Add-on FPUs
In the 1980s, it was common in IBM PC/compatible microcomputers for the FPU to be entirely separate from the CPU, and typically sold as an optional add-on. It would only be purchased if needed to speed up or enable math-intensive programs.
The IBM PC, XT, and most compatibles based on the 8088 or 8086 had a socket for the optional 8087 coprocessor. The AT and 80286-based systems were generally socketed for the 80287, and 80386/80386SX based machines for the 80387 and 80387SX respectively, although early ones were socketed for the 80287, since the 80387 did not exist yet.
Starting with the i486 (Intel dropped the '80' prefix from the 48 series substituting 'i' instead), in x86 chips the floating-point unit was integrated with the CPU, something true for almost all later x86-architecture processors. One notable exception is the i486SX; it was also unusual in that no actual coprocessor was available. Early examples of the i486SX was a full CPU with an integrated FPU; If during hardware testing the FPU of a i486 chip failed while the rest of the CPU hardware passed the FPU would be disabled and the chip would be packaged as a lower cost i486SX. If the yields of i486DX chips were high enough, a fully working FPU would be pysically disabled to meet the demands for the i486SX. Eventually i486SX chips were specifically manufactured without the FPU on the die. Later boards where the i486SX chips were soldered directly onto the board, it was not possible to replace the entire chip with a fully functional i486DX chip. Instead the i487SX chip was marketed as the coprocessor for these boards that had coprocessor sockets. In reality the i487SX chip was a full i486 chip that completely disabled the original i486SX chip and took over all CPU operations for the board. The i487SX was almost electrically identical to the i486SX. The sole difference was an extra pin whose sole purpose was to disable the existing i486SX when installed. The i487SX could be used as a substitute i486DX by clipping off the extra pin.
In addition to the Intel x87 series, several other companies manufactured co-processors for the x86 series. These included Cyrix which marketed its FasMath series as higher performance but fully x87 compatible, and Weitek which offered a high-performance but not fully x87 compatible series of coprocessors.
In addition to the Intel architectures, FPUs as coprocessors were available for the Motorola 68000 family line. These FPUs, the 68881 and 68882, were common in Motorola 68020/68030-based workstations like the Sun 3 series. They were also commonly added to higher-end models of Apple Macintosh and Commodore Amiga series, but unlike IBM PC-compatible systems, sockets for adding the coprocessor were not as common in lower end systems. With the 68040, Motorola integrated the FPU and CPU, but like the x86 series, a lower cost 68LC040 without an integrated FPU was also available.
Also, there are add-on FPUs coprocessor units for microcontroller units (MCUs/µCs)/single-board computer (SBCs)' which serve to provide floating-point arithmetic capability in systems that might not otherwise possess said functionality. The difference in these types of FPU coprocessors, when compared to more traditional floating-point coprocessors such as the 80x87 series, is that these add-on FPUs are host-processor-independent, possess their own programming requirements, and are often provided with their own integrated development environments (IDE)s.
Read more about this topic: Floating-point Unit