage and/or DX4 operation, verify that these jumpers are actually on the motherboard. If not, the motherboard probably omitted the voltage regulator. As a final check, look for the regulator itself _ a black rectangular transistor about index fingernail size with three leads soldered to the motherboard; this may be mounted on a small finned metal heatsink, and is usually near the CPU socket. If you find the regulator, the board should support 3-volt CPUs and you should also be able to find the jumpers for voltage settings; otherwise it probably does not have an on-board 3-volt source. If your motherboard doesn't support 3-volt CPUs, check out Tips for Upgrading Older 486 Computers.

IBM 486BLX (Blue Lightning) motherboards can be upgraded to 100 MHz with the IBM 486BLX3-100 CPU. Cyrix FasCache motherboards can be upgraded to 100 MHz with the Cyrix Cx486DX4-100 CPU. These clock tripled CPUs were the fastest made by their manufacturers, so that any higher performance would require motherboard and CPU replacement.

Clock-tripled DX4 processors are made by Intel ("P24C" at 75/100 MHz) and AMD (100/120 MHz). Initial versions used write-through on-board (L1) cache (16 kbytes in the Intel CPUs/8 kbytes in the AMD). Later versions (Intel i80486DX4WB/SL and AMD Am486DX4 Enhanced/SV8B) have enhancements including write-back cache. Selection of X2 or X3 bus clock operation is by motherboard jumper settings controlling the CLKMUL (B-13) pin state. Enhanced versions from both manufacturers are pin compatible with one another. These DX4 processors have generally been surpassed by the following 5x86 CPUs from AMD and Cyrix.

The AMD Am5x86-133 ("P75") CPU is a clock quadrupled version of the DX4. It has a 16 kbyte write-back cache and other enhancements, and the CLKMUL pin state determines either X3 or X4 operation (the X4 condition corresponds to the X2 condition for the Intel DX4). This CPU is popular with upgraders because it is now inexpensive and can usually be overclocked successfully to 150 or 160 MHz. Note that 150 MHz operation is obtained with the 3X setting and a 50 MHz CLK on the motherboard, and 160 MHz from 4X with a 40 MHz CLK, and so the motherboard must permit selection of these clock speeds for such overclocking to be possible. Disregarding floating point unit (FPU) performance, when operated at 133 MHz benchmarks are comparable to a Pentium 75 (hence the P75 designation); at 160 MHz, benchmarks are between the Pentium 90 and Pentium 100. FPU performance, however, is somewhat lower than the comparable Pentium's. Manufacturer data.

The Cyrix 5x86-100/120 MHz CPU uses 5th generation architectural features in a 486DX compatible processor, including a 16-kbyte, write-back, 4-way associative cache. In benchmarks, it outperforms the AMD 5x86 at the same clockspeed but, unlike the AMC chip, cannot normally be overclocked (although this is subject to correction as more experience becomes available). The 100 and 120 MHz versions can be operated with X2 (2 x 50 = 100) and X3 (3 x 33 = 100 MHz / 3 x 40 = 120 MHz) multiplication. Performance at 100 MHz is at about Pentium 75 level. A 133 MHz version was also produced in limited quantities which used a 4X multiplier with a 33 MHz CLK for 133 MHz internal rate, but this version is no longer available. These CPUs may have integrated heatsinks (without fans) mounted on the CPU. Manufacturer data.

The "enhanced" versions of the DX4 and the 5x86s from both Cyrix and AMD will usually run in older motherboards not designed for them, but with lower performance since features such as the SL enhanced (SMM) mode, Stop Clock and write-back L1 cache are automatically disabled; in other cases, however, they do not run satisfactorily and either a BIOS upgrade or new motherboard may be necessary. Since you can get close to 100 MHz Pentium performance with either the Cyrix or AMD 5x86s and their prices are lower than the comparable Pentium CPUs, buying a new motherboard optimized to use them is a practical upgrade option.