Math Coprocessors

Until the arrival of the Intel 80486 microprocessor, PC CPUs only contained integer calculation instructions. Floating-point calculations had to be done in software, which was much slower.

To address this shortfall, Intel designed a separate math coprocessor, also commonly called a Floating Point Unit (FPU). The table below shows the various math coprocessors that were available to DOS PC owners:

Microprocessor Supporting Math Coprocessor
Intel 8088/8086 Intel 8087
Intel 80286 Intel 80287 IIT/ULSI 2C87
Intel 80386SX Intel 80387SX
Intel 80386DX Intel 80387DX
Chips & Technologies Super MATH
Cyrix 387DX
Cyrix CX-83D87 FasMath
Intel RapidCAD
LC 4C87DX Green Math
Xtend 83S87
Cyrix Cx486DLC Cyrix 487DLC

Intel 8087

The original Intel 8087 was the only coprocessor available for 8088- and 8086-based PCs. It came in three versions: 8087-5, 8087-2 and 8087-1.'

Intel 80287

The Intel 80287 was...


The 2C87 was IIT (also sold by ULSI) was available in two versions - 12.5 MHz and 20 MHz.

Cyrix 387DX

The 387DX was Cyrix' first coprocessor for 80386 systems. It was pin-compatible with the Intel 80387 math coprocessor, and was soon replaced with the more advanced CX-83D87.

It was released in 25 MHz, 33 and 40 MHz versions.

Cyrix FasMath CX-83S87

The CX-83S87 was designed to complement 386SX-based systems and was available in 16, 20, 25, and 33 MHz variants. Like its big brother, CX-83D87, these were the fastest co-processors you could buy for 386 systems. Any CX-83S87s manufactured after 1991 got the 387+ internals.

The CX-83S87 is designed to be used alongside the Cx486SLC CPU.

Cyrix FasMath CX-83D87 / 387DX+

The CX-83D87 (called 387DX+ in Europe) was introduced in 1989. It is the fastest 387-compatible coprocessor, available in 25 MHz, 33 MHz and 40 MHz variants, and provides up to 50% more performance than the Intel 387DX. The 83D87 also offers the most accurate transcendental functions of all coprocessors. It is the 387 clone with the highest degree of compatibility to the Intel 387DX. Unlike the Intel 387DX, the 83D87 (and all other 387-compatible chips as well) does not support asynchronous operation of CPU and coprocessor. To reduce power consumption the 83D87 features advanced power saving features. Those portions of the coprocessor that are not needed are automatically shut down. If no coprocessor instructions are being executed, all parts except the bus interface unit are shut down.
Benchmark tests have shown that FasMath chips with a grey top run slightly faster than those with a black top, and also faster than the DLC versions of this chip.


The 3C87SX math coprocessor from IIT was designed to sit alongside a 386SX CPU, and came in 16, 20, 25, 33 and 40 MHz versions.


The 3C87 math coprocessor from IIT was designed to sit alongside a 386DX CPU, and came in 20, 25, 33 and 40 MHz versions.


The XC87SLC math coprocessor from IIT was designed to sit alongside a 486SX CPU, and came in 33 MHz version only.

Cyrix 487DLC

Because the Cyrix Cx486SLC/DLC CPUs didn't come with an internal FPU (floating-point unit), motherboards that supported these Cyrix CPUs sometimes also provided a Cyrix math coprocessor socket. These were designed to accommodate the 487DLC coprocessor. These came in 33 and 40 MHz variants.


The 4C87DLC math coprocessor from IIT was designed to sit alongside a 486DX CPU, and came in 33 and 40 MHz versions.


The XC87DLC math coprocessor from IIT was designed to sit alongside a 486DX CPU, and came in 33 MHz version only.