Comment by adrian_b

A significant difference in 80387 versus 80287 & 8087 was that in 387 you could no longer select the "projective" behavior for infinities (where positive and negative infinities are identical).

This feature had not been included in the IEEE standard, so it was no longer implemented.

Testing whether this feature works or not was used in the programs running on an 80386 CPU to detect whether the attached FP coprocessor was a 287 or a 387 (because the hardware allowed both; 387 was launched later than 386, so initially a 386 had to be coupled with a 287, if a hardware FPU was needed).